Mobile phone adhesive use in a single smartphone may involve over 10 adhesive application points, spanning six major modules; however, during SQE factory audits, inspectors often only possess knowledge of one adhesive testing method.
More subtly, many adhesives that pass initial in-process inspection may subsequently exhibit failures in downstream processes—such as OCA repeatedly degassing causing the adhesive layer to “peel off,” lubricating oil applied after camera focus adjustment causing the adhesive to adhere to the oil film, failure to cure battery PUR adhesive before housing installation, or the bottom fill adhesive being “burned out” during rework reflow soldering on the motherboard.
I. Display-Module Mobile Phone Adhesive: Chronic Damage During Defoaming
Display-module mobile phone adhesive choices fall into three primary types: OCA optical adhesive (full-lamination), polyurethane hot-melt adhesive (frame sealing), and UV protective adhesive (lead wire fixation).
The key parameters of OCA optical adhesive are transmittance, haze, and refractive index. In mainstream industry products, the transmittance exceeds 91%, the haze is below 0.5%, the refractive index ranges from 1.47 to 1.49, and the adhesive layer thickness ranges from 100 to 250 μm. Non-uniform adhesive layer thickness can lead to Mura, i.e., uneven display performance. For UV-curable OCA, the peel force after curing increases from 1,000 gf/25 mm to over 1,200 gf/25 mm.
Repetitive debubbling is the most common form of “chronic peeling.” During debubbling, the combination of high temperature and high pressure (80–100 °C, 5–8 kg/cm²) induces repeated stress relaxation in the adhesive layer, thereby reducing its adhesion strength. In the second debubbling cycle, the same conditions trigger viscoelastic creep in the adhesive layer, accompanied by molecular chain slippage and interfacial stress relaxation, leading to localized decreases in adhesive strength. By the third debubbling cycle, micro-peeling zones have already emerged at the edges, which subsequently manifest as bubble rebound or edge lifting during subsequent aging tests. When the debubbling temperature exceeds the effective softening temperature of the OCA, the adhesive layer softens and flows under high temperature; the high pressure then exerts transverse compression on the adhesive layer, resulting in a reduced effective bonding area after cooling.
Inadequate removal of adhesive residue during rework constitutes another critical failure point. Using a blade to remove adhesive makes it difficult to achieve 100% removal; any residual OCA film (5–10 μm) has already undergone aging and crosslinking, and its surface-active groups have been depleted. When new adhesive is applied, the actual adhesive layer in contact is the aged layer rather than the glass substrate. During peel force testing, the measured values may appear normal; however, after aging, cracking occurs between the new and aged adhesive layers, manifesting as interlayer delamination. This type of failure cannot be detected during incoming material inspection.
Excessive bonding pressure can also lead to batch failure. Overpressure can cause the OCA to be thinned, result in insufficient filling at ink segment transitions, and induce stress concentration after curing. Also, overpressure may cause false bonding—where the material temporarily flows under high pressure to fill the gap, but upon pressure release, elastic rebound causes edge lifting.
Figure 1: Relationship between the number of OCA defoaming cycles and the edge peeling force retention rate (repeated defoaming induces viscoelastic creep, leading to progressive decay of the peeling force)
Table 1: Key Control Points for Adhesives Used in Display Modules
| Resin type | What are the key priorities? | What does IQC inspect? | What does the production line need to protect against? |
| OCAoptical cement | Transmittance, haze, adhesive layer thickness, refractive index matching | Transmittance> 91%, opacities <0.5%, adhesive layer thickness ± 15 μm, edge peel strength sampling test (difference between the outer 10 cm and the center <15%) | Defoiling temperature ≤ 80 °C, re-foaming rate <5%, thickness variation at five sampling points after bonding <20 μm; rework requires plasma cleaning. |
| Polyurethane hot-melt adhesive (frame) | Final adhesion, temperature control, and moisture-cured environment | 7-day final viscosity (not initial viscosity) and melt viscosity stability | Set the adhesive application temperature to 145–150 °C (±2 °C) and the humidity to 30–70 % RH; avoid adhesive overflow to prevent blockages. |
| UV protective adhesive (lead frame/IC) | Curing degree and bonding strength | UVDSC layerwise curing degree (difference between inner and outer layers <3%) and peeling force after curing | UV irradiation: 400–600 mJ/cm², wavelength 365/395 nm, with supplemental irradiation in shaded areas |
II. Camera Module: Focusing Oil and Laser Thermal Shock
Mobile phone adhesive choices for camera modules include UV adhesive (for lens fixation), low-temperature epoxy adhesive (for holder bonding), and epoxy adhesive (for VCM dispensing). An optical axis deviation exceeding 5 μm can trigger a false detection by PDAF; the curing shrinkage rate of conventional adhesives ranges from 3% to 5%, which is sufficient to cause axial displacement exceeding 15 μm.
Curing in the shaded regions of UV adhesives remains a persistent challenge. UVDSC testing reveals that the curing degree of the outer layer reaches 95.70%, whereas that of the inner layer is only 92.24%. Due to insufficient UV penetration in these shaded areas, the curing of the inner layer in free-radical UV adhesives consistently lags behind. Cationic UV adhesives can achieve a shrinkage rate of approximately 2%, albeit at a higher cost. For industrial-grade UV adhesives, the single-spot application amount can be controlled within ±0.02 mg of 0.15 mg; however, a layer thickness exceeding 20 μm may compromise the precision of OIS optical image stabilization by up to ±0.5 μm.
The curing shrinkage of epoxy adhesive is equally critical. For conventional epoxy adhesives, after 100 hours of aging at 85 °C, lens displacement can reach 5–8 μm, whereas industrial-grade low-shrinkage adhesives (Tg=112 °C, tensile modulus=85 MPa, at 25 °C) can limit this displacement to 1–2 μm. According to publicly available literature, a displacement exceeding 5 μm may trigger erroneous PDAF detection. The lens is fabricated from H-K9L glass (CTE ≈ 7.1×10⁻⁶/°C), while the holder typically employs PBT material (CTE ≈ 100–140×10⁻⁶/°C); the difference in CTE between these materials can span up to two orders of magnitude. Even when a metal mount is used, this CTE mismatch remains the primary driving force for focal plane drift following curing shrinkage of the epoxy adhesive.
Regarding post-process damage, the residual lubricant in the focusing mechanism is the most challenging root cause to pinpoint. The lubricating oil in the focusing mechanism can splash onto the inner wall of the holder or the edge of the lens; during dispensing, the adhesive is applied directly onto this oil film. After UV curing, the interface between the adhesive layer and the glass does not form chemical bonds, but instead the adhesive adheres to the oil film. The initial peel force may appear normal; however, after aging at 85 °C, the oil film spreads, leading to interfacial delamination, which manifests as lens loosening and defocusing.
Thermal shock induced by laser welding can likewise cause damage to the adhesive layer. After the holder is bonded to the base, certain structures require laser welding to secure metal components; the instantaneous temperature of the laser can reach 300–500 °C, and this heat can be conducted into the adhesive layer that has just undergone initial curing. During the post-curing stage (24–72 h), epoxy adhesives or UV adhesives subjected to secondary high temperatures may experience chain scission, leading to softening of the adhesive layer and a decrease in Tg; alternatively, excessive crosslinking can result in an increase in Tg, causing the adhesive layer to harden and become brittle. These two effects represent competing mechanisms; following thermal shock, the adhesive layer typically exhibits reduced toughness and a loss of cushioning capability during impact events.
The issues from rework residues do not manifest immediately. During lens rework, the lens must be removed; however, due to the strong adhesion between the UV adhesive and the glass, the adhesive layer may tear during removal, leaving a thin adhesive residue (5–10 μm) on the glass surface—essentially imperceptible to the naked eye. When new adhesive is applied, it adheres to the residual layer of the old adhesive rather than directly to the glass. Over time, as the materials age, cracks develop between the new and old adhesives, ultimately leading to complete detachment of the lens.
Figure 2: Relationship between the curing shrinkage rate of the epoxy adhesive and lens displacement (low-shrinkage adhesive can limit displacement to within 2 μm, whereas conventional adhesive exceeds the PDAF misjudgment threshold).
Table 2: Key Control Points for Adhesives Used in Camera Modules
| Resin type | What are the key priorities? | What does IQC inspect? | What does the production line need to protect against? |
| UV adhesive (lens fixation) | Curing degree of the shaded area, dispensing precision, and adhesive layer thickness | UVDSC layered curing degree (difference between inner and outer layers <3%), single-point adhesive dosage: 0.15 ± 0.02 mg, adhesive thickness: ± 3 μm | After focusing, perform plasma degreasing followed by UV dual-sided/rotational irradiation; for photoresist thickness>20 μm, employ OIS lag prevention. |
| Low-temperature epoxy adhesive (Holder bonding) | Low stress, curing shrinkage, and displacement control | (1) Tg and modulus (DSC/DMA, type test); (2) Lens displacement <2 μm after 85 °C/100 h aging (type test) | Implement segmented temperature ramping in the curing temperature profile to avoid thermal shock; maintain a minimum interval of ≥24 h between laser welding and dispensing operations. |
| Epoxy adhesive (VCM dispensing) | Curing shrinkage rate, CTE matching | Fluorescence spectrum (450 nm decay <2%), photoinitiator type (excluding benzophenone) | After rework, measure the residual thickness of the adhesive layer (<5 μm), then perform plasma cleaning after adhesive removal. |
III. Battery Module: The Time Trap of PUR Adhesive and Electrolyte Corrosion
Battery-module mobile phone adhesive options include thermal conductive adhesives, PUR hot-melt adhesives, and sealants. The thermal conductivity coefficient of thermal conductive adhesives ranges from 0.85 to 2.5 W/m·K, and their bonding strength exceeds 2.5 MPa.
The 72-hour aging test represents the most critical challenge for PUR adhesives. While the initial tack of PUR hot-melt adhesives meets specifications, their final tack requires 72 hours to stabilize. Under high temperatures, the final tack of PUR adhesives may decrease by over 70% (according to publicly available literature, a specific PUR adhesive brand exhibited a final tack of approximately 7.5 MPa at 80 °C, which declined to about 1.9 MPa after 7 days, corresponding to a retention rate of approximately 25%, under ASTM D1002 test conditions). The melt viscosity of PUR adhesives is temperature-sensitive; fluctuations of ±5 °C in temperature can adversely affect coating uniformity.
Hastening production line throughput is the most common mistake. The final adhesion of PUR adhesive requires 72 hours to stabilize; however, in order to meet production targets, the assembly line proceeds to case loading and screw fastening within 4–6 hours after adhesive application. The pressure applied during screw tightening expels the uncured PUR adhesive, resulting in uneven adhesive layer thickness. More critically, once the case is sealed, the adhesive layer becomes confined within a narrow space, preventing moisture ingress, which halts the curing reaction and means the final adhesion can never reach the design specification. After six months of use, users may experience battery loosening and abnormal noises in their mobile phones.
Electrolyte splashing constitutes a chemical attack. During the electrolyte filling process, the electrolyte (containing LiPF6) inevitably splashes onto the thermal conductive adhesive or sealant along the edges of the battery casing. The electrolyte penetrates the adhesive layer and undergoes a swelling reaction with the polymer chains of polyurethane or silicone, causing the adhesive layer to soften and expand. Initially, the bonding strength remains normal; however, after three months, the volumetric expansion rate of the adhesive layer exceeds 20%, and the bonding strength drops below 30% of its initial value, leading to failure of the battery pack sealing.
Hot pressing accelerates the sedimentation of fillers. After battery pack assembly, certain structures require hot pressing (80–100 °C) to ensure tight adhesion between the cells and the casings. During hot pressing, the alumina/boron nitride fillers in the thermal conductive adhesive accelerate their sedimentation: in the upper adhesive layer, where the filler distribution is sparse, the thermal conductivity decreases from 2.0 W/(m·K) to 1.2 W/(m·K); in the lower layer, where the fillers are densely packed, the CTE mismatch becomes more pronounced. When the battery generates heat, the insufficient thermal conduction in the upper adhesive layer can lead to localized overheating of the cells.
Figure 3: Curves showing the evolution of the ultimate adhesion of PUR hot-melt adhesive over time and at different temperatures (7.5 MPa ultimate adhesion achieved after 72 h at 25 °C; retention rate at 80 °C is only approximately 25%).
Table 3: Key Control Points for Adhesives Used in Battery Modules
| Resin type | What are the key priorities? | What does IQC inspect? | What does the production line need to protect against? |
| heat-conducting glue | Thermal conductivity, filler dispersion, CTE matching | Thermal conductivity: 0.85–2.5 W/m·K (variation <10%); filler settling test (difference between upper and lower layers after 24 h of static standing); electrolyte resistance: 48 h | The hot-pressing temperature shall not exceed the Tg of the resin; employ a filler dispersion process (three-roll grinding / planetary mixing) to prevent filler sedimentation during hot pressing. |
| PUR Hot-Melt Adhesive (Securing/Sealing) | Final adhesion, moisture-curing, temperature sensitivity | Initial tack + melt viscosity (mandatory test on incoming material); 7-day final tack (type test; not the same as initial tack) | After coating for ≥24 hours in the housing, the time gate shall be set to automatic locking; coating performance will be affected if the relative humidity is between 30–70% RH and the temperature fluctuation exceeds ±5 °C. |
| gasket cement | Flame retardancy, chemical resistance, elongation at break | UL94 V0: resistance to electrolyte immersion; hardness (Shore A) | The liquid filling station and the dispensing station are physically separated to prevent swelling caused by electrolyte splashing. |
IV. Motor/Acoustic Module: Curing Time Deviation
For motors and acoustic modules, mobile phone adhesive choices are primarily silicone rubber, polyurethane adhesive, and UV-curable adhesive. The speaker must meet a sealing rating of IPX7 or higher for waterproofing requirements.
The recommended thickness range for the adhesive layer is empirically determined to be between 0.1 mm and 0.5 mm. A layer that is too thin (less than 0.05 mm) may lead to stress concentration due to curing shrinkage, increasing the risk of localized failure; conversely, a layer that is too thick (exceeding 1 mm) can induce internal stresses caused by uneven curing. These internal stresses, when persistently acting on the adhesive layer, may result in creep-induced cracking. The hardness of silicone rubber (Shore A: 40–60) is significantly lower than that of epoxy adhesive (Shore D: above 80); consequently, in thin layers, shrinkage-induced wrinkling is more pronounced. It is therefore recommended that the thickness be no less than 0.5 mm.
Vibration testing should also not be overlooked. The mobile phone motor must withstand over 100,000 vibration cycles, and the adhesive layer may exhibit creep under sustained shear stress. The elongation at break of the silicone rubber should exceed 550% (according to ASTM D412, dumbbell-shaped specimens), while its compressive permanent deformation must be kept within 15% (70 °C/22 h).
It is common to observe vibration testing being conducted on uncured organic silicone rubber within production lines. Since the moisture-curing process of organic silicone rubber requires 24–48 hours, production lines often perform gas tightness and vibration tests within 2–4 hours after applying the adhesive in order to meet production schedule requirements. During this period, the adhesive layer may develop a surface crust while remaining liquid internally; during vibration testing, this internal liquid adhesive can be ejected, contaminating the acoustic cavity or the motor coil. Following the testing, voids form within the adhesive layer; although the initial gas tightness test may pass, prolonged use by the end-user allows moisture to penetrate these voids, leading to motor corrosion and speaker voice failure.
In the process of laser marking of sealants, many manufacturers do not attach sufficient importance to this step. After the assembly of the motor/speaker, the housing requires laser marking (e.g., QR codes, model numbers). The instantaneous temperature of the laser can reach 200–300 °C; if the marking area is located near the edge of the sealant, the heat may cause carbonization of the sealant surface layer. Once the sealant surface is carbonized, the molecular chains break, leading to the formation of micro-cracks. While the device may still pass airtightness tests, after six months of use, these cracks can propagate, resulting in failure of the waterproof performance.
Changes in surface roughness during rework constitute a form of cumulative damage. When a speaker’s acoustic cavity requires rework due to inadequate waterproofing, the metal cover must be pried open. The waterproof sealant is then forcibly stripped away, leaving a residual adhesive film on the aluminum surface of the acoustic cavity, while also altering the micro-roughness of the aluminum surface (evidenced by changes in the Ra value). Upon application of new sealant, the aluminum surface no longer exists in its original activated state, thereby weakening the anchoring effect. Consequently, the waterproof performance of the acoustic cavity after rework drops to only 60%–70% of its initial performance level.
Figure 4: Time-dependent curing degree curve of silicone rubber (surface skin formation occurs at approximately 4 h; complete internal curing requires 24–48 h; vibration testing must be conducted outside the curing window).
Table 4: Key Control Considerations for Adhesives Used in Motor/Acoustic Modules
| Resin type | What are the key priorities? | What does IQC inspect? | What does the production line need to protect against? |
| Organic silicone sealant (sealing/ acoustic cavity) | Curing time, airtightness, vibration resistance | Airtightness leakage rate <1 Pa/min (mandatory inspection for incoming material); hardness (Shore A) 40–60 (mandatory inspection for incoming material); no cracking after 100,000 cycles of vibration (type test); compressive permanent set <15% (type test) | Apply coating for ≥24 h under vibration testing (surface skinning = internal curing), maintain a laser marking distance of ≥3 mm from the adhesive layer, and measure surface roughness after rework. |
| Polyurethane adhesive (structural bonding) | Vibration resistance, creep resistance, and bonding strength | Adhesion strength retention rate after vibration; elongation at break | Prior to vibration testing, confirm that the adhesive layer has fully cured to prevent internal voids that could lead to subsequent water infiltration. |
| UV adhesive (local fixation) | Curing degree and temperature resistance | UVDSC crosslinking degree and strength after high-temperature aging | Ensure adequate UV exposure to avoid residual liquid in shaded areas. |
V. Main Board/Connection Module: Rework can cause secondary damage.
The mobile phone adhesive range for motherboard modules includes underfill, conductive adhesive, UV adhesive, and potting compound. Underfill is a critical material for BGA and Flip Chip packaging; its viscosity is typically below 1,000 mPa·s, and it relies on capillary action to fill the gaps beneath the chip, with a filling time of less than 30 seconds.
Filler settling leads to uneven electrical performance. In the bottom-filled paste, spherical silica fillers account for 58%–70% of the total filler content, with particle sizes ranging from 0.5 to 3 μm. These fillers carry a negative charge, which, in conjunction with the protrusions on the copper posts, forms localized galvanic cells; this phenomenon causes the fillers to migrate toward the tin electrode and settle. Following this settling process, the local CTE of the paste layer increases from 31 ppm/℃ to over 41 ppm/℃, and the mismatch in thermal expansion coefficients further exacerbates the stress at the solder joints.
The volume resistivity of the bottom fill compound shall be greater than or equal to 1.0×10¹⁶ Ω·cm, the dielectric strength shall be greater than or equal to 35 kV/mm, and the Tg shall typically range from 80 to 165 °C. The total ion content shall be less than or equal to 20 ppm, and the chloride ion content shall be less than or equal to 5 ppm, in order to prevent electrochemical migration.
Reflux rework poses a secondary risk to underfill material. When reworking a BGA chip on a motherboard, the following steps are required: chip removal, underfill cleaning, re-application of underfill, new chip placement, and reflow soldering. However, the glass transition temperature (Tg) of underfill varies depending on its formulation: approximately 80–100 °C for flexible/low-stress variants, 120–140 °C for standard variants, and up to 150–165 °C for high-Tg heat-resistant variants (specific values shall be verified against the BOM model). The peak temperature of lead-free reflow soldering ranges from 245–260 °C. During the initial reflow process, the underfill has already been exposed to high temperatures once. During rework, the underfill undergoes a second reflow cycle; once its temperature exceeds its Tg, the material softens, losing its mechanical support for the solder joints. This can cause solder balls to migrate in the molten state, leading to bridging or cold solder joints. More subtly, repeated exposure to high temperatures may induce thermal degradation of the underfill, with degradation byproducts contaminating the solder pads. Consequently, the cold solder joint rate increases significantly after new chips are mounted.
Solvent penetration in EPI coating constitutes a chronic damage mechanism. When EPI coating is applied to a motherboard, the solvent can penetrate the already cured underfill or UV adhesive. The solvent, acting similarly to a plasticizer, infiltrates the adhesive layer, thereby reducing its glass transition temperature (Tg) and modulus. After the EPI coating cures, the adhesive layer softens; when the motherboard undergoes vibration or bending, the BGA solder joints lose adhesive support, and mechanical stress is directly transferred to the solder balls, leading to a reduction in their fatigue life.
Over-polishing of the solder mask can damage the solder pads. During BGA rework involving solder mask removal, operators use a copper wire brush or sandpaper to polish away residual solder. Excessive polishing can compromise the ENIG (electroless nickel and gold) layer on the surface of the PCB solder pads, exposing the underlying copper. When new underfill solder is applied, its adhesion to the exposed copper is significantly lower than its adhesion to the nickel-gold layer. Over time, the solder mask layer may peel away from the edges of the solder pads, allowing moisture to penetrate through these gaps and lead to solder joint corrosion.
Figure 5: Effect of reflow soldering cycles on the modulus retention rate of bottom-fill adhesives with different Tg values (high-Tg formulations exhibit significantly superior resistance to secondary reflow soldering compared to standard formulations)
Table 5: Key Control Points for Adhesives Used in Motherboard Modules
| Resin type | What are the key priorities? | What does IQC inspect? | What does the production line need to protect against? |
| Underfill | Filling rate, ion content, CTE matching, filler settling | Mandatory inspection for supplied materials: viscosity (rotational viscometer), appearance (no agglomeration/no discoloration); initial batch inspection / outsourced inspection: capillary filling time <30s, volume resistivity ≥ 1×10¹⁶ Ω·cm, total ion content <20 ppm (chloride ion content <5 ppm); type test: cross-sectional microscopy with 5 fields of view for filler dispersion comparison. | The rework temperature shall be below the Tg of the adhesive (to prevent overtemperature during secondary reflow soldering); replace physical grinding with plasma cleaning for adhesive removal, and protect the ENIG layer. |
| conducting resin | Volume resistivity, contact resistance, silver powder batch | Volume resistivity ≤ 1.0×10⁻² Ω·cm, contact resistance ≤ 5.0 mΩ, particle size and oxidation degree consistency of silver powder | Store in a cool, dry place away from light; use promptly after opening to prevent oxidation of the silver powder, which could lead to resistance drift. |
| UV adhesive (local reinforcement) | Crosslinking degree and solvent resistance | UVDSC curing degree and triple-protection paint compatibility testing | The primer of the three-protection paint must be fully cured for ≥24 h prior to coating, to prevent solvent penetration that could lead to plasticization. |
| Three-Protection Paint | Coating thickness, coating integrity, dielectric strength | Thickness: 25–75 μm; Salt spray test: ≥1,000 h; Dielectric strength: ≥25 kV/mm | Prior to coating, confirm that the base adhesive has fully cured to prevent solvent penetration from reducing the Tg. |
VI. Housing/Protection Module: Both assembly and disassembly present dual considerations.
Housing-module choices within the mobile phone adhesive range include waterproof silicone, acrylic AB adhesive, and pressure-sensitive adhesive. High-end smartphones are required to meet the IPX8 rating; the specific water depth and exposure duration shall be specified in the product datasheet (typically above 1.5 meters and for over 30 minutes).
The issue of material degradation in storage originates in the warehouse environment. When moisture-cured instant-curing adhesives are exposed to air for over one hour, they absorb water vapor, leading to premature polymerization; this results in the adhesive layer becoming brittle, with its strength decreasing by 50%. For low-temperature cured single-component epoxy adhesives, if the storage temperature remains above 25 °C for extended periods, the ultimate crosslinking density may decrease by 30%–50%. When rubber-based adhesives are exposed to prolonged sunlight, ultraviolet radiation can disrupt their molecular chains, potentially reducing their strength to merely 30% after three months.
IP68-rated waterproof silicone compounds (e.g., a specific brand of modified silicone) can exhibit an elongation at break exceeding 800%, with a Shore A hardness ranging from 30 to 50. The typical mixing ratio for acrylic AB adhesives is 10:1 or 1:1; inadequate mixing may result in incomplete curing, localized tackiness, or insufficient strength.
The issue of overfitting in the middle frame pressing process is systemic. When the middle frame of a smartphone is pressed against the back cover or screen, excessive pressure from the fixture can compress the waterproof adhesive to 30%–50% of its original thickness. This results in an excessively thin adhesive layer with insufficient elastic energy storage, thereby compromising its ability to absorb drop impacts. More subtly, excessive pressure can cause the adhesive layer to spread laterally, clogging drainage holes or speaker ports; this often leads to user complaints regarding water ingress or reduced sound output.
Forced disassembly using a pry tool causes gradual degradation of the waterproof sealant. During after-sales service, technicians may use a plastic pry tool to forcibly separate the mid-frame from the rear cover; this action stretches and tears the waterproof sealant, leaving residual layers of sealant on both surfaces of the mid-frame and the rear cover. During reassembly, a weak interface forms between the new sealant and the residual old sealant. Even if the subsequent airtightness test passes, after six months, the old sealant layer ages and degrades, causing the new and old sealants to separate, thereby compromising the waterproof performance.
Localized pressure buildup at screw fastening points constitutes a random failure source. During screw tightening, the waterproof seal material adjacent to the screw head undergoes localized compression. If the screw torque distribution is uneven—resulting in some screws being overtightened—the seal layer thickness around those screws may approach zero, creating a leak path. During IPX8 testing, high-pressure water can then infiltrate through these seal layer defects around the screws. Such failures exhibit strong randomness: within the same production batch, some devices may withstand water exposure while others may fail, making SQE traceability challenging.
Figure 6: Relationship between the compression ratio of waterproof silicone and its waterproof performance (a compression ratio range of 50–75% represents the optimal window; excessive compression can lead to extrusion and sealing failure)
Table 6: Key Control Points for Adhesives Used in Housing Modules
| Resin type | What are the key priorities? | What does IQC inspect? | What does the production line need to protect against? |
| Waterproof silicone (mid-frame / back cover) | Elongation at break, hardness, and water resistance rating | Elongation at break> 800% (for a specific brand of modified silicone), Shore A hardness 30–50, tested after IPX8 aging | The pressing unit is equipped with a pressure sensor; the rubber thickness is maintained at the original 60–80%, and overpressure may cause overflow, leading to blockage of the drainage holes. |
| Acrylic AB Adhesive (Structural Bonding) | Mixing ratio, curing uniformity, hardness | Mixing ratios of 10:1 or 1:1 (deviation <3%), consistent hardness after curing, and no localized tackiness. | After mixing, thoroughly stir and degas the mixture, while monitoring the curing temperature profile to prevent incomplete curing in localized areas. |
| Pressure-sensitive adhesive (buffering/temporary fixation) | Peel strength, adhesion, storage stability | 180° peel strength ≥ 18 N/25 mm, adhesion retention ≥ 48 h/1 kg, storage condition: 5–25 °C | Use as soon as possible after opening; avoid moisture exposure or pre-aging under UV light; use the instant-curing adhesive within 1 hour. |
| Instant adhesive (local fixation) | Curing rate, storage conditions, and embrittlement risk | To be used within 1 hour after opening; storage temperature <25 °C; strength retention rate after high-temperature aging | Exposure to air for over 1 hour may result in a 50% reduction in intensity; use with caution in high-temperature environments. |
Cross-module Comparison: Post-Processing Risks of Six Types of Adhesives
UV mobile phone adhesive: Curing in shaded areas remains a persistent challenge, and the subsequent processing stages are particularly concerned about tar contamination and thermal shock from laser welding. Inspection must employ UVDSC for layered curing degree measurement; surface-only evaluation is insufficient.
Epoxy adhesive: curing shrinkage rate ranges from 2% to 3%; in subsequent processes, the primary concerns are rework due to overtemperature during reflow soldering, and damage to solder pads caused by degreasing and polishing. Key inspection parameters include Tg measurement via DSC and modulus measurement via DMA.
PUR adhesive: The final adhesion can only be measured after 72 hours; in subsequent processing stages, the primary concern is the risk of casing installation and electrolyte splashing before complete curing. At elevated temperatures, strength degradation can reach up to 74.8%.
Organic silicone rubber: moisture-curing requires 24–48 h; in subsequent processes, the primary concern is to ensure complete curing before performing vibration testing and laser marking of the adhesive. The hardness should be controlled between 40 and 60 on the Shore A scale.
Instant-curing adhesive: Should be used within 1 hour after opening; in subsequent processing stages, prolonged exposure to air and high-temperature aging-induced embrittlement are the primary concerns.
AB adhesive: The mixing ratio determines curing uniformity; in subsequent processing stages, the primary concerns are overfitting during pressing—which can lead to extrusion—and forceful disassembly—which can cause interfacial damage.
Three-Party Coordination: Suppliers, IQC, Production Line
An 80% failure rate of mobile phone adhesive is not due to material issues, but rather the combined effect of three factors: improper storage by the supplier, inadequate edge inspection by the IQC, and damage caused by subsequent processes on the production line. The supplier must ensure proper storage and transportation of the adhesive, along with maintaining its molecular structure and reliability data. The IQC must shift its focus from random sampling of incoming materials to evaluating edge characteristics and post-unsealing stability. The production line must transition from a “just ensure the adhesive is applied” approach to a “ensure the adhesive remains undamaged during subsequent processing” approach. During handover, the following key points should be verified: the supplier’s material report must include the dual 85% retention rate and edge peel strength; the IQC must conduct sampling according to AQL standards and clearly mark the unsealing date; the production line must follow FIFO (first-in, first-out) inventory practices and implement a time-based lockout mechanism (e.g., automatic locking of the housing assembly process if the PUR adhesive has not been applied for 24 hours). Adhesive layers from reworked batches should be tested separately and must not be mixed with the first-piece samples.
The three-point linkage mechanism utilizes the “Adhesive Batch Traceability Ledger” as its foundation, with the supplier batch number flowing through the following stages: raw material receipt report → IQC sampling records → production line usage records → downstream process parameter records. In the event of an anomaly at any stage, both upstream and downstream parties can conduct cross-verification, thereby eliminating information silos.
Batch Traceability: Sample Retention and Correspondence
Mobile phone adhesive failures often occur in batch fashion. It is recommended that, upon receipt of each batch of adhesive, the IQC retain 50 g of samples, seal them properly (matching each sample to its corresponding production batch), and label them with the opening date, storage conditions, and supplier batch number. In the event of an anomaly on the production line, the retained samples from the same batch can be traced to retest adhesion force, curing degree, and viscosity, thereby rapidly determining whether the issue stems from the material or from subsequent process stages.
Quick Root Cause Analysis: Five-Step Diagnosis
When a mobile phone adhesive layer fails on-site, follow this troubleshooting sequence:
Step 1: Observe the failure symptoms
- Bubble rebound / edge curling → Suspected over-venting or inadequate bonding pressure
- Lens loosening / out-of-focus → Suspect oil contamination in the focusing mechanism or laser thermal shock
- Loose battery / abnormal noise → Suspected that the battery housing was installed before the PUR had fully cured
- Motor rusting / speaker distortion → Suspected vibration testing before the silicone sealant has fully cured
- BGA bridging/solder joint failure → Suspected overtemperature during reflow soldering during rework
- Waterproofing failure → Suspected over-pressing during lamination or forced prying during disassembly
Step 2: Review post-process records
- Deaeration temperature / number of cycles / re-aeration rate
- Oil removal/laser welding interval after focusing
- PUR coating time to casing loading / time gate record
- Apply silicone to the vibration test specimen.
- Reprocessing temperature curve / Degumming method
- Pressing pressure / Screw torque / Disassembly method
Step 3: Verify incoming material data
- Re-testing of sample aliquots from the same batch: peeling force, curing degree, and viscosity
- Supplier Report: Double 85 retention rate, edge delamination force, molecular weight distribution
- Opening Date/Storage Conditions Record
Step 4: Identify the root cause
- Re-testing of incoming material passes + non-compliance in subsequent process → Root cause: injury to adhesive during subsequent process
- Re-testing of supplied materials failed → Root cause: Supplier material issue
- Incoming material meets specifications + downstream process complies → Root cause: Improper selection of adhesive type (e.g., using conventional epoxy adhesive instead of low-shrinkage adhesive)
Step 5: Closed-loop improvement
- Post-process issues → Revise SOP and add error-proofing measures (time gate, pressure sensor)
- Material issues → Switch to different supplier batches and tighten IQC sampling inspections
- Component selection issue → Joint R&D for re-selection and BOM update
After the implementation of the corrective actions, three consecutive batches (or as required by the company’s “Change Management Procedure”) shall be continuously monitored to verify the effectiveness of the measures; the issue may be closed only after it has been confirmed that the failure mode no longer recurs.
Selection strategy: Domestic or imported
For mobile phone adhesive selection, the choice of OCA optical adhesive should not follow a one-size-fits-all approach. For mid-to-low-end smartphones, the repair market, and commercial displays, domestic products are fully adequate, offering significant cost-performance advantages; however, for OLED flexible displays, automotive displays, and high-end flagship devices requiring IP68 waterproofing, the high-temperature and high-humidity reliability of imported OCA remains a critical threshold. When selecting materials, two key metrics should be considered: the retained adhesion force after dual 85°C/1,000 h exposure, and the edge adhesion force degradation. While domestic products still lag behind imported counterparts in these two metrics, this gap is narrowing. It is recommended to conduct A/B comparative tests every six months to dynamically optimize the BOM.
epilogue
When inspecting a mobile phone adhesive, first identify the type of adhesive, then determine the appropriate inspection method, and finally, do not forget to ask: “Has it been damaged during subsequent processing steps?”
JOME manufactures the full JOME product catalogue in its own factory for customers in more than 30 countries. Readers continuing from What adhesive should be used for PE will find the Shanghai JOME Industrial adhesive portfolio covers the full portfolio, and project questions go to the contact the JOME technical team.