Feasibility Study of Protective Encapsulation for Vulnerable PCBA Components

Abstract

This study evaluates the protective performance of different adhesive types for vulnerable components on smartphone mainboards. The investigation included an industry survey of commonly used adhesives, an assessment of adhesive properties, and the selection of suitable candidates for testing. Board-level encapsulation trials were conducted to evaluate adhesive spreading and coverage. Push-force testing was used to determine the increase in component attachment strength and to verify the impact-resistance performance of the encapsulation process. The study also evaluated post-cure repairability and estimated the additional cost of applying protective adhesive to each PCBA.

1. Introduction

Factory production-line data showed that component damage accounted for 0.013% of smartphone PCBA scrap in 2017, representing a loss of nearly RMB 100,000. Analysis of scrapped boards indicated that most damaged components were located near the PCB edge. These components are vulnerable to impact during PCBA handling, depanelization, transfer, testing, and other production operations.

At the time, smartphone products had no dedicated protection for these vulnerable components. Protection depended mainly on standardized operator procedures and operator experience. To reduce PCBA scrap caused by component impact, adhesive encapsulation was proposed as a protective measure for selected components on the mainboard.

2. Adhesive Investigation

2.1 Analysis of PCBA Impact Damage

Production failure data were analyzed to identify the components most frequently damaged by impact. The top three component categories were proximity sensors, passive components, and geomagnetic sensors. Together, these categories accounted for 73.29% of the recorded damage, as shown in Figure 1.

Damaged components were mainly distributed along the PCBA edge and around connectors or positioning holes. Tall components and isolated components also had a relatively high probability of impact damage.

Figure 1 | Analysis of PCBA component types affected by impact

2.2 Industry Survey of PCBA Encapsulation

Teardown analyses of flagship smartphones from several major industry brands showed that the PCBA assemblies used protective adhesive encapsulation.

Further analysis of the encapsulated component locations identified the following typical applications:

1. Small components around board-to-board connectors.
2. Small components located along the PCB edge.
3. Components requiring repeated insertion and removal, such as coaxial connectors.
4. Tall components, including proximity sensors.
5. Fragile components with glass bodies.
6. Other components considered vulnerable to mechanical damage.

2.3 Adhesive Types and Parameters

Common adhesives used in smartphone products include bonding adhesives, underfill, thermally conductive and electrically conductive adhesives, UV-curable adhesives, encapsulants, sealants, and instant adhesives. For protective encapsulation of SMT components on a PCBA, the adhesive types considered suitable for testing were underfill, bonding adhesive, encapsulant, instant adhesive, UV adhesive, and sealant.

Bonding adhesive has relatively high hardness and is generally difficult to remove, making repair impractical. Because repairability is an important requirement for smartphone mainboards, bonding adhesive was excluded from further testing.

Instant adhesive is relatively brittle and has low peel and impact strength. Its heat and moisture resistance are also limited, so it was considered unsuitable for smartphone mainboards.

UV adhesive requires optical access for curing. Areas beneath components and other shadowed or inaccessible locations are difficult to cure properly. Tall components such as proximity sensors can block the UV light and create shadowed regions, further reducing curing quality. UV adhesive was therefore excluded.

Three adhesive categories were selected for the study: underfill, encapsulant, and sealant.

Seven adhesive products were evaluated. The underfills were LT 3811, DB 6519A, DB 6582, and DB 6588. The encapsulants were LT 3838T and FL FH8020. The sealant was SY 14280. 

Several adhesive parameters are particularly relevant to PCBA encapsulation:

1. Viscosity: Viscosity describes the internal resistance generated between molecules when a liquid flows. It is commonly expressed in centipoise (cP or CPS). One pascal-second is equal to 1,000 CPS.
2. Hardness: Hardness is the ability of a material to resist indentation by a harder object. The hardness of cured adhesive is generally expressed using the Shore D scale.
3. Glass-transition temperature, Tg: Tg is an important material property. Many material characteristics change significantly near the glass-transition temperature. Tg is commonly measured using thermomechanical analysis (TMA) and is expressed in °C.
4. Coefficient of thermal expansion, CTE: CTE describes the dimensional change of a material caused by temperature variation. It is expressed as the change in length per unit temperature change under constant pressure, commonly in ppm/°C.

3. PCBA Component Encapsulation Test Plan

3.1 Overall Test Program

The encapsulation evaluation consisted of the following steps:

1. Identify the component types and locations associated with impact damage on the smartphone mainboard.
2. Evaluate PCBA adhesive properties and applications, then select the adhesive models for testing.
3. Conduct adhesive spreading tests.
4. Perform component push-force testing after encapsulation.
5. Evaluate repairability after the adhesive has cured.
6. Estimate the cost of the encapsulation process.

3.2 Component Types Selected for Encapsulation

Based on the PCBA impact-damage analysis, three component groups were selected for encapsulation: proximity sensors, small isolated components located along the board edge, and small components around connectors. These three groups accounted for more than 60% of the total number of damaged components.

3.3 Encapsulation Locations and Coverage

The selected encapsulation locations included proximity sensors, small isolated components near the PCB edge, and small components around connectors. The target components were those inside the red boxes shown in Figure 2.

Figure 2 | PCBA adhesive encapsulation locations

According to the applicable PCB design rules, the minimum distance between a component and the board edge was 0.4 mm. The minimum distance between a small component and a connector was also 0.4 mm. The selected adhesive therefore needed sufficiently high viscosity to prevent excessive spreading and migration.

The lateral spreading width of the adhesive had to remain below 0.4 mm. At the same time, the adhesive had to completely cover a small component after curing to provide effective impact protection. The required vertical coverage therefore had to exceed the component height. The target conditions were:

• Width-direction spreading: W ≤ 0.4 mm
• Height-direction coverage: H ≥ component height

Figure 3 | Sample encapsulation location and coverage range

4. Test Procedure and Results

4.1 Test Preparation

The test samples consisted of 14 PCBAs from one project and seven bare boards. The main equipment included:

• Dispensing equipment: BOW B-300S
• Spray-dispensing equipment: AXXON AU77S
• Dispensing valve: V-7000
• Electronic balance: JAS-200, resolution 0.001 g
• High-temperature oven: C4-180
• Push-pull force gauge: Aidebao HP-500, 0–500 N
• Hot-air rework system: HAKKO 850B, 0–800 °C

The adhesive samples were LT 3811, LT 3838T, DB 6519A, DB 6582, DB 6588, FL FH8020, and SY 14280. 

4.2 Adhesive Spreading Test

The spreading criteria were defined as follows:

• Width-direction spreading: W ≤ 0.4 mm
• Height-direction coverage: H ≥ 0.8 mm

The test component was a 1608-C device. The test equipment consisted of the BOW B-300S dispensing machine and the AXXON AU77S spray-dispensing machine.

For the width-direction test, adhesive was dispensed onto a bare board. The diameter of the adhesive deposit was measured immediately after dispensing and again after curing. The two measurements were compared to determine the spreading in the width direction.

For the height-direction test, adhesive was sprayed onto 1608-C components mounted on the PCBA sample. The height of the cured adhesive was measured and compared with the component height. 

The spreading-test analysis produced the following observations:

1. The width-direction spreading of LT 3811, DB 6519A, and DB 6588 exceeded 0.4 mm.
2. LT 3811, DB 6519A, DB 6588, and DB 6582 flowed substantially beneath the components, resulting in a cured height below 0.8 mm, the thickness of the 1608-C component.
3. Underfill showed good spreading performance, while the encapsulant and sealant showed limited spreading.
4. Adhesive spreading was inversely related to viscosity: lower-viscosity adhesives spread more readily.

The spreading-test conclusions were as follows:

1. Underfill spread more than 0.4 mm in the width direction and remained below the component thickness in the height direction. It was therefore unsuitable for encapsulating small components near the board edge or around connectors.
2. The width-direction spreading and cured height of the encapsulants and sealant met the test requirements.

4.3 Reliability Test

The reliability target was to increase the component push force by at least one time, corresponding to an increase of at least 100% after encapsulation.

Seven adhesives were tested: LT 3811, DB 6519A, DB 6588, DB 6582, FL FH8020, LT 3838T, and SY 14280. Four component types were selected on the test boards: proximity sensors, 1608 resistors, 1608 capacitors, and 1608 inductors. After the adhesive had cured, push-force testing was performed and the increase relative to the unencapsulated condition was calculated.

SY 14280 was tested after curing for 24 hours at room temperature. The other adhesives were cured according to their specified process parameters. 

The test results showed the following:

1. For all four component types, the push-force increase achieved with SY 14280 was less than 100%, below the test target.
2. For all four component types, LT 3811, DB 6519A, DB 6582, and DB 6588 increased push force by more than 100% after encapsulation.
3. For passive components, including resistors, capacitors, and inductors, both underfill and encapsulant increased push force by more than 100%.
4. For proximity sensors, underfill increased push force by more than 100%, while encapsulant and sealant increased it by less than 100%.
5. The average push-force increase obtained with underfill was greater than that obtained with encapsulant and sealant.

The push-force test conclusions were as follows:

1. Because the increase achieved with SY 14280 was below the test target, sealant was considered unsuitable for mainboard encapsulation and was not evaluated further.
2. Because encapsulation of proximity sensors with encapsulant produced a push-force increase below the target, encapsulant was considered unsuitable for tall proximity-sensor components.
3. Passive components can be protected using either underfill or encapsulant.
4. Proximity sensors can be protected using underfill.

4.4 Repairability Test

The repairability test evaluated whether components could be normally reworked after the adhesive had cured. The test requirement was that encapsulated components remain repairable.

After PCBA encapsulation and curing, a HAKKO 850B hot-air tool was used to heat the samples. Underfill removal was evaluated at 60 °C, 80 °C, 120 °C, 150 °C, 180 °C, and 240 °C. Encapsulant removal was evaluated at 60 °C, 80 °C, 120 °C, 140 °C, 180 °C, and 240 °C. Repairability was assessed according to the difficulty of peeling or removing the adhesive at each temperature. 

The repairability analysis produced the following results:

1. LT 3838T and FL FH8020 encapsulants could be heated to 140 °C and peeled away relatively easily. Their repairability was good.
2. DB 6519A, DB 6582, DB 6588, and LT 3811 underfills could be removed after heating to 180 °C. Their repairability was considered acceptable.
3. The repairability ranking from easiest to most difficult was: LT 3838T > FL FH8020 > DB 6519A > DB 6582 > DB 6588 > LT 3811.
4. Higher adhesive hardness and higher Tg increased the difficulty of repair.

All six tested adhesives could be repaired normally and provided acceptable repairability. Encapsulants were relatively easier to remove than underfills. From a repairability perspective, DB 6519A was the preferred underfill and LT 3838T was the preferred encapsulant.

4.5 Encapsulation Cost Estimate

The encapsulation cost consisted of labor, adhesive material, energy consumption, and critical spare parts. The investment cost of dispensing equipment, curing ovens, and other production equipment was excluded.

The encapsulated locations included small components along the board edge, isolated small components, small components around connectors, and proximity sensors.

LT 3838T encapsulant was selected for board-edge components, isolated small components, and components around connectors. DB 6519A underfill was selected for proximity sensors.

The estimated labor cost was approximately RMB 0.13 per unit. Adhesive material cost was approximately RMB 0.30 per unit. Equipment energy consumption was approximately RMB 0.01 per unit, and critical spare-parts cost was approximately RMB 0.01 per unit. The total estimated cost was RMB 0.44 per unit.

The estimated additional cost of PCBA encapsulation was therefore RMB 0.44 per board, excluding the initial investment in equipment.

4.6 Adhesive Curing Conditions

Adhesive technical data sheets generally specify recommended curing temperatures and times. The maximum curing temperature was typically 130 °C, with a curing time of 10–15 minutes.

• DB 6519A, DB 6588, and LT 3811 underfill: 130 °C for 10 minutes
• DB 6582 underfill: 130 °C for 15 minutes
• LT 3838T and FL FH8020 encapsulant: 130 °C for 10 minutes

Verification showed that the adhesives cured normally at the recommended temperatures and times. The curing temperature was substantially lower than the PCBA reflow-soldering temperature of approximately 250 °C, so the curing operation did not affect the PCBA.

Because the recommended curing conditions were adequate, the study did not further optimize the curing parameters and instead used the adhesive manufacturers' recommended conditions.

4.7 Dispensing Method

The underfills had relatively low viscosity and good flowability, so either contact dispensing or jet dispensing could be used.

The encapsulants had higher viscosity and poorer flowability. They could not be applied effectively using contact dispensing and therefore required jet dispensing. When LT 3838T and FL FH8020 were applied using a needle, the adhesive accumulated on the component surface instead of forming a surrounding encapsulation layer. 

Figure 4 | Contact dispensing trial using encapsulant

4.8 Summary of Dispensing Trials

Dispensing trials were conducted using four underfills and two encapsulants. Contact dispensing was used for the underfills, while jet dispensing was used for the encapsulants. 

Figure 5 | PCBA adhesive dispensing trial samples

5. Conclusions

1. Underfill and encapsulant can protect vulnerable components on smartphone mainboards and reduce PCBA scrap caused by impact damage.
2. Underfill is suitable for proximity sensors and IC-type components and can be applied by contact dispensing or jet dispensing. Encapsulant is suitable for small components along the board edge and around connectors and must be applied by jet dispensing.
3. Sealant is unsuitable for encapsulating components on smartphone mainboards.
4. After curing, underfill and encapsulant increased the average component push force by at least 100%.
5. Both underfill and encapsulant met the normal repairability requirements.
6. PCBA encapsulation was estimated to add approximately RMB 0.44 per unit, excluding the initial equipment investment.

6. Further Work

Encapsulant was being introduced to smartphone mainboards for the first time in this evaluation, and the number of test samples was limited. Batch validation on active production projects is therefore required to further refine the encapsulation process.

Future work should focus on the following areas:

1. Complete the encapsulation process design, including component selection, encapsulation coverage, dispensing paths, and mechanical-clearance requirements.
2. Select suitable projects for process validation, introduce the encapsulation process, track production performance, and optimize the process based on the validation results.
3. Complete production-equipment planning and provide technical and quality training for operators.
4. Study established industry practices and further improve PCBA encapsulation process capability.