The influence of covering film on the conformability of flexible circuit boards
Dec 03, 2024
In the flexible printed circuit (FPC) manufacturing process, laminating the coverlay is a critical step that significantly impacts the quality of the FPC. Improper material selection or process parameters can lead to issues such as poor encapsulation or incomplete lamination.
In the past, traditional lamination presses were commonly used for coverlay lamination, where multiple products were stacked and laminated in a single cycle. However, modern processes increasingly utilize fast presses to replace traditional presses for coverlay lamination. Fast presses allow better control over resin flow, making it easier to detect and resolve issues early. Nevertheless, the requirements for coverlay specifications, process parameters, and compatibility with circuit lines are very high. Therefore, coverlay materials must undergo conformability testing to prevent batch-quality issues.
In the flexible printed circuit (FPC) manufacturing process, laminating the coverlay is a critical step that significantly impacts the quality of the FPC. Improper material selection or process parameters can lead to issues such as poor encapsulation or incomplete lamination.
In the past, traditional lamination presses were commonly used for coverlay lamination, where multiple products were stacked and laminated in a single cycle. However, modern processes increasingly utilize fast presses to replace traditional presses for coverlay lamination. Fast presses allow better control over resin flow, making it easier to detect and resolve issues early. Nevertheless, the requirements for coverlay specifications, process parameters, and compatibility with circuit lines are very high. Therefore, coverlay materials must undergo conformability testing to prevent batch-quality issues.
2. Experimental Process
2.1 Experimental Materials
Copper foil thickness: 12μm, 18μm, 35μm, 50μm, and 70μm.
Coverlay specifications: Materials produced by Shengyi Technology with the following grades:
0515, 0518, 0525, and 1025The first two digits indicate the polyimide (PI) thickness:
"05" = 12.5μm
"10" = 25μm
The last two digits indicate the coverlay adhesive layer thickness.
Resin flow specifications: Resin flow indicators of 0.11mm, 0.13mm, 0.15mm, 0.18mm, and 0.20mm.
|
Parameter |
Lamination Temperature |
Pre - lamination Time |
Lamination Time |
Layer Pressure |
|
A |
180°C |
10s |
60s |
100kgf/cm² |
|
B |
90s |
|||
|
C |
150s |
|||
|
D |
60s |
130kgf/cm² |
||
|
E |
90s |
2.3 Experimental Process
Copper foil preparation: Control the copper foil plating thickness on a single-sided panel. Etch the required circuit patterns based on the design specifications. The circuit patterns consist of parallel lines with line width/spacing of 2 mil, 3 mil, 4 mil, 5 mil, and 6 mil, with a total size of 100 lines × 1 cm.
Post-etch cleaning: After etching, perform basic cleaning and brushing on the panels to simulate the actual flexible PCB (FPCB) processing workflow.
Coverlay application: Attach the coverlay onto the etched circuit patterns as per the material combination requirements. Perform lamination using different sets of lamination parameters. After lamination, observe the conformability effect of the coverlay under a 30x magnifying glass:
If there are air bubbles between the lines or at the line edges, it is considered poor lamination and marked as "X".
If no air bubbles are observed, it is considered good and marked as "√".
| Line Thickness | 2 mil Line | 3 mil Line | 4 mil Line |
|---|---|---|---|
| Molding Time (s) | 60, 90, 150 | 60, 90, 150 | 60, 90, 150 |
| Excess Glue - 0.11mm | ×, ×, √ | ×, ×, √ | ×, √, √ |
| Excess Glue - 0.13mm | ×, ×, √ | ×, ×, √ | ×, √, √ |
| Excess Glue - 0.15mm | ×, √, √ | ×, √, √ | √, √, √ |
3.2 Effect of Resin Flow Amount on Conformability
From Table 1, it can be observed that increasing the resin flow amount helps improve conformability, but extending the fast press forming time has an even greater effect on improving conformability. For instance, when the fast press time is 150 seconds, even with a resin flow of 0.11mm, good conformability can be achieved.
Therefore, when dealing with coverlay films with lower resin flow, adjusting the forming time can achieve good conformability, thus avoiding defects or material scrap caused by insufficient resin flow. On the other hand, using coverlay films with higher resin flow allows for a reduction in fast press time, improving production efficiency.
3.2 Effect of Resin Flow Amount on Conformability
From Table 1, it can be observed that increasing the resin flow amount helps improve conformability, but extending the fast press forming time has an even greater effect on improving conformability. For instance, when the fast press time is 150 seconds, even with a resin flow of 0.11mm, good conformability can be achieved.
Therefore, when dealing with coverlay films with lower resin flow, adjusting the forming time can achieve good conformability, thus avoiding defects or material scrap caused by insufficient resin flow. On the other hand, using coverlay films with higher resin flow allows for a reduction in fast press time, improving production efficiency.
3.3 Effect of Line Width on Conformability
From Table 1, it is evident that when the line width and spacing are larger, even with lower resin flow amounts and shorter lamination times, good conformability can be achieved. By adjusting the fast press forming time according to the line width, production efficiency can be further improved.
| Line Width | 2 mil | 3 mil | 4 mil |
|---|---|---|---|
| Lamination Time | 60, 90 | 60, 90 | 60, 90 |
| Pressure (kg/cm²) | 100, 130 | 100, 130 | 100, 130 |
| Excess Glue Amount (0.11) | ×, × | ×, × | √, √ |
| Excess Glue Amount (0.13) | ×, × | ×, × | √, √ |
| Excess Glue Amount (0.15) | ×, √ | ×, √ | √, √ |
Based on extensive testing, the key factors affecting coverlay conformability include fast press lamination time, resin flow amount, line width, fast press pressure, coverlay adhesive layer thickness, and the ratio of copper foil thickness to line width. Among these, when the pattern is fixed, the most significant influences are fast press time, resin flow amount, and the ratio of coverlay adhesive layer thickness to copper foil thickness. Therefore, in the FPCB coverlay lamination process, it is essential to choose the appropriate coverlay and resin flow amount, and adjust the fast press parameters accordingly to achieve good conformability.
