How to Avoid Copper Peel Off

Hot Air Solder Leveling (HASL) involves coating the surface of the exposed copper with a layer of tin alloy to prevent oxidation and provide a good soldering surface for subsequent assembly. The basic process of HASL is to briefly immerse the PCB board into a high-temperature liquid tin bath. Through the action of flux and high-temperature tin, a copper-tin alloy (IMG) is formed. Then, on guide rails, the board is lifted out while high-pressure gas blows away excess tin from non-copper areas and levels out any uneven solder on the pads.

The HASL process provides good solderability. However, during the actual HASL operation, there is a tendency for copper peel-off: in single-sided PCBs this most commonly occurs to plated holes, and in double-sided PCBs to through-hole pads with narrow annular rings, especially for long slotted holes. This copper peel-off has its main causes:

1. The HASL tin bath temperature of 275-300°C greatly exceeds the board's glass transition temperature (TG point). When subjected to the high temperature of the tin bath and the high-pressure gas jet, the board experiences strong stress. Without the mechanical support from wide surface solder pads, the copper in PTH holes can easily separate from the hole walls.

2. During HASL, copper and tin combine to form a copper-tin alloy, which can corrode away some of the copper, thereby reducing adhesion to the board substrate.

Improvement and Optimization Strategies

To illustrate, let's use the analogy of climbing a cliff. If both hands can hold onto the protruding face of the cliff, and the gripping area is larger, climbing becomes easier.

Similarly, the copper surface within the hole walls behaves similarly. When both top and bottom layers have solder pads (typically recommended to have 0.3 mm or larger annular ring), the adhesion is stronger, and the occurrence of copper peel-off is significantly reduced.

Conclusion

For single-sided or double-sided PCBs with small solder pads, consider switching to double-sided PCBs (with solder pads larger than 0.3mm) or adopting ENIG surface treatment. Gold-plated solder pads have a smoother surface and are suitable for BGA boards.

If conversion to double-sided PCBs isn't feasible due to insufficient clearance or other reasons, consider using ENIG.

For cases where copper in hole walls is unwanted, opt for non-copper-plated holes. If connectivity between both sides is needed, use vias at the edges of the solder pads (for high-current applications, ensure an adequate number of vias are added).