热处理法改善PCB内铜镀层间裂纹缺陷问题

Solution to crack defect appearing between copper layers in PCB during heat treatment

[目的]随着通信产品向高频化、高速化和高密度方向发展,填孔覆盖电镀(VIPPO)工艺在印制电路板(PCB)制造中得到广泛应用。但这类PCB在无铅焊接时,其盖孔Cu镀层与底层Cu镀层之间易产生开裂,导致产品失效。本文拟通过热处理来解决这一问题。[方法]PCB塞孔树脂和板材树脂经预处理后于120℃下热处理,然后通过260℃的回流处理来模拟高温焊接。检测了不同条件热处理及回流不同次数后两种树脂的热膨胀系数(CET)及玻璃化转变温度(Tg),分析了热处理不同时间后PCB样品的质量损失,并采用扫描电镜观察了回流处理后不同Cu镀层之间的裂纹情况。[结果]热处理能有效降低塞孔树脂和板材树脂的热膨胀系数,优化它们的玻璃化转变温度。经热处理的PCB样品在经历6次回流处理后,微盲孔拐角处盖孔Cu镀层与底层Cu镀层之间无开裂现象。[结论]在PCB样品完成树脂塞孔及盖孔电镀后分别进行热处理可有效改善因Cu镀层间开裂而导致的失效问题。

[Introduction]As the communication products develop towards high frequency,high speed,and high density,the via-in-pad plating over(VIPPO)process has been widely adopted in the manufacturing of printed circuit board(PCB).However,these PCBs are susceptible to cracking between over Cu coating and bottom Cu coating during lead-free soldering,resulting in product failure.The present study was conducted to address this issue by heat treatment.[Method]The pretreated plug-in resin and board resin for PCB were subjected to heat treatment at 120℃,followed by reflowing at 260℃ to simulate the high-temperature soldering process.The coefficient of thermal expansion(CET)and glass transition temperature(Tg)of the two kinds of resins after heat treatment under different conditions and being reflowed for different times were measured.The mass loss of the resins after heat treatment for different time were analyzed.The cracking between different Cu coatings after reflowing was observed by scanning electron microscopy(SEM).[Result]The CET values of plug-in resin and board resin were greatly decreased after heat treatment,and their glass transition temperatures were also optimized.No crack was found between over Cu coating and bottom Cu coating at the corner of microvia on heat-treated PCB after 6 cycles of reflow.[Conclusion]The failure of PCB products caused by the cracking between different Cu coatings can be effectively mitigated by heat treatment after resin plugging and cover plating,respectively.