无铅波峰焊中PTH焊点吹孔成因探讨

Discussion of Blow Hole in Lead-free Wave PTH Solder Joint

借助实际案例,并利用X线透射技术(X-Ray)、金相切片、扫描电子显微镜(SEM)和热机械分析(TMA)等物理分析手段,研究了PTH孔质量,如孔铜厚度和孔壁的润湿性能等对生成焊点空洞的影响。结果表明,PTH孔铜厚度太薄,不足以抵抗焊接时PCB基材的热膨胀而被拉断,导致藏匿在PCB内部的潮气和未完全挥发的助焊剂等在焊接时受热膨胀,并通过孔断处向焊料内部喷发进而形成空洞或吹孔。而PTH孔表面由于喷锡层厚度太薄造成与孔铜完全合金化,亦可降低孔壁可焊性,导致焊料润湿爬升困难,进而形成空洞。

With the X-Ray transmission system, the micro-section, scanning microscopy（SEM） and thermal mechanical analysis（TMA） techniques, the effect of the PTH quality, such as the thickness of the plated cooper, the wetting performance of the PTH, on the blow hole or the void generation in the PTH solder joints was studied based on the practical case. The results showed that the thin plated cooper was cracked due to the bad resistance to the thermal expansion of the PCB during the soldering process, which lead to the moisture in the PCB and the uncompletely volatilized fl ux expand under the soldering thermal. The expanded residual gas erupted to the solder via the PTH fracture and generated void or blow hole. In addition, the thin tin fi nish on the PTH surface was completely alloyed with the base cooper, which reduced the solderability of the PTH and lead to the solder diffi cult to climb, thus generated void eventually.