Effect of reflow technology and surface finishes of PCB on solder spreading

A key process in surface mount technology is soldering, when a metallurgical joint is formed between the molten solder and metal surfaces. Solders in molten state have to show good spreading action together with good wetting of substrate. Molten solder flows or spreads on the metal surface to ensure good future formation of solder joint. Monitoring of solder spreading is one of the tools which can be used to solderability and wettability quantification. Measurement of spreading of molten solders is one of the tools that can be used to estimation of their wettability and solderability. Each solder has a different degree of spreading on the metal surface. Degree of spreading mainly depends on combination of flux, solder alloy, soldered substrate. This study is focused to investigate effect of reflow technology and PCBs surface finishes on spreading of molten solders. In our experiment we used four types of surface finishes (lead free HASL., passivated cooper, ENIG and immersion Sn), five types of solder pastes (one lead Sn62Pb36Ag2 and four lead free Sn96.5Ag3Cu0.5, Sn95.5Ag4Cu0.5, Sn96.5Ag3.5, Sn42Bi58) and three types of reflow technologies - continuous hot air soldering, continuous infra radiation soldering and vapor phase soldering.