Preparation Of SnAgCu Lead-free Solder Paste And Investigation On Reliability Of Board-level Package

According to the requirements of surface mount technology èthe properties of theoriginal Sn3.0Ag0.5Cu lead-free solder paste has analysed. It has many defects, suchas lack of wettability, hot slump easily, and printing difficultly and so on. To solve thisproblem, a type of flux with excellent performance has been developed by changingthe components. Then, the performance of solder paste which was prepared by theimproving flux has been tested and compared with the commercial solder paste.Moreover, it has been tested the performance of low Ag lead-free solderpaste[&(Ag)”1%] which has been developed by optimized flux and low Ag solderpowder by board-level process adaptability experiment and micro-solder jointsthermal cycling experiment.It has opitimized the formula of flux by adjusting its composition, the resultsshow that through changing the constituents, ratio and content of rosin, the wettabilityand anti-hot slump ability has been improved, and the weld residual has been reduced;It has been increased the activity of solder paste and reduced corrosion throughchanging the constituents, ratio and content of activator; Choosing new kinds andratio of surfactant to improve the spreading rate significantly and ameliorate thewettability..According to IPC-TM-650Test Methods Manual and IPC/J-STD-005standand,solder paste was tested by solder ball, wettability, and slump and corrosionexperiment and compared with the commercial solder paste. Test results show that thesolder paste has shown excellent performance, even better than commercial solderpaste in anti-slump test. The solder ball test has reached the first grade, the wettabilitytest reached the second grade, both the collapse and hot collapse is0.15mm, andnon-corrosive.The process performance test results of low Ag lead-free solder paste show thatthey have good print quality and their sevice life both greater than4h. The printingtimes of high Ag and low Ag solder paste are close. The silver content decreased hasno siginificant effect on the print quality; After reflow soldering, low Ag lead-freesolder paste has good weldability, its intermetallic compound thickness below thehigh silver lead-free solder paste, proved that reduction of silver content make theintermetallic compound thickness is reduced. The thermal cycling of low Ag lead-free solder paste test results show that withthe reduction of silver content, the proportion of voids and crack is reduced. And onBGA chip, the both sides of the ball are easier to crack than the middle of the ball.After1200thermal cycles, wih lower silver content, the variation rate of the thicknessof IMC is reduced.