Spreading Kinetics of Molten 60Sn40Pb on Higher Melting Temperature Pb-Sn Alloy Substrates
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where the apparent activation energy Q was 4.2 kcal/mole for 0 and 16 kcal/mole for h. The time exponent m (negative for 0 and positive for h) decreased with temperature from - 0.2-0.3 at 205'C to -0.05 at 2600 and then increased again at higher temperatures. The magnitude of Q for 0 is in accord with that for the viscosity of molten Pb-Sn alloys and that for h with a combined liquid-solid diffusion involved in the dissolution. Further work is however needed to identify unequivocally the mechanisms which govern the wetting in these duplex Pb-Sn alloy systems. 1. INTRODUCTION There has been greater emphasis in recent years on chip-on-board technology using flip chip soldering to meet I/O requirements and gain performance enhancement in the system level. The challenge is to join chips with high melting point solder bumps to a printed circuit board (PCB). In flip chip soldering, high melting point Pb-base solders such as 3Sn97Pb or 5Sn95Pb have been used to interconnect a semiconductor to the ceramic chip carrier, whereas almost all of the electronic components are joined to the PCB using the lower melting point Pb-Sn eutectic. Since flip chip soldering requires a processing temperature above 300'C, it can not be used directly to join components on the conventional FR-4 printed circuit board. This is because most conventional PCBs are made of epoxy-fiber glass composites whose glass transition temperature is - 125TC. Although the board material can withstand a brief exposure to the Pb-Sn eutectic soldering temperature, it would decompose at the higher melting temperatures of the high Pb alloys. To solve this problem, a "duplex" soldering concept has been devised to bridge 5Sn95Pb (or 3Sn97Pb) from the chip side and the eutectic from the PCB side. Only little is known regarding the kinetics of the corresponding wetting and dissolution process and the microstructures which develop. The objective of the present investigation was therefore to provide information on these aspects pertaining to the joining of duplex Pb-Sn alloys. The work focused on the kinetics and microstructure associated with the wetting of three high Pb alloy (3, 5 and 10 wt.% Sn) substrates by molten 60Sn4OPb drops. 189
Mat. Res. Soc. Symp. Proc. Vol. 390 01995 Materials Research Society
2. EXPERIMENTAL 2.1 Materials and Reflow Procedure The low melting alloy employed for the liquid drops was commercial 60Sn4OPb (TM = 183'-190'C). Three compositions of higher melting point Pb-Sn alloys were used for the substrates, namely commercial 3Sn97Pb, 5Sn95Pb and 1OSn9OPb. The melting temperatures of these alloys are 312'-318o, 305'-312' and
2680-2990, respectively. As-received bars of the alloys were cold rolled to 0.5 mm thick sheet, from which 7 mm x 15 mm sections were cut for the substrates and 0.5 mm squares for the reflow drops. Following cutting, the pieces were all ultrasonically cleaned in ethyl alcohol. The substrate sections were then annealed for 30 min. at 200 0C in a vacuum of 10-3 torr. In preparation for reflowing, both the 60Sn4OPb cube
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