Shape and Behavior ofa Molten Solder by VPS Reflow on Narrow Copper Terminals for QFP.

概要

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The precoating 0.65-0.3mm pitch (P) pads for QFP with an eutectic solder were studied by using VPS. When the solder paste was printed in proportion to the pad's surface area and then reflowed, swelling on the 0.4P and bridging of the 0.3P pads were observed. When more paste was supplied, swelling began on the wider pads. It was concluded that, under the specified reflow conditions and solder, a Critical Solder Amount (CSA) exists corresponding to each pad width, below which the molten solder can have a regular rounded oblong shape. The CSA was measured from a cross-section area of a regular portion on a pad, which had a swell. The CSAs have a remarkable correlation with the pad's widths. The cross-sections showed a smooth surface and their contact angles seemed to change in correspondence with their pad width. As a conclusion, a VPS reflow solder should be clean and behave as a free-liquid, based on these observations and the conclusion that oxidation in the vapor phase of VPS apparatus is nearly zero, which is clarified in the preceding report.
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