Effect of Substrate Preheating on Solderability Performance as a Guideline for Assembly Process Development Part 1: Baseline Analysis *

Abstract

A study was performed which investigated the wettability of 63Sn‐37Pb and 96.5Sn‐3.5Ag solders on copper and gold ‐nickel plated Kovar ™ using a rosin ‐based, mildly activated (RMA) flux, a water soluble organic acid flux (WS ),and a low residue (LR) flux. The quantitative metric was the contact angle, θc, measured by the meniscometer /wetting balance technique. The first part of the study (Part 1) examined wetting performance following continuous exposure to 25°C prior to testing. Then, a preheating step was introduced into the experimental procedure after flux application, but preceding the actual wettability test in order to simulate a factory reflow process; these results are presented in Part II of this study. Contact angles for the 63Sn‐37Pb solder (215°C) on copper were 22±2° with the RMA flux, 12±5° for the WS flux, and 31±6° for the LR flux. Increasing the 63Sn‐37Pb solder temperature to 245°C improved wettability with the RMA and LR fluxes, but no change was observed with the WS fulx. Theii 96.5Sn‐3.5Ag lead ‐free solder exhibited poorer wettability on copper compared with the 63Sn‐37Pb alloy, with contact angles of 41±2° (RMA), 63±15°(WS) and 39±4°(LR). For the gold ‐nickel plated Kovar™ substrates, the 63Sn‐37Pb solder at 215° had contact angles of 15±3°, 35±6° and 29±6° for the RMA, WS and LR fluxes, respectively. The values were reduced at the higher test temperature (245°). The 96.5Sn‐3.5Ag solder also exhibited good wetting performance on the gold ‐nickel plated Kovar™ specimens compared with copper. Analysis of the interfacial tension parameters, γSF‐γSLand γLF ,exemplified the importance of γLF as well as the condition of the surfaces (γSF ) on wettability performance. A so ‐called ‘combined analysis’ of the 63Sn‐37Pb and 96.5Sn‐3.5Ag wettability data on either copper or gold ‐ nickel plated Kovar™ substrates was used to predict the solder temperature dependence of wettability for the three fluxes and two base materials.