Surface Mount International Conference Proceedings


LEAD-FREE SOLDER FINE PITCH STENCIL PRINTING

Author: Cindy Melton
Company: Motorola Corporate
Date Published: 9/10/1996   Conference: Surface Mount International


Abstract: The 25 mil (0.635 mm) pitch Quad Flat Pack (QFP), with terminal counts from 88 to 200, has become the industry standard IC package for leading edge electronic assemblers. The continuing miniaturization trend in electronics is forcing manufacturers to evaluate ever increasing densities through the use of finer pitched packages. This fine pitch packaging trend requires the re-evaluation of stencil printing materials and processes. The implementation of a lead-free soldering process is another issue that needs to be considered in developing future stencil printing processes. Recent global activity in restricting the use of lead has electronic manufacturers concerned. The inherent properties of lead-free solder alloys compared with conventional SnPb is another aspect that needs to be investigated. Furthermore, it is advantageous to understand if these differences provide a processing advantage in fine pitch stenciling. The purpose of this study is to investigate the fine pitch QFP printing properties of a, drop-in replacement, lead-free solder alloy and quantify the reliability of this attachment. In parallel, Ball Grid Array (BGA) package assembly and reliability using the lead-free alloy is examined. In both cases, the results of the lead-free solder alloy is compared to a conventional eutectic SnPb alloy. The terminal pitches evaluated in this study range from 0.5 down to 0.3 mm for the Thin QFP (TQFP) and from 1.27 to 0.5 mm for the BGA packages. Printing was performed with both a lead-free solder, SnInAg, and with conventional eutectic, SnPb, solder. Double sided, 18 mil thick, FR-4 substrates were used. The pad finishes evaluated were Ni/Au, immersion SnBi and an organic imidazole. The quality of print, paste volume, die shear strength and thermal shock performance were used to assess the manufacturability and reliability of these fine pitch components at board level. Stencil thickness, stencil aperture sizes and the stencil printing parameters were adjusted to determine process feasibility at these fine pitches using solder stenciling. The primary limitation for fine pitch printing of TQFP packages was found to be the stencil thickness and the aperture size, while this was not the case for BGA packages.



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