Direct Attachment of Leadless Chip Carriers to Organic Matrix Printed Wiring Boards

Abstract

A test program was performed that compares the reliability of various leadless chip carrier (LCC) solder joint configurations under conditions of temperature cycling from -55 to +125°C. Since there is a coefficient of thermal expansion mismatch between the ceramic body of the LCC (6 ppm/°C) and that of an epoxy-glass printed wiring board (15 ppm/°C), it was of interest to determine the optimum solder joint configuration that results in maximum reliability after extensive exposure to temperature extremes. This configuration was determined to be the one that results in a 45° solder fillet; consequently, the distance the printed wiring board (PWB) pad extends beyond the edge of the LCC is a critical requirement. For an 18 pad LCC, this distance is 40 mils. Information on the use of various substrates for leadless chip carriers is also discussed.