Capacitor Cracking due to Thermal Shock in Wave Solder Processing

Abstract

Increased use of surface mount ceramic capacitors in wave solder applications has resulted in a number of field failures due to migration of microcracks through the capacitor. Cracks are initiated in board assembly by a number of processes including excessive forces of assembly equipment, board warpage and/or deflection, rework temperatures, and wave solder profiles. Thermal shock from wave solder contributes approximately 20–25% of the failures. The majority of capacitor manufacturers recommend no greater than 100°C delta between pre‐heat and wave solder temperature during processing. This can be difficult to achieve when processing a range of board sizes. A delta of 120°C allows for different size boards without using a custom thermal profile for each. This study shows no increased failure rate for X7R 1206 capacitors between the two process deltas of 100°C and 120°C. The suggested delta of 100°C is based on a 15000 psi tensile strength of the dielectric and is conservative. The temperature profile may be extended to include the more versatile 120° delta without compromising the reliability of the component.