The Combined Effect of Assembly Pitch and Distance to Neutral Point on Solder Joint Thermal Cycling Life
Author: Jean-Paul Clech Company: EPSI Inc. Date Published: 9/25/2016
Abstract: Cycles-to-failure versus Distance to Neutral Point (DNP) data for SnPb and lead-free assemblies under Accelerated Thermal Cycling (ATC) conditions are observed to follow three main trends: 1) no DNP dependence; 2) a power-law dependence with an exponent near -1; 3) a power-law dependence with an exponent close to -2. The first two trends are at a significant departure from standard Coffin- Manson types of models for SnPb and lead-free assemblies (e.g., IPC 9701). Deviations of DNP test data from the standard models can be significant and have serious implications for board designers: a) solder joint life based on Coffin-Manson type of models may be under-estimated by a large factor - and packages rejected - for new designs using a larger die or package; b) moving to smaller die or packages, the existing models may over-estimate the reliability gains associated with smaller size components. This leaves board designers with significant uncertainties and reliability risks. This paper resolves the above differences by means of a simple strength-of-materials model that provides physical and quantitative insight into the combined effect of assembly pitch and DNP on thermal cycling life. The pitch, which affects the stiffness of the assembly, is a significant factor that is not accounted for in standard models. The proposed “pitch and DNP” life model accounts for the pitch stiffness effect and is validated against numerous ATC datasets.