A method of reliability assessment on creep corrosion for immersion silver finished PCBs

This study focuses on a method of reliability assessment for creep corrosion on immersion silver-finished PCBs utilizing the mixed flow gas (MFG) test and clay test. Four kinds of locations were defined to conveniently describe the location from which the dendrite corrosion products were growing: IL refers to the location that corrosion products grew from a hole or pad edge with solder mask; IIL refers to the location that corrosion products grew from a hole or pad edge without solder mask; IIIL refers to the location that corrosion products grew on a hole or pad; and IVL refers to the location that corrosion products grew from solder mask. The IL dendrite corrosion product is a key factor to result in a short circuit. It is better to use the maximum length of the IL dendrite corrosion products as an assessment parameter. The failure criterion can be the distance (or half the distance) between the edges of two holes (or pads) or from one hole to another pad. The regression equation of the maximum length of the IL dendrite corrosion products and time can be used to predict remaining life. IVL dendrite corrosion products are potentially dangerous to result in a short circuit. The growth rate of corrosion products, which refers to the length that the corrosion products grow per unit of time, can also be an assessment parameter to describe the growth features of corrosion products. The thickness gain of copper for monitoring corrosion rate can be measured by cross-section method. Two cases show that the thickness gains conform to lognormal distribution.