Thermal Stress
Repeated exposure to the high temperatures required to melt tin-silver-copper solders subjects electronic assemblies to measurable thermal strain. Standard lead-free reflow cycling typically involves peaks reaching 245 to 260 degrees Celsius. These temperatures are much higher than those used in older lead-based processes.
Each trip through the oven forces the materials to expand and contract, which can weaken the mechanical bonds within the board.
Material Impact
Substrates must be engineered to withstand these heat spikes without blistering or losing their adhesive properties. During lead-free reflow cycling, the resin system undergoes rapid expansion that can stress the copper plating in the holes. If the material has a low decomposition temperature, it may begin to break down chemically after the third or fourth cycle.
This degradation leads to a loss of insulation resistance between high-voltage traces.
Reliability Testing
Engineers use these cycles to simulate the worst-case conditions a product might face during manufacturing and repair. Evaluating a board after multiple rounds of lead-free reflow cycling reveals the long-term durability of the vias and solder joints. Cross-sectional analysis often follows these tests to look for microscopic cracks in the copper barrels.
Passing this evaluation is a requirement for qualifying new materials for automotive or aerospace use.
