Meaning
Material loss limits for the deformation of mounting surfaces during high temperature manufacturing processes define the financial responsibility for mechanical failure in assembly. This substrate warpage scrap allowance accounts for the tendency of thin organic or ceramic plates to bend when exposed to soldering heat. If the curvature exceeds a few microns, the silicon chip cannot be attached correctly.
The allowance is a standard part of the yield model in modern microelectronics.
Thermal Deformation
Heating and cooling cycles in the reflow oven cause different materials in the stack to expand at different rates. This mechanical stress leads to the physical bending that the substrate warpage scrap allowance is designed to cover. Thinner substrates used in mobile devices are more prone to this issue than thicker industrial versions.
Control over the ramp rate of the oven can minimize but not eliminate this bending.
Assembly Failure
Misalignment of the contact pads on the chip and the substrate results in open circuits or shorted connections. When a unit is rejected because it is too curved to be bonded, it is charged against the substrate warpage scrap allowance. This scrap represents a loss of both the substrate itself and the labor invested up to that point.
Monitoring the flatness of incoming materials is a critical quality step.
Supplier Liability
Contracts specify who pays for the lost parts once the agreed percentage of failure is surpassed. If the substrate warpage scrap allowance is set at two percent and the actual failure rate is four percent, the material supplier or the assembly house must often provide a credit. This creates a strong incentive for the manufacturer to improve the material properties and the thermal profile of the process.
Clear measurement standards for flatness are required to enforce these terms.
