Printed Circuit Board (PCB) for Surface Mount Technology (SMT)need to be choosen wisely taking into consideration factors such as CTE (Coefficient of Thermal Expansion), cost, dielectric properties and Tg.
When designing surface mounting board (PCB), the selection of substrate is basically determined by the type of SMD components to be used. In any Electronics manufacturing or PCB assembly, when leadless ceramic chip carriers (LCCC) are mounted on printed circuit boards made out of glass epoxy substrates, solder joints cracking is generally seen about 100 cycles. The cause of the excessive stress is the CTE differential between the ceramic package and the glass epoxy substrate.
There are three different approaches to solder joint cracking problems:
Using a substrate with a compatible CTE;
Using a compliant top layer substrate; and
Replacing leadless ceramic packages with leaded ones.
The most widely used substrate for SMT Printed Circuit Boards, is glass epoxy. It entails no CTE compatibility problems when used for plastic surface mount packages. However, This provides the solution for commercial applications only.
The most commonly used substrate for pcbs for military applications is one with a CTE value compatible with that of the ceramic packages that has been specified. Each PCB substrate option has its own advantages and disadvantages. Designers need to be carefully balances the constraints of cost with reliability and performance needs. In addition, solder masks and via hole sizes should be selected carefully.
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