Thermo-mechanical analyses of Printed Board Assembly during reflow process for warpage prediction

Abstract

One of the essential requirements for handheld devices such as smart phone, digital camer, and note-PC is the slim design to satisfy the customers' desires. PCB (Printed Circuit Board) should be also thinner for slim appearance. However, the PCB bending stiffness decreases as the PCB becomes thinner, which is one of the most difficult concerns to engineers. Especially, PCB deforms severely during reflow (soldering) process where the peak temperature goes up to 250°C. Therefore, it is necessary for thermo-mechanical quality/reliability engineers to predict PCB deformation at high temperature before PCB manufacturing. The purpose of this paper is to predict PBA (Printed Board Assembly) deformation based on thermo-mechanical finite element analysis by using the following capabilities. First, PCB is modelled in detail in order to obtain the interlayer stress caused by CTE or elastic modulus mismatch between each layer. Also, the contact boundary condition between PCB and rails in the reflower is considered because PCB deformation along the rails cannot be predicted in advance. Last, PCB multiple array placement can be controlled by the user to find the optimized PCB array for minimum PCB warpage. From the numerical results, it is seen that thermo-mechanical analyses of PBA based on detailed modeling can give the engineer PCB warpage prediction.