Study on SMT Quality Monitoring by Auto Optical Inspection

Solder paste is the main interconnect material to use between FBGA type packages and main boards. It is consisted of different types of solder powers (in case of type 3 size is 25~45um) and flux. Many paste parameters, such as paste composition, printing stencil aperture and reflowing profile, influence the printing performance and the reflow process of solder pastes. Flux in solder paste is classified into mainly water-soluable and no-clean flux depending on the applications. No-clean flux contained solder paste has been used in most of applications such as module, SSD etc. But, water-soluable flux contained solder paste is required in SiP module applications to prevent any possible corrosion failure due to flux residue in underfilled module. However, water-soluable flux is very sensitive by humidity in storage condition. Activation and viscosity of flux can be degraded by uncontrolled environment and caused unexpected rotation to resulted in poor solder joint self align. In general, the rheology of solder paste is complicated and affect the shape of solder joint depending on ratio of moisture absoption. If the mixture of solder paste absorb the moisture, the solder paste could be easily slumped and package component is slipped or rotated during the reflow process, result in poor solder joint quality such as non-wet. There are several methods to detect the poor solder joint quality after SMT, such as X-ray and Optical Inspection. The benefits of X-ray inspection are broad in scope due to the ability of X-rays to see through packages including. But X-ray method is restricted to used in massive production due to its poor detection resolution and low efficiency of analytical time. Especially, the poor self-align quality like non-wet joint could not be easily detected by X-ray only after SMT, so the failure samples could be outflow to customers. The main purpose of this study was to investigate effective monitoring methods for replacing sampling X-ray measurement. The existing automatic optical inspection (AOI) equipment can be used to check out the defect of the electronic products, such as solder bridging, non-wet and so on. In this paper, the solder joint self-align trend (amount of rotation) was calculated by M-AOI (Mounting-AOI of package before reflow) and S-AOI (Solidifying-AOI after reflow) data. The trends of M-AOI and S-AOI was compared with non-wet failure rate at final test and X-ray inspection result for relevant properties of solder paste such as viscosity, thixotropy, acidity and so on. AOI data shows well-matched trend with amount of rotation of package and non-wet failure rate of solder joints with regard to floor time of solder paste. As a result, new monitoring method using AOI is proposed and confirm usefulness to replace the sampling X-ray measurement as massively useful tool to check the joint quality after reflow process, and also will be used to control the moisture level of solder paste to ensure the proper rheology for SMT assembly.