SMTA International Conference Proceedings

Using SPI, AXI, and CT X-Ray Data To Improve SMT Process with QFN Devices

Authors: Stephen Chen, Tho Vu, Hung Le, Alan Chau, Elliott Le, Phuong Chau, Hao Cui, Raymond Tran, Roy Chung, Bryan Goble, Nadarajan M Singaram, Golden Xu, Zhen (Jane) Feng, Ph.D., David A. Geiger, Murad Kurwa, and Evstatin Krastev, Ph.D.
Company: Flextronics International Inc. and Nordson DAGE
Date Published: 10/14/2012   Conference: SMTA International

Abstract: Quad Flat No Lead (QFN) is extremely popular because of their low cost, low stand-off height and excellent thermal and electrical properties. The challenge for the industry is to achieve the best possible QFN solder joint quality. Here are some very important topics. What is the best way to use test and inspection techniques? How to minimize voiding in thermal pads by changing design rules in order to meet stringent customer requirements? How to reduce the use of Mechanical Cross Sectioning as it destroys the valuable PCBs and is time consuming?

In this project, we focus on developing the best test procedure for identifying solder joint defects in QFNs and improve our SMT process. The test methods include SPI, AXI, and 3D CT Board Level X-Ray Inspection, also called Limited Angle or Partial CT (PCT). All these methods are completely non destructive. Our test assembly has a large number and variety of QFN devices.

We analyzed data from Solder Paste Inspection (SPI), AXI (5DX), and Board Level CT X-Ray (Virtual Cross Sectioning) using nine QFN package types (24 components). The goal is to look for a correlation between SPI and AXI using more than 6,400 attribute and variable data points and also to compare the data with 2DX and Board Level CT. This is a very challenging project requiring advanced SPC techniques. Finally, based on the test results, prepare an action plan for SMT process improvements intended to take place within several months.

Our QFN SPI-AXI Pearson Correlation does not show a strong relationship, however, attribute data has shown good results: 100 % of the packages show correlation for more than 90% of the pins, while 80.6% of the packages showed correlation for 100% of the pins. SPI is the first test machine in the SMT process -- implementing SPI in the SMT line results in significant reduction of components to the repair line.

Key Words: 

SPI, AXI, CT X-Ray, QFN Devices

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