When to Use Aoi, When to Use Axi, and When to Use Both

With today’s technology embracing ever-smaller chip components, array packages with hidden solder joints, and numerous components on both sides of the printed circuit board assembly (PCBA), manual visual inspection is running out of steam to adequately address these challenges. AOI (automatic optical inspection) and/or AXI (automatic X-ray inspection) are starting to be used at a majority of today’s manufacturing sites. The key question is: When is it best to use AOI, when to use AXI, and when to use both? This paper will answer that question by addressing several issues for selecting the best inspection strategy. The paper will present data from many studies Agilent has performed in the quest to find the optimal test / inspection strategy. The paper will also make recommendations on which of these strategies to use in different situations. INTRODUCTION “When to use AOI, when to use AXI, and when to use both?” is unfortunately not an easy question to answer. It is not a simple matter of looking at just a few characteristics and then having the answer. The reality is significantly more complex, and there are many factors to consider to get even close to an answer. This paper attempts to share data and new thoughts on this question. Please see it more as guiding principles than absolute truth. Today’s test engineers have significantly more challenges than just a few years ago. The board complexity is increasing with more components, more joints, higher densities, new package technologies such as area array packages, and 0402 and 0201 chip components. The higher component and joint counts create more defect opportunities which lead to lower yields for a given defect level. At the same time, there are more test and inspection alternatives today with new technologies such as Solder Past Inspection (SPI), Automatic X-ray Inspection (AXI), and Automatic Optical Inspection (AOI). These inspection technologies are well established and provide real choices among capabilities and benefits. Boundary-Scan test technology has also emerged as a popular electrical technique to complement InCircuit Test (ICT) and Functional Test (FT). While these new tools offer more choices, they also pose a new dilemma. Which is the right test / inspection strategy? Which is the right combination of these tools? This paper will focus on the selection of AOI and / or AXI. Today, most test engineers have a good understanding of the optimal use of ICT, Boundary-Scan and Functional Test. It is assumed that these test strategies are used to complement the major inspection strategies discussed in this paper. It is also outside the scope of this paper to discuss Solder Paste Inspection (SPI) in detail. The main focus of this paper is selecting inspection strategies for volume production. Prototype testing poses unique challenges because of the need for fast test/inspection turn-around times for program development and the typically low quantities of boards manufactured. Prototype testing strategies are outside the scope of this paper.