THT AOI Inspection

Through Hole Technology for assembling electronic circuits is often described as an old fashioned technology that is losing its popularity and being replaced more and more by SMT, surface mount technology. No wonder, as electronic devices where SMT is used are more compact, thinner, lighter, faster and consume less energy. SMT is also much more efficient from an assembly point of view and surface mount device components cost less. Basically the replacement of THT by SMT is nothing recent but has been on-going since  the introduction of SMT in the late 1980s but increased significantly in the last decade. Except for high-power circuitry where SMT is being combined with THT, we can say that MT is the current widely applied assembly technology. Typically, the main focus of most vendors is to invent and develop equipment that is specialized in packaging, placing, solder reflowing, reworking, inspection etc. of SMD parts. Only a small number of vendors are developing and offering THT equipment. This equipment is mainly focused on the soldering part of the assembly process.

Human Eyes

In PCB (Printed Circuit Board) assembly process, QC (Quality Control) is required to increase yield. One of the most popular QC tools is vision inspection. Because of the small footprint of SMD components and the density of SMT circuits, the popularity of SMT resulted also in popularity of AOI, automatic optical inspection systems. Research proves that AOI systems can typically find 4-5 times more defects than human eyes. Given that machines work consistently and without breaks, AOI systems became mandatory due to their inspection advantages and quick ROI (Return Of Investment) thanks to cost reductions.



There are numerous vendors developing AOI systems varying from low-cost component only inspection- to high-end full inspection machines. Most of these machines are focusing on SMT inspection. With this specialization it means that the optics (hardware) and algorithms (software) are designed for such inspection. Unfortunately, this means that the development of optics and algorithms for THT inspections is being ignored or at least is not getting the attention it deserves. No wonder, as usually market trends are followed closely and people tend to chase after the “big bucks”.



By looking into current market trends, we can see that thru-hole technology is still being actively applied, especially in the automotive industry. Modern cars contain more and more electronics and some of them are 100% electric. So there is an ever increasing demand for power electronics in the automotive industry and their sub-contractors. This automatically means that there is an increasing need for visual inspection of these electronics.



Where some AOI vendors ignored THT inspection and where others abandoned further development of their existing capabilities, across the years Mek (Marantz Electronics Ltd.) further developed its AOI systems to be able to inspect THT components and complement it with reliable THT solder joints inspection. The first Mek  AOI system (formerly called Marantz) was developed in 1996 for its own use. Audio entertainment fans know Marantz from the high-end amplifiers and receivers. These audio devices consist mainly of analogue components in their THT form. So obviously the first AOI system developed by Mek was designed with the ability to inspect such components. And as modern audio systems still contain analogue THT devices (high-end audio systems like Marantz, Denon etc.), Mek has continued developing creative algorithms and optics for such inspection.


Component Side

At Mek we inspect both component side and solder side of the THT (partially) assembled PCB. Biggest challenge for inspecting components side is that THT components tend to move (bend) easily in any direction while component shapes and colours can also differ strongly. It is practically useless to use centroid data (CAD data) as the components can float from their central position. Also, components can be very tall causing them to appear blurry in the camera image. Needless to say, Mek has developed special search, filter and decision algorithms that overcome these component side challenges and find defects reliably.


Solder Side

Regarding the solder side of the THT, the challenge here is to find all related defects in both types of soldering process: wave and selective. These solder defects can be: shorts, no pin no solder, pin visible but no solder, no pin but with solder, solder that is attached only to pad but not the pin, too much solder, lack of solder, bad shape solder and circumferential wetting problems (where solder covers less than 75% of the pad). Mek has the optics and the special, rule based algorithms that can find each of the mentioned THT solder defects. By projecting light omni-directionally and from different angles and colours, an ideal profile of the solder can be created. Combining the resulting images with the various rule based algorithms, a reliable judgement can be made in an efficient and effective way.


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