When is it your fault?

When is it your fault?In a feature on electronics manufacturing Jon Mainwaring examines the importance and the differences in inspecting product.
Today’s customer demands a lot from manufacturers. Gone are the days when he expected to take a significant amount of his purchases, especially of the consumer electronic variety, back to the shop because they would not work properly.
If a customer buys a faulty product today, then it is highly likely that the experience will be influential in his choice of manufacturer the next time he buys. People can no longer afford the time to ask for refunds, or repairs, and when they do they remember the inconvenience.
In the field of electronics, manufacturers are also aware of the cost of repairing, or recalling, products that have failed in the field. They know that a lot depends on quality, which is why they are prepared to pay millions on inspection equipment for their printed circuit board (PCB) manufacturing operations.
But there are various approaches to inspection. Sample inspection is one such approach that is usually allied to Statistical Process Control (SPC), a system more successfully implemented by Japanese manufacturers than by Western companies. The idea is to check that the production process is working properly by not only sample inspecting products, but testing variables such as how much solder is being used on each board, or the temperature of a reflow oven. If the measurements reveal only tiny amounts of variation, then everything must be fine.
Western electronics manufacturers, however, are not quite so happy to place so much trust in the production process. And so, while many of them use SPC, most favour the much more expensive approach of one hundred per cent board inspection.
There are many ways to test a PCB. You can electrically test all the solder connections to make sure they are all there, or you can perform a fully functional test to see whether the board does what it is supposed to do in the field. What these tests do not tell you, however, is whether a board’s solder joints have been made correctly; there may be an electrical connection, but the physical link might be so weak that it will soon break.
One type of inspection that allows you to almost guarantee the integrity of solder joints involves the use of X-rays.
Hewlett-Packard has been working in the field of PCB inspection by X-ray for a very long time. Despite X-ray machines being criticised for being too slow for in-line use in PCB manufacturing and too expensive, the company has pressed ahead with its development of X-ray equipment. “The market acceptance of these things is huge. Sales have been going through the roof,” says Colin Charette, business project manager at HP’s Manufacturing Test Division. “HP wouldn’t have invested [in X-ray] if it didn’t think it was going to be big.”
Charette believes the company’s latest machine, the 5DX Series 2, is going to further increase interest amongst PCB manufacturers. “X-ray used to be slow and expensive, but we’ve actually made dramatic improvements with equipment,” he says. “In the last six months we’ve produced a system that can do 100 per cent test.”
One major change has been the knock-on effect of advances in charged coupled device (CCD) cameras. The last camera HP used before the Series 2 had 512 by 512 pixels. “We went to a 1000 x 1000 camera and increased the area you could look at on a board by a factor of four,” says Charette. “Now you can look at 400 solder joints instead of 100, in the same time.”
J?rgen Stephan, project manager for X-ray technologies at Siemens’ Central Department for Technology, believes that X-ray inspection is best for certain applications that cheaper automatic optical inspection (AOI) cannot handle. AOI is an acceptable method for checking physical defects on a PCB, but it is best to leave solder joint inspection, especially on devices that obscure their solder joints like ball grid arrays, to X-ray. “AOI can be used to detect tombstoning [where components pop up after soldering] and missing components,” he says. “It is cheaper than X-ray, but it does cause many false alarms.”
For instance, AOI machines can be confused by the lighting conditions in a factory, whereas X-rays cannot. “If the lighting is a little bit different, then you can get different images with an AOI system,” says Stephan. He also points out that X-ray is not influenced by the surface conditions on a PCB. How much flux a board has on it, and how clean it is, can also lead to false alarms.
Stephan makes it quite clear that X-ray inspection is very important for certain kinds of products. “One component that is always being inspected by X-ray is the air bag sensor used in cars,” he says. Should an air bag fail in the field, the automotive electronics supplier that made the sensor could expect a great deal of grief.
But Andy Pidduck, managing director of Hampshire-based R.L. Pidduck which represents AOI manufacturer Orbotech Schuh in the UK, still thinks PCB makers would do better to stick with AOI. He believes the issue of solder joint integrity in BGAs is just a smokescreen put up by X-ray inspection proponents. “[PCB manufacturers] we’ve spoken to have told us that it is not common to get bad solder joints if the chip is fitted in the right place,” he says.
And, although X-ray inspection is getting faster, Pidduck believes that AOI will always remain ahead. “Don’t forget, we’re working to improve speed as well,” he points out.
Pidduck also raises the question of safety on the shop floor. Although he is sure that all current X-ray machines meet safety standards, he says: “A lot of the engineers we’ve spoken to aren’t comfortable working with high frequency radiation machines.”
Lastly, Pidduck dispels the notion that the systems he sells are affected by lighting. “The Orbitech VT026 uses camera flash level xenon lighting, many factors above ambient light,” he says, explaining that this makes any changes in the lighting levels on the shop floor irrelevant. “Our machines work in a very controlled environment.”


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