Getting your EMS provider involved early in the the design process

Lee Cushen, head of test engineering with Axiom, said: "You'd be surprised how many companies send us a design which we can't test. We want good products to leave the factory, so we do as much test as we can, but we need to work with the customer." So what do you need to bear in mind when you design a board that is going to be manufactured elsewhere? Rule number one is engage early with your contractor. Phil Randall, senior test engineer with TTims, noted: "People are getting better at this. We used to have designs 'coming over the wall'; now, we have a chance to enter into dialogue. It's one of the things with which we're making progress." Matthias Hoffman, senior test engineer with Plexus, added: "The 'testability' of a product should be an important focus during design. It is key to improve the diagnostic capabilities and to reduce the number of potential errors. At this stage, it is possible to influence the design and adapt test requirements; for example, setting additional test points and using pull-up or push-down resistors on inputs and outputs. Focusing on test during the design phase will also allow for the optimisation of the manufacturing process." Looking to kick start this process, EMS providers will have information telling the engineer how to design with prototyping and post manufacture test in mind. Cushen said Axiom has an on site PCB designer who can 'run the rule' over the customer's design. "We also issue design for manufacture and test (DFM/DFT) guidelines, so they know what they should be doing." Randall concurred. "We have a list of DFM/DFT guidelines, which tells companies the way to design things into their boards or how to make them easier to assemble. But it's just the starting point. If it's a mainly digital board, there won't be the need for too many test pads; if there is a lot of passives, the test process will benefit from a lot of test points." Cushen agreed in part. "The basic thing is to give us as many test pads as possible; that gives us the option to use a range of tests." What role does JTAG play in manufacturing test? Cushen continued: "We can take advantage of JTAG, but it does need a connector to be included that allows us to interface with the components. Some customers will design using JTAG, but won't provide access for test. "It's not the 'be all and end all'," he noted, "but it's a good tool when designed in and used properly. However, it has to be accompanied by other functional tests." Randall said many designers still see JTAG as 'new'. "It's about 20 years old, but we still hear that it's 'new technology'. Sometimes, it's the only test that will give the results you need. The skill, however, is to know which test to apply." And EMS companies have a range of tests which can be used, as Hoffman explained. "There are numerous testing technologies from which to choose, including automatic optical inspection (AOI), automated x-ray inspection, in circuit test, functional test and flying probe. With a good test strategy, several of these methods are combined systematically and tailored to the individual assembly requirements. Through targeted planning, the details of each test are included in the manufacturing process, so production is disturbed as little as possible. Testing must be done quickly and with purpose." Randall noted: "Ideally, you'd want every point covered; each board has a list of components and a list of solder joints. You'd like to place a tick next to each." Cushen said 90% of the products manufactured at Axiom will be inspected using such tests as AOI and flying probes. "There are no issues involved in adjusting an AOI programme," he pointed out. "Flying probes also allow you to check the Bill of Materials and things like component values; you can pick up a lot of information using this approach." Randall said which test is selected depends on the board's core technology. "Some solutions are okay for one board, but not for another," he claimed. "Between us, we need to choose the right test for the board. But remember, the more access you provide, the better the test." Part of creating the test strategy is to decide whether or not your design needs 100% test. "Some designs can get away with sample testing," Randall continued, "but they tend to be in much higher volumes than we handle, so we do 100% test." Cushen added: "Most customers will want all production to be tested. That can be done using a proper fixed test approach, but we do need to determine what needs to be tested." In Hoffman's opinion, which tests are used and when depends on many factors. "It makes sense, because of the high level of complexity of modern assemblies, to combine different tests in order to get an error free product. "Each test engineer must understand the test methodology thoroughly and understand the product to be manufactured as a whole. They need to have visibility of the production process, to assess risks and calculate costs, because the use of tests is not without risk; misplaced tests lead to higher costs or slowing the process unnecessarily." What tips do the experts have? One theme is careful use of real estate. "People are always pushing components closer to the edge," Randall said. "Often, this is because they need to. We have to be careful, but test fixturing does need to be borne in mind." Cushen agreed. "Don't put things too close to the edge of the board; they can get damaged when the board is clamped. This could be a 'showstopper'; in the worst case, needing a complete redesign." Hoffman repeated the call to get involved with your EMS company as soon as possible. "EMS companies often have particularly good experience, because they do not just do production test. They can also be involved in the planning and development phase, which would include the design of a product specific test strategy." Randall said: "Documentation is key. It has been one of problems over the years and can make or break the project. If you have accurate information, it makes life straightforward." And bear in mind the need to test your product for the environment in which it will be used. "Where's your market?," Cushen asked. "North America ranges from cold in the north to hot in the south, so you might need to subject the device to environmental stress. Designers need to think about things like that." Hoffman concluded: "Too much test can be just as bad as not enough test. There is no 'one size fits all' solution and specific tests should always be chosen on a case by case basis."