December, 2016
www.us-
tech.com
Page 47
Test Tactics to Reduce the PCB Bone Pile H
By Peter van den Eijnden, Managing Director, JTAG Technologies
aving worked within an established compa- ny in the world of PCB assembly testing for many years we still come across companies
(OEMs) who will tell us, “We don’t need to test, because we demand that the contract manufactur- er (CM) only send us 100 percent working boards.” But, demanding and receiving are two different matters. No one can totally accomplish this, and even if they could, what does it actually cost? There’s an old saying in the engineering industry: “If it hasn’t been tested, then it doesn’t work” — rather, you should assume it doesn’t work. Most people however assume the opposite, and that is where the problems start.
But Testing’s Not My Problem An OEM might not know or care
if its boards are being tested or not, but it should. For example, the CM might only perform a cursory func- tional test using some type of hot mock-up, which may only power up the board and see if it appears to be working correctly. If the board fails this test, it is cast aside and another is built to replace it until the order is complete. Depending on the CM’s process quality and yield, any num- ber of boards could be hitting the “bone pile,” leaving the OEM with significant extra expense. However, if it costs less to diagnose and fix a board rather than to create an entire- ly new one, then of course it makes sense to test more vigorously. The degree and type of testing is
important to consider. A simple design like a flashlight can be tested by turn- ing it on and off. To extend this test it could be turned on and off many times to check the integrity of the principal moving component — the switch. This is known as a functional test. Other
Rack-mountable JTAG TAP controller.
how that information is derived should be consid- ered. Independent fault coverage assessment sys- tems are also available that can take inputs from a
variety of test systems and aggregate the results. These require a great deal of understanding and maintenance and are generally suited to large organizations with a dedicated DFT staff.
Determining Test Strategy When defining the optimal test system, be
sure to ask the following questions:
l What is the maximum size of the unit under test (UUT)?
l Are the boards mainly analog or digital? l Do the boards support JTAG/boundary scan? l Are there any unusual aspects to test, e.g. RF or high-voltage?
Continued on page 50
FLYING PROBE TESTER
FASTEST & MOST ACCURATE Granite chassis eliminates vibrations
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Ultra-compact benchtop test fixture.
types of testing include structural test- ing, which breaks down the test process to check the smallest elements of the design. In a flashlight, this could be the continuity of the conductors to the switch, the switch isolation and closed resistance, the voltage of the batteries, and bulb impedance. This approach to testing gives a much more precise reason for device failure. The decision to test must be
based on hard facts and knowledge of the manufacturing process. Both the OEM and CM must discuss design for test (DFT), fault coverage, diagnostics resolution (test system performance), and other factors before determining the most effective test method. Most good test systems will now provide a fault coverage assessment figure, but
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