The cost of overlooking design-for-test
At the design and layout stage the assumption is often made that manufacturing will be flawless, allowing the development team to concentrate on functional tuning of the product. Unfortunately the test engineer at the manufacturing house is seldom given due consideration during design.
The result is that when manufacturing personnel test the assembly of that particular board, it is a difficult, if not impossible, task. The critical access or test points that would make the board more easily testable are not there. If DFT requirements had been considered, the probes of a flying probe tester would have incorporated the necessary contact points to make contact on the circuitry for measurement purposes, as shown in figure 1.
Figure 1: DFT's main objective is to provide contact points on circuitry for probes of flying probe tester.
By not giving assembly and manufacturing those critical access points, the result is low test coverage and, in worst cases, the board will go through manufacturing missing all the critical tests it needs, such as 30 per cent test coverage when a typical PCB should be 70 to 80 per cent accessible with testing probes. In the example shown in figure 2, a high-frequency, high-speed board lacks the necessary extra test points or the possibility of making the pad sizes larger.
About eight hundred 0201-type passive components are at the centre of this DFT issue. There are land pattern sizes for the 0201 component in this particular design. There are also IPC recommended sizes for the land pattern. The 0201 component used on this particular board is almost half the size in the Y direction. Instead of being 0.6mm, it is 0.325mm. The standard 0402-type package size is about half the size of the 0201 component itself.
At the PCB design stage, the 0201 pad was made smaller for signal integrity purposes. The test point and shape of the pad affect the impedance. If the pad were thicker or bigger, then the impedance would be less. In this design, the 0201-type pad is made smaller to avoid impedance mismatch, and as a result, there is no additional area on the pad for test probes to hit.
Figure 3a shows the smaller pad sizes used for the 0201 in this design. There is no extra land area that can be used to probe the circuitry and measure this component's values. However, there were vias covered completely with solder mask to prevent solder from flowing into the vias.
Figure 2: High-frequency, high-speed PCB lacks extra test points or possibility of creating larger pad sizes.
Figure 3a: Smaller pad sizes for 0201 passive components used to avoid impedance mismatch. But no pad area is left for test probes to hit and measure 0201's values.