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JANUARY 2022 I DESIGN007 MAGAZINE 33 Bare board testing involves the measure- ment of both capacitance and resistance of the finished circuit board. • Capacitance testing involves testing for opens and shorts by applying current then probing each net to measure the induced capacity. • Resistance testing measures the electrical resistance of the conductor within the net by probing both ends of the net. e circuit board's copper conductors will generally deliver a low resistance measure- ment but the measured resistance will also be impacted by the copper conductor's length and its cross-sectional area. As an example, when measuring two nets of the same material and the same cross-section, and one conductor is twice as long as the other, the longer net will deliver an increased resistance value. On the other hand, if there are two nets of the same material and even though they are the same length, if the copper conductor on one of the nets is twice as thick as the other, the thicker conductor will measure that net at half the resistance of the thinner copper conductor. Bare Board Test Methodologies Two methodologies are commonly applied for bare board testing: fixed probe and flying probe. Both fixed probe fixture test and flying probe testing can save the frustration of over- looking circuit board manufacturing defects. So, which test method will be best? It really depends on the volume of units to be tested and the economics of scale. When the circuit board quantity is minimal or the product is subject to further refinement or revision, the flying probe test is a logical choice. e cost of a flying probe test is calculated per unit so the cost of setup and processing will be mini- mal, but throughput will be slower. When the product has been stabilized, however, and the requirement is for hundreds or thousands of units per week or month, then the setup costs for developing the fixture and programming for fixed probe testing can be justified. Fixed Probe Testing For high-volume multilayer PCB applica- tions, a system is commonly employed that is programmed for simultaneously testing all nets on the circuit board. e contacting fix- ture, referred to as bed-of-nails (Figure 1), is furnished with spring-loaded probe terminals that access every net on the finished circuit board. In preparation for test, individual boards are placed onto a custom fixture configured with an array of probe contactors aligned to test lands provided on the board surface. For more complex circuit board applications, the board will be placed between opposing probe platforms, pressing the board from the top and bottom with hundreds of spring-loaded con- tactors. e actual test cycle is very quick regard- ing "go and no-go" determination, but the primary benefit is the test system's capabil- ity of identifying the failure type and specific defect location. Although fixed-probe test- ing is fast and efficient, the development of the dedicated fixture and testing program is costly and, if the board undergoes any changes, Figure 1: Bare board test fixture with bottom side probe access.

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