Balancing Quality and Cost Print E-mail
Written by Mark Verbrugge   
Saturday, 31 August 2013 00:51

Not all electrical test equipment is created equal, especially when time and money are factored in.

Question: What types of testing options are available to verify electrical integrity of flexes?

We all want assurance that what we buy is of the highest quality, but how do we ensure that what we produce meets the expectations of our customers? Electrical verification is one of the best ways to provide the confidence that what we designed for is what we get.

Let’s review the most common forms of electrical verification with an eye on cost compared to the value gained.

Continuity. Rarely completed without a companion IR (insulation resistance) test, this is the most common and cost-effective electrical verification procedure. This test verifies that the circuit meets the design schematic. Each conductor within the flex circuit is checked for continuity from point to point within each net. Typical requirements are for a resistance of no more than 5Ω per conductor. Test operational voltages are in the 20 to 150VDC range. Realize that some very long or narrow conductors (less than 5 mils in width) can exceed the 5Ω threshold using standard test equipment. (These situations may require advanced four-probe test equipment to factor out probe/internal test equipment resistance.) Asking for less than 5Ω will lead to a higher reject potential of perfectly acceptable boards.

Insulation resistance. Measured in megaohms, this test also has an accompanying voltage requirement. (100 to 500VDC is common.) This test compares each net to all other nets within the circuit, comparing the resistance between each one to a known value. Typical resistance values are 50 to 250MΩ. Test speed is quick, with most common test equipment requiring a few seconds or less to complete both continuity and insulation resistance. The combination of continuity and IR is the best inspection value.

Impedance. Sometimes called AC resistance, basically this test measures the opposition to current within a conductor when voltage is applied. The results are stated in ohms, with 50 to 110Ω being the most common desired value. Typical tolerances are +/-10%; going below this tolerance can add significant cost, as yields will suffer. Test equipment consists of a TDR (time-domain reflectometer) or oscilloscope. Impedance can be quite difficult to measure within a flex circuit and is usually tested on a representative coupon. The coupon must have a structure identical to the circuit with the same conductor width and spacing and internal separation between signal and ground. Coupons should be placed within the processing panel as close to the flex circuits as possible. As this test verifies the structure of the flex circuit meets the specification, testing each individual flex is not required. Costs are low, as only one or two tests are required per processing panel.

In circuit. Usually the most complicated and costly to fixture, simply put, we verify the fully populated flex functions as intended. All component values are checked, and in the case of active components, their outputs are proved. Rarely automated, this test usually requires a dedicated fixture, programming of test equipment and always significant test time.

Dielectric withstanding. I mention this one last, as it is an uncommon test procedure. Often called “Hi-Pot” or “High Potential,” we can think of this as an IR test on steroids. Unlike IR testing where we measure resistance between nets, here we are looking for leakage current. Typical leakage current thresholds are set in the 15 to 30 milliamp range using voltages of 500 to 1,500 volts (AC). This test requires a given amount of time per test, usually 10 to 30 seconds. This is where costs can rise in a hurry! Example: If we have a “simple” flex with 20 individual nets, each tested for 30 seconds, and we test each net against all others, we are looking at a test time of 10 minutes per part! The test time can rapidly exceed the entire time it took to produce the entire flex. With this type of testing, it pays to do a thorough review up front and determine what nets need to be tested and group them so we can limit the test to one cycle per part. Alternately, we can test the basic construction by creating a representative coupon of our closest conductors and test one or two coupons for failure. I have seen rare cases where generated capacitance has actually caused a “false reject”. Flexes have such thin substrates (as thin as 1 mil) that a heavy shield over a wide conductor can be read as a dead short. This is especially true when there are large plane areas on adjacent layers.

Test equipment. Testing adds cost without adding directly to the value of our flex. We need to review our testing procedures to get the best value for our investment. Total order quantity is crucial in selecting our test-tooling options.

Automated test equipment has real advantages for prototype or low-volume jobs. Sometimes referred to as flying probe, these testers do not require dedicated fixturing but operate based on programming at the CAD/design stage. Excellent for panel-based testing, circuit verification can be determined prior to value-added processing such as SMT or connector installation.

Dedicated fixtures are almost always required for flexible PCBs with components installed (Figure 1). Additionally, testing a board after it has been excised from the panel prevents any conductors damaged in the blanking process from escaping. Speed is good, but fixturing costs can be high, depending on circuit complexity.
Not all electrical test equipment is capable of all possible test parameters. It is important to review the manufacturer’s equipment and work to define a test regime that is not only effective at verifying electrical integrity but cost-effective as well. IPC-9252, “Requirements for Electrical Testing of Unpopulated Boards,” is a great resource. Cost considerations aside, electrical verification is a valuable tool to build customer confidence in your product.

Mark Verbrugge is a field applications engineer at PICA Manufacturing Solutions (picasales.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . He and co-“Flexpert” Mark Finstad ( This e-mail address is being protected from spambots. You need JavaScript enabled to view it ) welcome your questions.

The Flexperts will present at PCB West, the Silicon Valley's largest printed circuit board trade show, Sept. 24-26, at the Santa Clara Convention Center. www.pcbwest.com

Last Updated on Wednesday, 04 September 2013 00:06
 

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