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WP-Tests-Flying-Prob What Electronics Manufacturers Test and Inspect For, and Why It Matters Authors: Dan Kreider, Test Engineer, Axiom Electronics; John Kim, Manufacturing Engineer, Axiom Electronics October 2017 Test is extremely important when manufacturing low-volume/high-complexity electronics in critical applications, such as secure communications, target acquisition and destruction, or in space applications where the assembly is usually on a one-way trip that will not receive ser- vice calls. This white paper addresses what manufacturers test for and details the important testing and inspection routines from receiving the Printed Circuit Board (PCB) to testing the completed Printed Circuit Board Assembly (PCBA). Defective ICs and bad fabs are rare, and ICs. Boundary scan can be an effective test they can be difficult to detect with our auto- mode for highly complex boards. mated testers. Flying probe test strategy basi- cally assumes the Printed Circuit Board (PCB) Open solder/missing components is the is good (tested at the PCB fabrication facility). most common assembly defect category. Both A defective IC may be shorted or open across flying probe and ICT (bed-of-nails) test for a set of pins. This may lead to a failure of an these issues, particularly for passive and dis- automated test like flying probe or ICT, or it crete components. Lifted pins on IC legs (or may affect a diode junction or capacitance open solder joints on BGAs) can be detected test, otherwise the defect may go undetected. in many cases. It is possible to measure the protection diodes built into many ICs to detect JTAG/boundary-scan testing is effective at open/shorted pins or defective components. screening for defective ICs and board conti- Test coverage is limited by the number of nuity issues such as open BGA solder joints. electrical nodes accessible on the board with JTAG models are available for many complex probes (or nails). Axiom Electronics Testing & Inspection Steps Electrical shorts are the second most com- Bare printed circuit boards are manufactured at a fab- mon assembly defect category. Tests are rication facility, and one of the first steps taken is to: developed to check specifically for shorts between adjacent pins of ICs and connectors. 1) Netlist Compare: The PCB fabricator validates the Shorts will often cause other routine tests to Gerber generated files against a netlist to compare the data. This is run with software tools such as Visu- fail, i.e. passive components connected to the alCAM, or Valor. shorted path in the circuit. Flying Probe test a. This is to confirm that all the data matches the helps to identify the presence and location of design database. a problem. 2) Flying Probe or In-circuit test: Once the PCB is Wrong-valued components are rare but im- built, it is retested by the PCB fabricator either via a pact an entire batch of PCBAs. The chances flying probe or in-circuit test method to compare the of catching the problem are very good if the manufactured board to the netlist again. component is a passive or discrete part that is a. This is a test that is initiated on the PCB (no measured parametrically (resistor, capacitor, components). The PCB will either pass or fail this diode, transistor). Capacitors are often in par- and good PCBs will ship to the assembler. allel so if a smaller-valued capacitor has an 3) Incoming inspection: Upon arrival at Axiom Elec- incorrect value or is missing, it may be difficult tronics’ assembly facility the PCBs will go through an to detect. Small-valued inductors are difficult incoming inspection. to measure but their presence is confirmed a. PCBs are reviewed under high powered micro- with a continuity check. scope for any imperfections before being released to the floor to build. The “NZT” feature of the SPEA flying prober helps identify defects by taking capacitance 4) AOI (automated optical inspection): Once the com- and/or resistance measurements at every ponents are installed on the PCB, it is inspected by accessible node on the board. In principle it is AOI for the following. capable of detecting any of the defect types a. Solder defects b. Lead defects above. c. Component presence and position d. Correct part / polarity Ask your supplier to conduct flying probe test e. Through-hole parts to save your valuable engineering resources f. Solder paste otherwise consumed with troubleshooting g. Component skew design and assembly errors. Axiom Electronics offers PCB design ser- vices, through sister-company DFX Design, which examines the PCB layout and makes minimal changes to allow full testing of the PCBA. On PCBAs we have worked with (high speed and controlled impedance), we have achieved over 99.3% coverage, thus improving manufacturing outcomes. AOI image (9 cameras) Axiom Electronics Test & Inspection 2 Axiom Electronics Testing & Inspection Steps (cont’d) At Axiom, 100% of PCBAs are inspected with AOI. 7) Functional Test: Axiom can design and build func- Traditional AOI machines use 5 cameras to inspect tional test equipment and run these test for cus- components, more advanced AOI machines use 9 tomers. This test powers up the PCBA and can test cameras to provide a more complete view of compo- specific or full functionality. (Note that most customers nents. utilize the flying probe test because of its speed.) 5) 3D X-ray inspection: Opens the window to viewing through component surfaces to solder joints hidden to visual inspection or AOI. 3D x-rays are excellent for use with BGA or through-hole components. a. Can be programmed to perform automatic X-Ray inspection on individual PCBAs. Operators can focus on other tasks while inspection is performed. b. Performs 2D X-Ray inspection with higher power settings (180kV, better resolution and deeper pen- etration), oblique views up to 70°, and full 3D (CT) scan capability. c. Provides process monitoring and failure analysis on state of the art components. Dan Kreider Test Engineer Dan handles all forms of test engineering, including pro- gramming the SPEA Flying Prober. He joined Axiom Electronics in 2009. Dan holds a B.S. Engineering from Harvey Mudd College, and an M.S. Engineering and MEM Engineering Management from Dartmouth College. 3D x-ray image of column grid array John Kim 6) Flying Probe: The most frequently used advanced Manufacturing Engineer method of testing a PCBA, once programmed it runs John is a specialist in 3D X-Ray inspection. He joined Ax- very quickly. This is similar to the flying probe run at iom Electronics in 2006 and holds a B.S. Electrical Engi- fabrication, but Axiom Electronics now checks compo- neering from Oregon State University. nents for: a. Opens, Axiom Electronics LLC ● 1-800-643-6601 b. Shorts, www.axiomsmt.com c. Polarity issues, [email protected] d. Bad/failed part. This test eliminates costly engineering time that is DFX Design ● 1-503-350-4120 wasted on diagnosing problems in a PCBA. www.dfxdesign.com [email protected] 3 Axiom Electronics Test & Inspection.
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