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34 The PCB Magazine • April 2015 tackled, especially for volume production, with this or similar technology. HDI Fixtures vs. Substrate Fixtures Quad and octal density universal equipment is used for standard HDI fixturing. Pin sizes for this type of fixturing are usually 90–110 µ with a length of 20–30 mm, therefore much smaller and shorter than standard universal grid fix- tures. To put this in to perspective the human hair has an average diameter of about 70 µ (mi- cron.) Pins in a substrate fixture of current gen- eration typically are 70–50 µ; the next genera- tion under development will reach 40–25 µ. Substrate fixtures are typically not used with universal test machines. They require a much higher grid density; therefore the grid itself is generated as a wired solution either to standard connectors or to a universal one-touch interface depending on the final machine used. HDI fixture usually test a complete test ar- ray, whereas substrate fixtures usually test only one or two images of the entire array and the machine will step the board through in mul- tiple tests. That is mainly done to achieve the high accuracy required to test those kinds of boards and the extreme cost and time involved to make these fixture. SPLITTING HAIRS: THE MANUFACTURE OF HDI AND SUBSTRATE TEST FIxTURES continues HDI fixture typically can be produced with- in 48 hours, the manufacturing time for a sub- strate fixture is usually 100–200 hours, and ex- treme cases can even take longer. The manufacturing process for the substrate fixture is much more complex than the stan- dard universal fixture. Accuracy is absolutely critical for the success of registering the product to the fixture. There are basically six steps in the manufac- turing process whether it is a hybrid design or a tension probe design: 1. DFM—Engineering 2. Drill 3. Electrode level—wiring, electroplating 4. Probe level—adding probes (wires) 5. Final assembly 6. Final QC and test setup Engineering In this step the test engineer utilizes CAD/ CAM tools to design the fixture. This includes the intelligence for the test machine, the wire mapping for the fixture and the programs for the CNC drill machines. Drill The next step involves the drilling and ma- chining of material to be used. The material used can be FR-4 for HDI fixture but is typically material without glass fiber enforcement for the substrate type. Accuracy is crucial in this phase. The plates are drilled and the holes are then optically measured for positional accuracy and size. Figure 2 illustrates the process. Electrode Level—Wiring and Plating In this process the actual wiring takes place. Using the wire maps the cable assembly is pro- duced which will interface to the test machine on one side and the test pins on the other. This process is very delicate and requires a high skill level. Electroplating is used to combat oxida- tion on the critical test area so that functional- ity is not degraded over time. Probe Level—Adding Probes/Pins In this stage the probes are added to the fix- ture. Due to the small size of the actual probes/ FeAture Figure1: Substrate fixture.