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Tiny Test Points and OSPCan They Coexist?
Agilent Board Test User Group Meeting 2012 – Cleveland, OH May 9, 2012 Presenter and Author: G...
Agilent Board Test User Group Meeting 2012 – Cleveland, OH May 9, 2012 Presenter and Author: Gary A. Manhart Principle Test Development Engineer Benchmark Electronics design develop deliver advanced technology
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Topics Be the best
Define test point size which can be reliably probed using present test fixture designs. Discuss factors which make using smaller test points challenging.
Discuss the impact of Organic Solder Preservative (OSP) finishes on test point contact. Discuss what options are available.
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Test Point Size BEI recommends using a 28 mil bottom-side and 30 mil top-side test point diameters.
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Lower Cost Fixtures Less Complex Fixtures Lighter Fixtures
UUT requires smaller size test points. What is the smallest test point allowed? Before the following board Benchmark would go down to test point sizes of a 24 mil. Guided probe fixture Diamond tooling pins
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Test Point Size Board has 17.7 mil test points.
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Customer said they had a Pneumatic Fixture designed before to test a board a 17.7 mil test points already. Differences with Pneumatic Fixture.
More accurate than a vacuum actuated fixture. More force can be applied to the UUT. Can weigh an excess of 200 pounds. Cost increase can be greater than $10K. Use Aux Connect to control actuation instead of vacuum ports.
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Test Point Size Be the best
Vacuum Fixture
Pneumatic Fixture
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Test Point Size Be the best
Pneumatic fixture hitting 17.7 mil test points had numerous contact issues.
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Test Point Size Contact issue is not caused by the fixture and test point registration, but by the solder mask covering VIAs.
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PCB built in a large panel from the PCB vendor. Each board had a code that told where in the large panel it came from. The boards farthest from the panel y-axis origin (boards in section B and C) showed the greatest misalignment.
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Test Point Size Be the best
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Test Point Size Be the best
PCB Position
Panel Position A
B
C
D
Contact Fail Total per PCB Position
1
5
27
31
10
73
2
1
44
17
1
63
3
0
45
23
0
68
4 Contact Fail Total per Panel Position
0
55
33
1
89
6 171 104 12
B
C
A
D
Contact failures per section of the panel. Contact failures were found on 10% of the build. design develop deliver advanced technology
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Organic Solder Preservative (OSP) Be the best
Organic Solder Preservative (OSP) is a coating that is put on a PCB to prevent corrosion of the circuit board copper layer exposed to the general environment. Coating is often thick enough to cause contact issues on a bed of nails fixture. OSP finishes vary from circuit board fabrication providers.
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Organic Solder Preservative (OSP) Benchmark uses of the following OSP finishes:
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RoHS Process Capable: Entek Plus 106 AX HT by Enthone OMI M-Coat by MacDermid Glicoat-SMD F2LX and F2LXPK by Shikoku Chemicals, Inc.
Non RoHS Process Capable:
Entek Plus 106 AX by Enthone OMI Entek Plus 106 AX HT by Enthone OMI M-Coat by MacDermid Glicoat-SMD F2LX and F2LXPK by Shikoku Chemicals, Inc.
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Organic Solder Preservative (OSP)
Board 1
UUT 14.5” X 23” with ~5600 probes
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This board did not use an OSP finish Vacuum Box Bottom side 28 mil square test pads 3 Sided Chisel probes Fixture worked very well and only required typical PM activity
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Organic Solder Preservative (OSP)
Board 2
Board was redesigned.
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Board size the same Probe count increased to ~6500
No issues with the previous fixture, a similar fixture was designed. Immediately during debug contact issues were found. Differences Probe count increased to ~6500 OSP board finish 28 mil square pads contained a VIA
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Organic Solder Preservative (OSP)
Board 2
Test probe changed from a 3 Sided Chisel to a Razor.
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Improved, but did not eliminate contact issues
Solder stencil was created. High failure rate of solder bridges Reduced solder volume on VIAs, contact issues returned Flux residue was found in the VIAs Water washed boards to remove the residue
Occasional contact failures were fixed by cycling the fixture test fixture multiple times.
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Organic Solder Preservative (OSP)
Board 3 Be the best
Created another version of this board. Board size continued to be the same Probe count dropped to ~4800 VIAs and board finishes were the same
Desire to remove water wash. Plug the VIAs. Adds $60 to PCB cost Change board finish. Increases cost Increasing the spacing between VIAs Not sure if pneumatic fixture would alleviate the need for the water wash Attempts to reduce the residue issue and allow better contact to the printed circuit board
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Organic Solder Preservative (OSP)
Board 3 Be the best
Vendor #1
Vendor #1
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Vendor #2
Vendor #2
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Combination of OSP and Small Test Points If your board requires small test points and/or OSP finishes a pneumatic fixture can be developed, but consider the following:
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What is the size of the board to be tested? Is the PCB fabricated in a panel? Will the board expand and contract during manufacturing? What is the spacing from test points and other board features?
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Combination of OSP and Small Test Points Be the best
Are test points VIAs? What is the center to center spacing for spring probes? What size test point is a fixture vendor comfortable hitting? Fixture vendor should be consulted.
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Summary Be the best
Still recommend 28 mil bottom-side and 30 mil top-side test point sizes. Test points 24 mil and smaller will be more costly than standard. For products with test points less than 24 mil, review the tolerances and stack ups for the PCB.
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Summary Be the best
Pneumatic fixtures are very accurate and capable of hitting 24 mil and smaller test points. Razor probes work better than standard probes at probing OSP. The risk to contact issues can be minimized further by applying solder to test points. If test points have VIAs enough solder must be applied to create a dome so flux residue buildup doesn’t cause a problem.
