|Design for Testability Guidelines
- All test targets are preferred to be on one side of the PCB
- Test Target Priorities: 1) Test Pads; 2) Component Leads and; 3) Via's
- Center to center spacing between test points or adjacent drilled locations should be a minimum of .100". Everett Charles is quite capable of .050" and .075" spacing, however, probe durability and contact aren't as reliable as .100". In addition, the tighter spacing increases the cost to manufacture.
- Test points are to be distributed evenly over the surface of the PCB. Areas of high probe density should be avoided.
- The preferred lead length is .062" or less.
- The preferred size of test targets is .035" (or .040" on top-side targets). Square pads are preferred, due to the increased target area (27%) over the round shaped targets. · Solder mask should not cover any test pads or vias.
- A solder bump should exist on test pads that are higher than the solder mask. Ideally, and if possible, solder paste should be applied to test pads at SMT screening to promote even and consistent solder bump formation.
- Test targets should be at least .190" away from the edge of the PCB or "break-away" areas. This will allow the gasketing material to meet evenly with the board surface and create a good vacuum seal.
- PCB's should have a minimum thickness of .062". This helps minimize board flex.
- Tooling hole diameters should preferably be .125". (.090" minimum). The tolerances should be held at +.002/-.001. The locations should be at least .100" from the edge of the PCB and placed in opposite corners.
- Tooling hole to pad/feature tolerance should be targeted at +/- .002".
- All via's should be closed. Excessive open vias and unfilled component holes cause air leaks inhibiting the vacuum seal. ECT can provide a mechanical hold down gate or lid to hold the board to the top plate during vacuum.
- Avoid placing nodes on SMT devices. This can cause damage to the device and provides minimal probe contact area.
- Avoid placing test points on excessively long leads (.090" or more). Additional machining may be required for leads of this length and probe contact is compromised due to deflection.
Copyright (c) 1998-2001 Everett Charles Technologies. All rights reserved. Everett Charles Technologies, 700 E. Harrison Ave., Pomona, CA, 91767, U.S.A.