Designing For Test – 11 Important Factors You Need To Consider

If you want to achieve the highest test coverage for a circuit assembly then certain elements need to be considered at the design stage. By following these guidelines, you will facilitate test program development, and the test application process will run faster and more smoothly.
In-circuit test considerations for circuit board design
Text version
Design FeatureRecommendationImportance (3=high)
Oscillators controllabilityThe control lines of oscillators should not be hard-wired directly to the voltage rail, ground, their own or other device's outputs. use pull down or pull up resistor [1k ohm or higher] on all oscillators enable pins in order to disable them during ICT process. Provision must be included in the circuit for the tester to separately control all clock signals.
Free-running clocks may cause test instability due to inability to squelch transients.
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Kelvin measurementKelvin measurement is utilized for high precision measurement of low value components (Resistors less then 100 ohms, Inductors less than 100mh, Capacitors less than 100nf). Two separate test points must be provided on each side of the parts that require kelvin measurement. If power up testing is desired, then the loading effect of the extra probes that are required for kelvin measurement may actually interfere with the functionality of the board specially on in-line resistors for high speed digital buses, low value components used in switching power supplies and sensitive analog circuits. One approach that has been used in the industry is to mount one sample of low value components out of circuit on the board to verify that the correct value were loaded in the pick and place machine during assembly. It is up to the designer to suggest provision of 4-wires on specific parts by consideration of board topology and functionality.2
Digital/hybrid device output controlEvery digital/hybrid device should have provisions to be tri-stated. If an output can not be shut-off provisions should be made to place that pin in a state that can be safely back-driven.3
Separate control on bussed devicesAll devices which share a common bus should have controllability over Chip enable or output enable separately.The ICT tester must be able to enable one device at a time one a bus and keep the rest of the devices disabled. 3
On board batteries isolationBatteries should be socketed or isolated from the rest of the circuit through jumpers. Batteries will affect the power-off tests at ICT.2
The ability to control signals The control lines of any device (/CE,/OE, etc) should not be hard-wired directly to the power rail, ground ,their own or other devices outputs. Pull up / Pull down resistors should be used instead.3
High voltage on the boardThe ICT testers are not designed to handle high voltages. If the circuit generates any high voltage more than 100V, it would be desirable to be able to shut down the high voltage during ICT if possible. 3
Unused and NC PinsInputs: Input pins should not be left floating.
Treatment in order of preference:
1. Individually pull high or low as appropriate.
2. Pull high or low in groups of up to 3 pins
3.Tie direct to rail as appropriate.
Add test point to additional nets created. Test points provide access to ensure inputs are in a defined state.
Avoid tying ANY enable/disable pins in groups or direct to rails. The ability to disable all devices improves test stability and avoids the need to back-drive.
Outputs : Allocate test point where possible
NC: Pins with no internal connection do NOT need a test point.
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PCB thickness
PCBs should be no thinner than .062".
A thin PCB requires special Major consideration to ensure the forces exerted by the spring loaded test probes do not cause the PCB to warp, possibly causing solder joints to crack. Special consideration can be given to areas of high probe density by using a hold-down gate and concentrating the push fingers in such a way as to minimize board flex (at an additional cost).
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Accessibility to Reset,hold,halt,single step,wait,interrupt on microprocessor, FPGA or ASICSTester can initialize and control devices with internal state machine.
If the procedures cannot be disabled, the tester does not
have explicit individual control over a device and
thus cannot perform a thorough test.
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Component Heights – CAD component height1) The component geometry height attribute in the CAD file should be the actual mounted physical height of a component.
2) Component Heights that are .060” or taller (on the bottom side of the board), typically require additional support plate milling.
3) Taller components should be placed on the top side of the PCB, opposite of the test point locations. If probing from both sides the taller components should be on the side with the least amount of test point locations.
4) Maximum bottom side component height is dependent on the tester system, and the decision of whether to use a Beef plate. If the component requires a clearance rout that goes through the plate(s), a Cap plate would be required to maintain a vacuum seal.
This Cap plate also requires additional offset around the clearance rout for mounting and sealing. For Agilent 3070 systems, there is an additional 1.0” of “room” beneath the Beef Plate. For non-3070 systems, the amount of clearance is subject to the pan depth.
The diagram below shows two examples where a tall bottom side component can protrude into the Bottom and/or Beef plates.
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