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How to reserve a test pad on a PCB?
發表時間:2009-06-22 此文章有30283人閱覽

 How to reserve a test pad while you layout new PCB.

              This paper is prepared by charles chan ADSYS Co. LTD. 2009/08/14 All rightreserve
Introduction:
Design a testable electronic products is major mission or challenge for smart design engineer. How to reserve a test PAD on each net of a PCB for ICT test? We try to develop rules for your reference as shown below: 
Rule 1: Single side fixture is better than double side fixture, due to the cost considerations.
Rule 2: The number of TEST PAD on top side will be less than the test pad on bottom side.
Rule 3: The priority to reserve TEST PAD will be reserved a TEST PAD firstly, then COMPONENT LEAD PAD, the last are non mask Via holes.
Rules 4: The pitch between 2 TEST PAD will not be less then 1.27mm( 50mils). Longer than 2.54mm(100mils)is prefer,  acceptable pitch is 1.905mm(75mils).
Rule 5: The distance between a TEST PAD and a component that the height is less than 3mm which is acceptable distance is 2.54mm. If the height of this component is higher than 3mm, the minimum space between TEST PAD and component needs 3.05mm.
Rules 6: Test pad will be uniform distribution on a PCB is prefer.
Rule 7: The diameter of a TEST PAD will not be less than 0.7mm(28mils), if the TEST PAD will be on top side of the fixture, the diameter will be bigger than 1.0mm. Round pad is prefer, the square pad also acceptable.
Rule 8: a un-use pin on a IC component may be needs a test pad.
Rule 9:Guide hole specifications, at least 2 guide holes with at least 3mm diameter on the longest diagonal of a PCB.
Rule 10: The distance between a screw hole and a test pad at least reserved 6 mm.
Rule 11: Each NET must reserve a test PAD on it.
Rule 12:  The distance of a TEST PAD to the edge will be at least 6 mm.
Rules13:The distance between a SMD CHIP bigger than 1206 to the edge will longer than 100mils.
Rule 14:The distance between a SMD CHIP smaller than 1206 to the edge will longer than 60mils.
Rule 15: The distance between a SOIC package IC, horizon at least 50mils,vertical at least 35mils as shown below:

            


                                           50mil                                    35mil
Rule 16:The thickness of a PCB smaller than 0.62”(1.35mm) will have curving problem. It needs a special treatment.
Rule 17: Avoid to reserve a TEST PAD on SMD component soldering PAD, it not only un- stable contact situation but also damage this components on PCB.
Rule 18: Avoid to use the lead of a component which is longer than 0.17";4.3mm or the hole bigger than 1.5mm as TEST PAD.
Rule 19:  The deviation between DEVICE UNDER TEST PADS and TEST PADS must smaller than 0.05mm.
Rule 20: The height of component in the contact probes side will be less than 6.5mm.
Rule 21: Do not leave a drilling hole on a TEST PAD. i.e. no Via hole on TESTPAD is needed。
Rule22: Settle a TEST PAD in each NET, avoid to use Via hole as a TEST PAD.
Rule 23: Un-use pins of IC & CONNECTOR will be leave a TEST PAD on it.
Rule 24:TEST PAD will not be in the body of a component. It is useless to reserve TEST PAD.
Rule 25: To avoid moving TEST PAD when layout a new version PCB, or you have to build a new fixture. It costs a lot of money.
Rule 26: GUIDE PIN diameter will be 2.8mm or 3.0mm∮.

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