On 02:01 PM 29/01/2005, Dennis Saputelli said:
saw something on the DXP list that got me thinking ...
how does the flying probe test work if vias are tented ?
anybody know ?
vias not probed ?
test before solder mask ?
the writer over there talked about a slight reduction in the mask rather than a full cover for this reason
Dennis Saputelli
In my experience flying probe testing can be done from component pin to pin. This works fine with gull wing devices, most of the 0805, 0603 etc packages and leaded parts - there would be little need to expose vias - they can be tented to allow a greater packing density without the risk of solder shorting during assembly. However there is a problem with QFN, BGA packages and the like. I guess the best you can do then is probe on a via close to the pin if necessary.
I guess in an increasing number of designs signals will go from an inaccessible pin to another inaccessible pin (or pins) - many nets may not have any accessible pin anywhere in the design. In this case you need to expose enough of a via to permit test access.
I guess in some cases there will be nets with no external access at all. If full node testing is required for post assembly in-circuit test then special test point vias will have to be placed. (Then we get into the whole issue of signal integrity vs test access. What about a really high speed serial link between two BGA devices, blind vias take the signal to and from an internal layer and only tracking is on this internal layer. To provide test access may degrade system performance.)
Brad V made a comment about not probing vias if possible. I have had the same advice a number of times. I generally try to place a separate test point surface pad on each net. However, it is not always practical to place a dedicated test point surface pad due to density. I thought some of the probe heads were supposed to be gentler on via barrel connections than others.
Much the same applies to bed of nails testing I s'pose - with the added constraint of ensuring appropriate probe clearance. I am not sure of the relative precision between flying probe placement and bed of nails probing - this would affect the amount of a via that should be exposed.
Ian
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