[sdiy] Importance of electrical test at PCB manufacturing?

[Dave Manley]

Colin f wrote:
>  
>   
>> One thing I have done in the past, when a proper test point 
>> is not available, is to pull up the design files and locate a 
>> via which will give me access to the point in circuit. With 
>> more complex boards there is almost always a via to tap in at...
>>     
>
> Was just about to say the same thing...
> I would add putting a silkscreen label next to vias that carry potentially
> useful troubleshooting signals will only cost you a few minutes when laying
> out a board, and could make life much easier at repair.
Also, make sure you do not have soldermask over your test vias.  It also 
helps to set your via size large enough to solder a fine wire into.  
Then you can clip a scope probe to the wire.  It also makes rework 
easy.  If your board is large also put tenth inch posts connected to 
ground spread around your board so you always have a place to clip your 
scope probe's ground lead.  If you are running at very high frequencies, 
you may want a ground pad next to any high speed signal you want to 
probe, so you can minimize the length of the probe's ground lead.  Probe 
manufacturers often make special ground clips that fit directly to the 
tip of the probe, or you can get creative with a paper clip bent into 
the proper shape.

In the 'old' days in my telecom equipment work we were required to put 
dedicated test points, a square pad 5-20 mils on a side, on every net 
for automated board test access - this was for 'bed of nails' testing.  
The PCB layout guys did not always appreciate having to place all these 
pads.   The boards were tested in a clam-shell that could simultaneously 
probe the top and bottom of the board.  Later, as the density of the 
boards became greater and there was not always room for dedicated test 
points, the test technology advanced to where they could directly probe 
vias.  The PCB guys liked this option.  :-)  The test vias were a little 
larger with no solder mask.  Part of the board release process required 
testability analysis that guaranteed there was complete test access to 
every node.  There are innovative test techniques for BGA devices, one 
which puts a metal plate over the device and drives it with a high 
frequency signal that gets capacitively coupled through the device, 
through the balls, hopefully through solder, and then to the trace, 
where the signal can be detected.

Dave (only been doing this as an engineer or manager since 1982, so what 
do I know?) Manley