Doug: I have found very little in written spec's covering test related issues
from any of the major organizations (IPC, etc.). Doesn't mean they're not out
there, I just haven't found them yet either. You can see that what little there
is out there is describing the pad-to-pad dimensioning without concern to
components covering test pads or body-to-pad spacing.
What I have found is that the ATE industry is abound with people who have
written their own Design for Testability Guidelines (probably in self defense,
as I have). This includes most of the turnkey programming and fixturing houses.
If you deal with one of them for your ICT development, try giving them a call.
As for probing accuracy, talk to the major pogo pin manufacturers. They have
lots of info on this subject. Some, like IDI and QA, have nice write-ups on
probe accuracy in their full catalogs or as hand-outs and have lots of sales
people who will be happy to talk your ear off on the subject. Likewise
fixturing houses should have data on their fixture kit accuracy (Everet Charles,
Quality One, TTI, etc.) and should be able to describe the overall tolerence
build-up.
Current "standard" recommendations state a .050 probe hitting a .035 pad.
(I believe the SMTA has this in writting somewhere...) Obviously, there are
smaller probes available (though you have to use them manually as most ICT
generation software will not take them into account) and today's fixture
accuracy will typically allow hitting smaller pads (down to .025 with some
consistancy). Smaller probes can be placed closer to your components, hence
less air-gap.
I have found the major issues affecting component body to pad location to be:
1. The mechanics: top side vs. bottom side probing (more slop required for top
side), and are you next to a tall part (ie: does the fixture have to be relieved
to allow fit?)
2. How well are your component body dimensions described in your CAD system?
I've typically had to work with my CAD folks to correct body size dimensioning
issues.
I'm not a CAD guy, but I seem to remember a design rule check for pad to
component outline?
- -Anyway, the gist of this is that you will probably have to do your own DFT
guidelines. Get your Test Engineer, CAD Engineer, Design Engineer and fixturing
house together and hash it out.
-- ORIGINAL MESSAGE ----
The following questions relate to ICT (In Circuit Test) of assembled rigid PWB's
using conventional .050", .075" or .100" probes used in test fixtures. (Flying
probes are not of interest due to volume of units built).
1) Is there an industry specification that address the air gap required from the
edge of ICT test pads to SMT component body's and/or component pads?
My dilemma: IPC-SM-782 section 5.7.2 states: "Test lands should be .025 minimum
from mounting land areas." This does not specify the spacing from the component
body nor does it seem sufficient to the land areas. Our vendors use IPC-A-610 to
inspect assembled boards. If the assembly is class III and a 1206 resistor is on
the board IPC-SM-782 section 5.7.2 would seem to allow a test pad to be .010"
under the part. Obviously, this is not what we want.
2) Is there another industry specification such as IPC, Bellcore, etc. that
addresses this issue? If so, please indicate the specification #. If you have an
internal specification that addresses this issue I would be grateful if you
could share what your parameters are.
3) Is there a specification that defines the probe pin accuracy or location
tolerance of .050", .075" and .100" probes used in a "standard" ICT test
fixture?
I've been reading IPC's tech-net and design-net mail for years and there is a
wealth of knowledge in this mailing list.
Thanks for any help you can provide. Doug
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