Reference for PCB test fixture design?

[Spehro Pefhany]

Is there any kind of 'bible' reference for semi-automated test fixture
design or is all this stuff embedded in supplier data and internal
company lore?

This kind of thing: https://i.imgur.com/GiZIVpj.png (pneumatic
actuated bed-of-nails)

Primarily interested in the mechanical part of the design such as
location tolerances and location features on the PCB, and test pad
sizes using the common sizes and end details of pogo pins etc.

I've found a few things llike this:

https://circuitcheck.com/wp-content/uploads/2019/04/Design_Guide_Introduction_to_Automated_Test_Fixtures.pdf

https://www.testfixtures.com/pcb-test-fixtures/

https://resources.altium.com/sites/default/files/uberflip_docs/file_1215.pdf

This is going to be a stand-alone unit.
--
Best regards,
Spehro Pefhany

[TTman]

Very steep learning curve for DIY. Goto a professional supplier and
swallow the cost.All mechanical data will be taken from your PCB gerber
files.

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[Don Y]

Ages ago, a firm I was with would make a test fixture for every
board we designed (quantities were low -- thousands -- so we
could afford to do in-house testing as part of the normal
manufacturing cycle).

We'd purchase a "press" -- a sort of clamshell affair into
which the DUT would be sited (using reference holes in the
PCB). Pylon/pogo pins mounted into the clamshell would make
contact with the circuit at key points for the test procedure
we'd designed. Having the capacity in-house made it a lot
easier (cheaper) to respond to manufacturing problems quickly,
instead of having to ship boards off to be tested, shake-n-bake, etc..

But, this was in the days of multilayer thruhole technology.
The pins were physically large (to be robust enough to handle
the repeated abuses). There may be a finer-grained technology
available, nowadays. But, I would imagine the resulting
fixture would also be considerably less "forgiving" (of the
types of folks who would be slapping boards in and out, all
day long).

Can you, instead, reduce the number of such "test points"
and use a connector (even if unpopulated) to exercise the
circuit? This is the approach I've taken with my recent
designs as there are, otherwise, way too many potentially
"interesting" points to probe via ATE.

[Rick C]

The testing I use is functional using the existing connectors on the board. The test fixture contains enough circuitry to fully exercise the DUT. In the case of the design I'm presently building 10,000 of this means an FPGA to stimulate the digital control and network side digital interface, an RS-422 receiver/driver for the I/O side digital interface and a loop back for the audio interfaces. The RS-422 interface gets tested easily through the FPGA and the audio circuit is tested as part of a loopback audio test at 20, 1000 and 20,000 Hz for frequency response, noise and crosstalk.

A bed of nails or clam shell tester would still require the design of a test circuit unless a very expensive generic test device were used. I think I paid $1,000 each for five test fixture boards 13 years ago before PCBs were so inexpensive and easy to get from Asia (and considered reliable). As it was, Sunstone did a terrible job with many open vias, some of which did not present until after the boards were in use. Today this would probably have cost more like $200 -$300 each assembled.

I can't see a need for a bed of nails or clam shell tester unless the tests have to include measuring intermediate points that simply can't be observed externally with the existing I/Os. Designs should be done to not have such issues, but I can see where that may be inevitable in some cases.

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Rick C.

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[Lasse Langwadt Christensen]

we usually have test point for pogo pins for everything that needs probing, because sticking a pcb in a tester with pogo pins and hitting "test" is faster and less error prone than having to plug an unplug connectors that also wear out

[Phil Hobbs]

The usual method (AFAICT from dealing with test engineers) is to make a
PCB with Mill-Max cage jacks, then a Delrin plate with matching holes to
support the pogo pins, whose tail ends go into the cage jacks so that
the pogos can be replaced easily. The Delrin has counterbored holes to
locate the pogos axially, so there's no sliding wear on the cage jacks,
which therefore last a long time.

There's a spacer round the outside to keep the board at the right Z
position as well.

The whole works goes on a piece of aluminum jig plate with four dowel
pins, and the top is held down with a lid like a waffle iron.

Minimum test pad size is typically 1 mm, and the method works best when
the pads are on the back of a one-side-stuffed board.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

[jla...@highlandsniptechnology.com]

We do pogo fixtures, but we just sort of do it. This one lets us test
a little board before we glob-top it.

https://www.dropbox.com/sh/h2mvej0ss02u4pr/AAD5nYsNsGq0pRhwMZSzQHOca?dl=0

The black block is machined delrin, and the white base was 3d printed.

PCB bare-board testing is mostly flying probes these days, not bed of
nails. Looks like automated test is going flying probes too. That
leaves the top side accessable. Finding component-level defects could
be interesting software.

We lose too many boards to probe slips. I'd like some sort of
pantographic, solid probing system for engineering tests.




--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

[Don Y]

On 7/17/2021 7:34 AM, Lasse Langwadt Christensen wrote:
> we usually have test point for pogo pins for everything that needs probing,

With newer packages (finer pitch, BGA, etc.) you have to deliberately
bring each "point to be probed" out to a spot on the board where you can
locate a genuine testpoint -- and may have to turn the crank on the artwork
to create such a testpoint!

Older throughhole technology made this simpler -- you could revise a test
fixture to site another pin and "find" a convenient place on the *existing*
artwork to grab a signal that would tell you what you wanted.

> because sticking a pcb in a tester with pogo pins and hitting "test" is
> faster and less error prone than having to plug an unplug connectors that
> also wear out

The connector on the *tester* is the item that sees the most wear.
So, you make *it* easily replaceable.

We test disk drives at one of the non-profits with which I'm
affiliated. Back in the days of IDE drives, this meant mating
a ribbon cable to each drive, running the test, then removing the
cable. When you are doing hundreds a week, it's not hard to
end up with a failing connector ON THE TESTER. So, you simply
replace that length of ribbon periodically as a preventative
measure (we've got almost as many of those cables as we do
drives -- as each drive came *with* a cable!)

You can also just create a connector *site* and let the fixture
probe the "pads" of an unpopulated connector.

But, as with deliberate test points, you have to know which signals
are GOING to be of interest when you layout the PCB. And/or
add smarts to the tester to be able to exercise and select signals
of interest at points that are convenient.

[I designed a sensor array that would detect the introduction
(by a lab technician) of blood samples -- 10uL -- into any of
60 "wells". It's easy to test that the design *appears* to
be working. But, verifying that each well will accurately
reflect a 10uL "load" requires some physical assistance
(you can't simulate a 10uL water mass electronically). As
the board would be potted (sterilization) once tested, it's
kinda important to know you're not potting something that needs
to be fixed!]

[Rick C]

Yeah, but you need the room for the test points. There is literally no room for any additional test points although there are a few I put in for debugging.

http://arius.com/photos/Top.png
http://arius.com/photos/Bot.png

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Rick C.

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[Rick C]

Also, don't you need test points for all the I/Os on the board? This test is going to be on a board that is powered and operational, no? If your design has a high I/O pin count it would require all those I/Os to be on probe points. That can be a lot of space on the board.

--

Rick C.

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[Spehro Pefhany]

Quantities are not huge at all, and testing is simple in this case. A
few voltages and currents measured and simulate some input signals.
I want to do some cool things like produce a .pdf test or "conformity"
report for each unit loaded right up onto the server via Wifi and tied
to the serial number. Should be easy with Python on a Rpi.

Right now I have a Tag-connect on the bottom of the board for
programming and those pads are bit close together (pad diameter is
0.787mm (31 mils). They have 3 locating pins to fit unplated holes
right there, so they work okay. https://i.imgur.com/DGxZWUz.png

I'm pretty confident of hitting an old-school 0.1" pitch 0.9mm
hole with a conical probe but not so much half those tolerances.

[Don Y]

I did that with my disk drive tester and laptop tester.
Let the actual device talk to the tester (via enet)
and exercise code loaded from the tester. When done,
tester talks to RDBMS to log results of test. Print
any sort of "receipt" you want...

> Right now I have a Tag-connect on the bottom of the board for
> programming and those pads are bit close together (pad diameter is
> 0.787mm (31 mils). They have 3 locating pins to fit unplated holes
> right there, so they work okay. https://i.imgur.com/DGxZWUz.png

Can you route signals to a "tab" that is later snapped off
the board? If you are only using this for manufacturing
test (and not repair depot), you don't care if the
connector/connection-points remain with the product
thereafter!

> I'm pretty confident of hitting an old-school 0.1" pitch 0.9mm
> hole with a conical probe but not so much half those tolerances.

Yeah, that's the problem I see. If you're trying to make something
*tiny* (my boards are 1.25 x 2.5" -- and components up and down!),
leaving space for a probe gets "expensive".

[Thankfully, my boards "stack" via connections around the edges.
So, as long as all the "interesting" signals go off-board as
part of this process, I can capture them there!]

The "presses" were reasonably well made -- it's not like there was
a lot of slop in the mechanisms. But, tolerances accumulate,
esp when you have "people" operating the mechanism.

[I worked on a 600-pin tester many years ago. Each pin was a ~0.1" pitch
coaxial connector. Put 600 of them together and you've got one helluva
BIG connector assembly (to the DUT). Motor driven screw-drive
mechanism to *pull* the DUT into the connector. No room for error.
But, lots of money to ensure that! :>]

[TTman]

SNIP

>
> Also, don't you need test points for all the I/Os on the board? This test is going to be on a board that is powered and operational, no? If your design has a high I/O pin count it would require all those I/Os to be on probe points. That can be a lot of space on the board.
>

Common practise is to feed the outputs back into inputs either by DFT or
linked pogo pins or connector setups...

[jla...@highlandsniptechnology.com]

We seeded our little mouse-bite board with about a dozen test points,
28 mil OD pads with 20 mil drills. The board just snaps onto a bunch
of conical-point pogos, but we included some guides to help. We push a
board down, hit a foot switch to power it up, look at a scope, and
it's tested.

This is all low impedance stuff, so we don't worry about leakage from
blood.

[Lasse Langwadt Christensen]

there's a huge number of different types of pogo pins, some can work with the pins of through hole connectors

[Lasse Langwadt Christensen]

with the low price of proto pcbs you could probably make it all out of FR4
possibly stacked to make up the height.

also makes in it easy in CAD to use the DUT PCB file as template and
add/remove things to make the fixture

[jla...@highlandsniptechnology.com]

https://www.dropbox.com/s/ljtp8kwqwun7pu2/Test_1.jpg?raw=1

I just slipped the pogos into the delrin block and soldered them to
the PCB.

[Lasse Langwadt Christensen]

they are usually used with sockets so they are easily replaceable
http://dirtypcbs.com/uploads/store/pogo-pin-P100-series.png
http://dirtypcbs.com/uploads/store/pogo-pin-tips.jpg

https://dirtypcbs.com/store/designer/details/ian/12/dirty-pogo-pins

[jla...@highlandsniptechnology.com]

On Sun, 18 Jul 2021 14:40:06 -0700 (PDT), Lasse Langwadt Christensen

It wouldn't be difficult to replace them, but they seem to be
reliable. We're testing maybe 100 boards a month, not thousands a day.

[Rick C]

On Sunday, July 18, 2021 at 12:44:18 PM UTC-4, TTman wrote:
> SNIP
> >
> > Also, don't you need test points for all the I/Os on the board? This test is going to be on a board that is powered and operational, no? If your design has a high I/O pin count it would require all those I/Os to be on probe points. That can be a lot of space on the board.
> >
> Common practise is to feed the outputs back into inputs either by DFT or
> linked pogo pins or connector setups...

Not sure what you are describing. By DFT I assume you mean some sort of mux on the inputs to accept a loopback from outputs. Two problems, one is it requires a balance between the input and output and the other is it adds circuitry to the board or required a connector which is what we are trying to avoid using. Why not just use the durn connectors that are on the board?

I posted links to the top and bottom layers the board I'm currently building. Literally no room for added pogo pads. The only issue with using the designed in connectors is they aren't easy to remove. That reminds me I need to have a tool designed for that.

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Rick C.

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[Spehro Pefhany]

On Sun, 18 Jul 2021 12:19:24 -0700, jla...@highlandsniptechnology.com
wrote:

>
>>I'm pretty confident of hitting an old-school 0.1" pitch 0.9mm
>>hole with a conical probe but not so much half those tolerances.
>
>We seeded our little mouse-bite board with about a dozen test points,
>28 mil OD pads with 20 mil drills. The board just snaps onto a bunch
>of conical-point pogos, but we included some guides to help. We push a
>board down, hit a foot switch to power it up, look at a scope, and
>it's tested.

Sounds pretty close to what I actually need. Maybe add some non-PTH
holes to the board for registration (outline is not clean).

I'm tempted to buy one of the press kits from the usual overseas
sources just for the halibut. About 3kg so the shipping won't be
cheap.

>
>This is all low impedance stuff, so we don't worry about leakage from
>blood.

Ha.. yes they're stabby little b*ggers.

[TTman]

DFT means design for test... sounds like it's too late for that.

[Rick C]

DFT was used as much as possible. If you saw the images, you can see there were severe space limitations. DFT doesn't mean push back on the requirements and tell your customer you want to design something else loosing the job.

This design tests very easily. As I indicated a test fixture is used that the DUT is plugged into. Works the champ.

--

Rick C.

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[Lasse Langwadt Christensen]

mandag den 19. juli 2021 kl. 08.22.37 UTC+2 skrev Spehro Pefhany:
> On Sun, 18 Jul 2021 12:19:24 -0700, jla...@highlandsniptechnology.com
> wrote:
> >
> >>I'm pretty confident of hitting an old-school 0.1" pitch 0.9mm
> >>hole with a conical probe but not so much half those tolerances.
> >
> >We seeded our little mouse-bite board with about a dozen test points,
> >28 mil OD pads with 20 mil drills. The board just snaps onto a bunch
> >of conical-point pogos, but we included some guides to help. We push a
> >board down, hit a foot switch to power it up, look at a scope, and
> >it's tested.
> Sounds pretty close to what I actually need. Maybe add some non-PTH
> holes to the board for registration (outline is not clean).

we use some of these: https://www.mekan.dk/wp-content/uploads/2015/06/Udklipkidfu.jpg

mounting holes on the pcb fits on guide pins and the plastic posts on the clear lid pushes
the PCB down when the top is closed

[jla...@highlandsniptechnology.com]

A quick touch with a Metcal stops the bleeding.

For a small board, maybe use one of the toggle pusher-down things that
machinists use. Foam on that and some alignment posts below, into your
registration holes.

Our test takes seconds, so we just push with a finger. If it fails, we
throw it away.

You could push it down manually and slide in some things from the side
to lock it down. Washers loose on spacers or something, or some fancy
machined thing. That would leave the top accessable for probing.

[Lasse Langwadt Christensen]

https://www.amazon.com/IIVVERR-Prototype-pl%C3%A1stico-tornillo-prototipo/dp/B08LDVBZWL

[jla...@highlandsniptechnology.com]

Cool. I just ordered some to play with.

[Joe Gwinn]

On Mon, 19 Jul 2021 08:19:09 -0700, jla...@highlandsniptechnology.com
wrote:

Toggle Clamp. There are too many kinds.

.<https://www.mcmaster.com/toggle-clamps/>

.<https://www.carrlane.com/product/toggle-clamps>

[Spehro Pefhany]

On Mon, 19 Jul 2021 09:41:16 -0700, jla...@highlandsniptechnology.com
wrote:

Those things are stupid cheap in China- like a dime, but nobody seems
to have a great drawing or 3D model.

I've seen them with a bar screwed to two of the release levers so one
side can be released at once.

Would be interested to know if they're useful