Testpoints for ICT testers

Warren Brayshaw schrieb:

There have been a lot of discussion on  the placement of pads and the

validity of testing regimes but very little on the second part of the

question.

 

As I understand it, most of the points contacted during testing are regular

pads.

To my knowledge, this is not true. Especially in high volume ICT,

testpoints always are separate SMD pads. This makes sure that these

areas are always free for the test needles and have a clean, smooth

surface. When using pads as test points, it is very difficult to get

reproducible contact - and the needles wear out much faster due to

horizontal forces.

So, how do you analyse the layout to determine if nets have  test points or

pads acting as testpoints or nothing suitable?

If there were special ICTP objects (as suggested), it would be no

problem to generate a list of covered and uncovered signals, including

statistics, with a small ULP.

Tilmann

On 12/30/2010 8:08 AM, Olin Lathrop wrote:

Wow, that was a amazing bit of sideways logic. I was reacting to your

rediculous assertion that you always need to give a tester access to all

nets. It only takes a single counter example to disprove that, and I

supplied several such counter examples.

 

Your response is to act as if I said you are always limited to doing things

as in the counter examples, which is completely illogical.

 

Of course there are many methods to test a board. The point is that you

don't always need to have access to every net, not that you should never

have access to any net.

Sorry, been traveling for some time, back in the office now.

If I used sideways logic then you are just as guilty of it.  I gave you

several counter examples where your methodology would not work and

several points on how your way can in some cases cause you to spend

recurring costs on the BOM, yet you choose to ignore those as well.  So,

perhaps we are both guilty of sideways logic.

I have never claimed that I want access to every net on a board in all

cases.  Sometimes I might (particularly if it's a board with no

microcontroller or reprogrammable logic, which I certainly admit is

rather rare).  In fact, in general I want to use a combination of

methodologies to do automated testing of boards.  Ideally, I'd use JTAG,

ICT test points, and yes your method as well where appropriate

(meaning where possible and where it does not incur recurring BOM cost).

  A mix of methods will give me the most thorough and cost effective

means to test a populated board.

In summary I still propose that CadSoft should consider a means to add

test points during layout without adding them explicitly to the

schematic, (much like the way we add vias).  This is supported in many

of the major CAD systems that Eagle competes with and follows the

methodology and division of labor used by many enterprises.

On 12/31/2010 2:01 AM, Tilmann Reh wrote:

Warren Brayshaw schrieb:

 

>> There have been a lot of discussion on  the placement of pads and the

>> validity of testing regimes but very little on the second part of the

>> question.

>>

>> As I understand it, most of the points contacted during testing are regular

>> pads.

 

To my knowledge, this is not true. Especially in high volume ICT,

testpoints always are separate SMD pads. This makes sure that these

areas are always free for the test needles and have a clean, smooth

surface. When using pads as test points, it is very difficult to get

reproducible contact - and the needles wear out much faster due to

horizontal forces.

 

>> So, how do you analyse the layout to determine if nets have  test points or

>> pads acting as testpoints or nothing suitable?

 

If there were special ICTP objects (as suggested), it would be no

problem to generate a list of covered and uncovered signals, including

statistics, with a small ULP.

 

Tilmann

You can use regular SMT pads, but in general you shouldn't use vias (or

at least the contract manufactures I use will not use them).

Tilmann is right, in general you want to use a dedicated test pad that

is separate from both your vias and your SMT pads.  The reasons are as

follows:

1.) CMs have problems with the ICT test probes sticking in the holes of

vias.  In some cases this will result in the board getting hung up on

the ICT test fixture when the fixture tries to open.  In my experience

very few CMs will want to try this for this reason.

2.) Some CMs will let you use SMT pads but they don't like it since they

don't know the height of the solder "bump" precisely so they have some

issues setting the ICT pin height.  They really don't like building a

fixture that has a mix of dedicated ICT pads (which are unsoldered) and

SMT pads with solder (or at least in my experience).  However, if your

board is such that you can get good coverage from just the SMT pads this

is an option.

3.) Complicating point 2 is the fact that in general you want your ICT

fixture probes to only interface with one side of the board (this is a

cost and complexity issue, you can test on both sides if you're

motivated).  If you've got a board with components on one side only,

generally you're going to find that you want the ICT test points on the

side opposite the components.  This also tends to take testing against

your topside SMT pads off the table.  For a design with components on

the top side only, you'll tend to be driven to putting all your ICT pads

on the bottom side.

-Michael

Warren,

Thanks for your suggestion.  It has proven to be the most workable near

term solution for placing ICT test points on schematics.  I built a

schematic symbol much like you suggested.  I used the point type symbol

pin, I assumed that you did the same. Putting it on a separate layer

really makes it workable.  The only (small) fly in the ointment is the

connection dot that Eagle places when you drop the symbol on a net.  As

you pointed out, it's completely invisible if you drop it on existing

connection dots and turn the test point layer off.  I suppose that Eagle

is generating a short (or even null) wire segment and joining it to the

wire you dropped it on and generating the connection dot in the process.

  Do you know of any way to modify this behavior of Eagle while still

getting the net connection to the test point?

Thanks again for the suggestion.

Regards,

-Michael

"Michael Sansom"  wrote

Warren,

 

Thanks for your suggestion.  It has proven to be the most workable near

term solution for placing ICT test points on schematics.  I built a

schematic symbol much like you suggested.  I used the point type symbol

pin, I assumed that you did the same. Putting it on a separate layer

really makes it workable.  The only (small) fly in the ointment is the

connection dot that Eagle places when you drop the symbol on a net.  As

you pointed out, it's completely invisible if you drop it on existing

connection dots and turn the test point layer off.  I suppose that Eagle

is generating a short (or even null) wire segment and joining it to the

wire you dropped it on and generating the connection dot in the process.

  Do you know of any way to modify this behavior of Eagle while still

getting the net connection to the test point?

 

Thanks again for the suggestion.

 

Regards,

 

-Michael

 

Hi Michael

Sounds as if you are doing it the way I suggested and don't have any ideas

to refine it any further. Since my initial response I have determined that

circles around the junction dots work best,in my opinion, (see attached).

You need to get the width of  the circle wire to something you like.

Regards

Warren

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On 1/15/2011 6:20 PM, Warren Brayshaw wrote:

Hi Michael

 

Sounds as if you are doing it the way I suggested and don't have any ideas

to refine it any further. Since my initial response I have determined that

circles around the junction dots work best,in my opinion, (see attached).

You need to get the width of  the circle wire to something you like.

 

Regards

Warren

 

Yes, I built my symbol with a circle as well.  Think I used a 5 mil wire

width (0.127 mm) and it looked fairly decent.

I still say the best solution for this is for CadSoft to create a

testpoint object in the layout tool that, like you suggested, works like

a via without a drill.  Then you could write a ULP to add unplaced

testpoints to the layout that the PCB designer could then place and

route.  Your ULP might optionally load a list of net names that are to

have test points added (there are third party tools that will analyze a

netlist and generate a list of needed testpoints).  If you wanted to get

really sophisticated your ULP could read your netlist and import the

BSDL files for whatever JTAG devices you have on the board and it would

skip placing testpoints for anything you can cover with a JTAG scan

chain.  BSDL is just a subset of VHDL so it can be pretty readily

parsed.  You'd probably also like the ability to attach a "no_test"

attribute on any nets that for instance, carried high speed signals

where a hanging testpoint might act as a stub and generate reflections

so the ULP would skip placing testpoints on those as well.  This is all

sort of blue sky thinking but if Eagle would support such a testpoint

object you could do all sorts of cool things.

Someone mentioned the possibility of having a ULP autoplace the

testpoints and route them.  Some might be interested in that, but

personally I'd be happy with just unplaced testpoints at the bottom of

the layout.  I think a human can do a better job of looking at the trade

offs deciding the best place to drop the testpoint and route it.

Anyway, thanks for your help.  This is now a part of my layout process.

-Michael