Mindready
(NI)'s Guidelines for Testing PCB's
INTRODUCTION
The following are guidelines to assist companies
who are laying out PCBs which they wish to test using bed of nails, both
in-circuit and functionally. They are of a general nature, but Mindready (NI)
will be happy to assist with specific problems as well.
Please use the links below to find a specific
section of the page which is of interest to you, or alternatively use the
scroll bar at the side.
1. NODE COUNT - ICT Vrs
FUNCTIONAL
2. WHERE PROBES CAN BE USED
3. PROBE SPACING & PITCH GUIDELINES
4.
PROBE STROKE
5. SUITABLE PAD SIZES
6.
VIAS - PRACTICALITIES OF PROBING
7. PROBE TIP STYLES AND
RECOMMENDATIONS
8. DELIVERABLES NEEDED TO MANUFACTURE
FIXTURES
9. FURTHER INFORMATION
Node Count - ICT Vrs
Functional
It used to be that laying out a PCB which could
be tested automatically was simple. All the components were through the board
and a simple drill file or artwork was all that was needed to manufacture the
fixture. It was assumed that a probe could be required for each node on the PCB
and this was easy to provide since all junctions easily accessible.
Now with surface mount technology (SMT),
everything is much more complex. It is no longer the case that all junctions
are easily available, in fact many never reach the bottom of the PCB, where
conventional probing takes place, and those that do may not be in the right
shape for a probe to make contact.
In-circuit testing has been the front line
method of test for PCBs, especially in sub contract manufacture. It ideally
requires a probe to make contact with each electrical junction (node) on the
PCB. There are various techniques to allow in-circuit testing to take place
with less than 100% access, however these significantly add to the cost of the
test system. So in-circuit testing and SMT to some extent conflict with each
other.
In contrast, functional testing requires much
fewer, typically 10-15%, nodes to be accessed and is therefore much simpler in
regards to access. Functional testing is often slower and can require less
general purpose test systems.
WHERE CAN PROBES BE
USED
A probe can access almost any through hole
component, but must never be used to access an SMT part directly ie at its
pads. Sometimes this may appear tempting if there appears to be a lot of spare
land at the pad, but in practice this is dangerous due to the variation in
placement.
Although it is common to talk of ‘shorts
and opens’ testing as the first part of an in-circuit test, in fact the
‘opens’ part refers to checking for where there should be a short eg
a fuse or a wire link. It is very rare to have a probe at each end of a copper
track in order to check it for an open, the assumption is made that the bare
PCB is supplied pre-tested.
One of the best answers to SMT is to introduce
test points. These are extra tracks and lands specifically added to bring an
inaccessible node out and provide access to it. Ideally a well designed for
manufacture product will have at least some test points.
However they can introduce EMC problems and
indeed can affect the operation of some sensitive circuitry. But worst of all
they have to be designed in from an early stage and this simply does not always
happen.
Another answer is to probe on the via holes.
With care this can work, but certain rules must be followed, especially
concerning leaving the vias uncovered and with the currect solder mask to
provide sufficient copper on which to probe, see later.
One answer to access problems may be to use
double sided fixturing. At first this seems appealing as it provides many more
opportunities to access hard to get at nodes. However double sided probing is
expensive and significantly harder to make reliable. Most of the problems come
from the difficulty of accurately lining up top sided probes with the PCB, on
the bottom side they are often lined up by reference or mounting holes. Double
sided fixturing can be used, but it should be treated with caution.
Another factor which can affect fixturing is
high components, on either side of the PCB. These at best can mean having to
mill out parts of the fixture plates, at worst they can make accessing some
nodes impossible. This is one of the reasons that fixture manufacturers like
Mindready (NI) insist on having a built example of a board before they begin to
make a fixture.
PROBE SPACING &
PITCH GUIDELINES
The industry standard probe has always been on a
0.1 inch pitch. This has evolved into a low cost, reliable item which is
available in a wide range of types. If the layout of the PCB permits then
ideally all probing should be done with probes no closer than 0.1 inch.
There is also an obvious need for various
different shapes on the end of the probes to cope with matching with the
different parts of a PCB e.g. components or flat copper pads. More information
is provide later about this.
For combined in-circuit and functional testing,
dual height fixturing is often used. This means that the in-circuit probes can
be normal probes, but the longer functional probes must have a much longer
stroke.
For any probe to work properly it must have
sufficient pressure applied so that it makes good contact with the PCB.
As the size of probe spacing is reduced, the
variety of probe styles which are available also reduces.
Probe pitch guidelines
|
Probe Pitch |
Standard/Long stoke
|
Notes
|
|
0.1 inch |
Standard and long |
Reliable, cheap, robust, many
types/styles |
|
0.075 inch |
Standard only |
Less tip styles available
|
|
0.050 inch |
Standard only |
Even less tip styles, less
robust |
PROBE STROKE
A common 0.1 inch probe will have a stroke of
6mm ie it is designed to compress by this length. The pressure exerted on a
probe in use will typically compress it to 2/3 of its full stroke.
Commonly a PCB will have a mixture of probing on
through hole components and on plain pads ie there will be variations in the
effective height the probes are called to meet. This, and other variations,
will mean that some probes have to compress more than others.
SUITABLE PAD SIZES
The size of the land that a probe must make
contact with obviously depends on the probe size and style. The following are
some guidelines used in Mindready (NI) :
Pad size guidelines
|
Pad Diameter |
Preference |
Notes |
|
0.060 inch |
Preferred minimum size |
|
|
0.050 inch |
OK |
Below this problems start
|
|
0.040 inch |
Just OK |
Take care if centre of land is
not solid |
VIAS - PRACTICALITIES OF
PROBING
Vias are a particular problem as they tend to be
small (0.040 inch typically) and with a hole in the middle. This might mean a
doughnut with a 40 thou diameter and a 24 thou centre hole which means the
strip of land left is 0.008 inch wide! If the centre is not filled ie unmasked
and allowed to fill with solder, then there will be probing problems.
The other vital constraint on vias is solder
masking. In order to probe these they must be reliably unmasked. This
means proper, planned masking and not relying on the PCB manufacturer or his
process to happen to leave them clear. The masking must be larger than the
actual via.
PROBE TIP STYLES AND
RECOMMENDATIONS
When a supplier like Mindready (NI) provides a
fixture, they will chose probe tip styles which appear to offer the best
connection. However there is no substitute for real life experience with the
actual PCB and you should be prepared to change some of these styles as you
pass significant PCBs through the fixture.
The following are the probe rules we tend to
follow, but remember they are only a starting point.
Probe tip style rules
|
Situation |
Tip style |
Notes |
|
Through hole components, bent and
soldered |
Tulip or crown ie 4 headed
|
Good self cleaning
properties |
|
IC legs, not bent |
Serrated crown |
Not so good cleaning, wider versions
too |
|
Flat test land or filled
hole |
Sharp single pointed probe
|
|
|
Hole in PCB, not via |
Chisel head |
Self centring |
DELIVERABLES NEEDED TO
MANUFACTURE FIXTURES
In order to provide a PCB test application
(program and fixture) we require certain items, the deliverables, which must be
of the latest revision. Incorrect or out of date deliverables will cause us
extra debug time and possibly extra time on your site. The following list is
designed to clarify what we need and why:
Gerber files. This means top and bottom
layer files, pad files, aperture or match file, solder mask and silk screen or
component identification. Sometimes we are sent a composite Gerber file, ie all
of these in one file. This can not be simply used to manufacture a fixture.
Drill file. The X, Y coordinates used by
the PCB manufacturer to drill the PCB. This provides the co-ordinates for our
drilling.
CAD schematics. These can usually be
imported into the fixture manufacturer’s own CAD and it makes component
identification much quicker.
All PCB and document revision numbers. It
is vital that to know that the correct information is being used.
Blank PCB. Sometimes it is necessary to
electrically trace a particular track and the blank PCB allows this to be
done.
Built PCB. This is used for in
manufacturing the fixture, mostly to check for component heights. It does not
need to be a working board.
Agreed number of tested PCBs. These must
be manufactured using the same process as the real PCBs ie hand soldered
samples are not acceptable where flow solder will be used. These are used both
to choose the correct probe types and to test the program. Thus if the
manufacture is different wrong probes may be used.
Fresh copy of the schematics. Often ones
supplied for quotation purposes are initial versions.
Test specification. Used for functional
testing.
FURTHER INFORMATION
If you have any further questions, Mindready
(NI) can be contacted on :
Phone - 028 9335 7300
Fax - 028 9335 7305
E-mail -
info@mindready.com
