Neil,
I agree, don't try to find a reason to use SPC. Use what works best for
you and the application. During Customer audits we have been asked
where our SPC data is and why don't we use it. SPC works well on larger
volumes, but when the volumes are small, 5 to 10 pieces, it is harder to
make SPC or worst yet 6 Sigma work.
Phil Nutting
-----Original Message-----
From: Neil Atkinson [mailto:[log in to unmask]]
Sent: Friday, October 05, 2001 11:06 AM
To: [log in to unmask]
Subject: Re: [TN] SPC @ WAVE SOLDER
I agree with everything Adrian says and I would never suggest that
processes
can be controlled by measuring the output. What I would say though is
that
SPC is not always the best way of controlling the input variables.
If something is critical then SPC is the last resort - the best way is
mistake proof e.g. wave height is an input variable of the whole wave
soldering process but is an output of the pump speed, solder volume etc
all
of which should be controlled using a closed loop system.
There are many ways to control a process which actually prevent problems
occurring much better than SPC and I would say mistake proof as much as
possible and if you can't mistake proof it then look at SPC.
I would also add that this question has been asked many times on the
TechNet
and seems to cause people many problems. If it is so difficult to find
a
good use for variable charts in PCB assembly, why do we keep trying.
Surely
SPC is a tool to be used when it is useful not a tool which we must find
a
use for at all costs...
Attribute charts are very useful in assembly because so much of what we
are
recording is counting not measuring e.g. solder joints, component
placements
etc. They are not a poor relation to variable charts, just another tool
with a different use. They are statistically sound and for some
purposes
can be very useful. e.g. comparing two different fluxes, two different
types
of process etc. this cannot be achieved with variable charts.
Regards,
Neil
-----Original Message-----
From: Adrian Irwin [mailto:[log in to unmask]]
Sent: 05 October 2001 10:16
Subject: Re: SPC @ WAVE SOLDER
I believe that the comments made by Neil are good ones and attribute
charts
definitely give some good feedback, however the use of attribute charts
to
measure defects is not a control chart as such, as the chart measures
the
output of the process and not the input variables, which ultimately
decides
the output result. Yes it is difficult to establish what the critical
input
variables are, but they can be identified depending on the type of
defects
you are getting.
For example, if you are using a double wave and soldering SMT devices,
you
may be experiencing dry joints on SOT23's. Experience and controlled
experimentation as proven that the pressure and height of the chip wave
are
critical factors for this defect type, therefore you could establish an
SPC
chart on these 2 factors. If you are experiencing solder shorts on
SOIC's
then it is possible to measure the time in the second wave as a key
factor.
However, the process window can be increased dramatically by ensuring
the
PCB design is correct, e.g pad size, rotation, minimum spacings, minimal
shadowing etc..
So an attribute chart is good at quantifying the process capability (the
process is not just wavesoldering, it includes design, component finish
variation, component package variation..... ) but may not assist in
controlling the input variables.
Hope this makes sense.
Neil Atkinson
<Neil.Atkinson@STAD To: [log in to unmask]
IUM.CO.UK> cc:
Sent by: TechNet Subject: Re: [TN] SPC
@
WAVE SOLDER
<[log in to unmask]>
10/05/01 09:15 AM
Please respond to
"TechNet E-Mail
Forum."
Scott,
Wave solder is always the process people turn to in when they want to
'implement SPC'. I would always answer your question with a question -
"What do you want to control?".
As you know there are many parameters which can affect the quality
leaving
a
wave soldering machine but most of them should be controlled by the
machine
e.g. temperature, speed etc and provided the machine is checked for
calibration these would not benefit from the use of SPC.
Once a process is set up correctly, the biggest influence on the quality
of
the output are the parameters which are difficult to measure (at least
using
SPC on line) e.g. board design, solderability etc. for this reason I
would
advise the use of attribute charts to measure the quality of the output.
I
know many quality people think this is not "proper SPC" but it is and
can
be
used to make improvements. I would use 'p' charts to measure defects
per
opportunities or np charts if you are prepared to have a separate chart
for
each board.
This type of chart also fits in well with the draft document IPC 9261
and
maybe one day we'll be able to benchmark each other against this
standard
(if anyone will release their data!!)
This type of chart will then give you a handle on how well the process
is
operating over time and what type of variability you see. The hard bit
is
then improving it! The chart obviously doesn't do this for you but it
does
help you decide whether you have REALLY improved the process. In making
improvements I often find it useful to have a number of copies of the
board
layout and mark them up as measles charts using different colours for
different faults. This simple technique is really useful at identifying
trends due to design etc. And the more information you record the more
useful they become e.g. pallet number, date, time, shift etc.
My final question would be why are you implementing SPC. If it is to
meet
a
quality requirement to use 'statistical methods' then I would suggest
that
you meet this by using a broad range of statistical tools to improve
your
process and not just go down the old 'we must implement SPC' route. For
example if you want to trial a different process, flux etc. use
statistical
methods (e.g. Chi Squared tests using contingency tables) to compare
results
and prove any change / improvement.
I would always advocate the use of statistics to prove a hypothesis but
only
use SPC when you need it, don't go looking for a use for it. If you are
already measuring data on a standard chart etc it may be worth looking
to
change that to an SPC chart and start working with limits etc.
Good luck,
Neil
-----Original Message-----
From: Rougeux, Scott [mailto:[log in to unmask]]
Sent: 04 October 2001 15:02
Subject: SPC @ WAVE SOLDER
In regards to SPC at wavesolder. I would just like to here from some of
the
people in this forum on what type of data they are collecting or
sampling.
Any info is beneficial. Thanks.
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