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.
#####################################################################################
Attention:
This message is for the named person's use only. It may contain
confidential,
proprietary or legally privileged information. No confidentiality or
privilege
is waived or lost by any mistransmission. If you receive this message in
error,
please immediately delete it and all copies of it from your system, destroy
any
hard copies of it securely and notify the sender. You must not, directly
or
indirectly, use, disclose, distribute, print, or copy any part of this
message
if you are not the intended recipient. STADIUM GROUP PLC and any of its
subsidiaries
each reserve the right to monitor all e-mail communications through its
networks.
Any views expressed in this message are those of the individual sender,
except where
the message states otherwise and the sender is authorised to state them to
be the
views of Stadium Group Plc. or one of its subsidiaries.
Although this message has been scanned by MailMarshall for known viruses
and
inappropriate content, we recommend that recipients employ appropriate
measures on
their systems to intercept any such material.
Thank You - Stadium Group Plc., IT Department - Tel: +44 (0)1429 266544
#####################################################################################
---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send the following message: SET
Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases >
E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for
additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700
ext.5315
---------------------------------------------------------------------------------
_____________________________________________________________________
This message has been checked for all known viruses by Star Internet
delivered through the MessageLabs Virus Scanning Service. For further
information visit http://www.star.net.uk/stats.asp or alternatively call
Star Internet for details on the Virus Scanning Service.
---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send the following message: SET Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------
|
