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'[EE]: Solder bridging on low volume PCBs acceptabl'
2001\05\18@043929 by Scott Newell

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What percentage of fine pitch surface mount boards should be expected to
return from the assembly house with solder bridges?

At my day job, we've taken a couple of new designs into low-volume (100
boards at a whack) production.  This is the first use of surface mount
technology at this company.

The current design I'm working on has three fine pitch parts: a TQFP-144, a
48 pin TSOP-II, and a 28 pin SSOP.  Eight of the 100 (supposedly machine
built) boards arrived today, and four had visible solder bridges, some
covering more than 2 pins.  All bridges were confirmed to be hard shorts
with a meter.

Is it considered acceptable industry practice to make the buyer do a
detailed physical examination?  These designs don't have provision for
boundary scan or self test, but all I needed was a unaided visual scan and
a meter to find all the shorts.  I can easily fix the boards with a little
solder wick, but it seems like I shouldn't have to.  (Considering the time
constaints and backorder status, we'll probably have to do our own repair
work in the short term, but I'm getting to where I just want to reject the
shipments outright.)

Comments?


thanks,
newell

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2001\05\18@054047 by Kevin Blain

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Those packages you mention are very easy to bridge, unsurprisingly, it comes
from using to much solder (either too big an aperture or to thick a solder
mask)

All of our Surface mount assembly is carried out in house here, and we don't
tolerate solder bridges in manufacture at all, therefore our assembly line
has some very good inspection people, which mean that a surface mount build
fault of any kind is very rare.

So to me, 50%  (or even 4%) of the boards having errors on seems outragous,
but maybe I am used to expecting too much.

Did you (your company) provide the solder mask? If so, this is probably the
cause of the problem, and smaller apertures will lead to improved results -
a couple of thou here and there makes a big difference!!

That said, I would expect any manufacturing facility, in house or external
to take some pride in their work, and a solder bridge between a couple pins
from time to time sometimes slip through the net, but between more than two
pins??????

Regards, Kevin



{Original Message removed}

2001\05\18@055955 by Michael Rigby-Jones

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{Quote hidden}

Generaly unacceptable.  If you are getting your boards stuffed externaly,
then part of the contract should be full visual inpsection.  Without any
electronic board testing, you might get the odd one that slips through the
net with a couple of pins shorted, but it sounds like your assemblers visual
inpsection is sadly lacking.

Mike

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2001\05\18@090344 by Douglas Wood

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If your vendor is capable of producing boards with that precision, then for
a small run you should accept nothing less then 100% working boards. If you
accept something like 95%, out of a run of 100 you'll have 5 bad boards.
That is a lot of bad boards for such a short run, IMHO. My experience with
surface mount stuff is that if the vendor isn't setup to work with
fine-pitch parts, no amount of testing will fix the drop-out rate.

Douglas Wood
Software Engineer
dbwoodspamKILLspamkc.rr.com

Home of the EPICIS Development System for the PIC and SX
http://epicis.piclist.com

{Original Message removed}

2001\05\18@090356 by Orbit Communications

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Michael Rigby-Jones wrote:

{Quote hidden}

Your PCB layout also effects yield from production. Were the boards reflow
soldered ?

If you have a good report with your contract assembler/PCB manufacturer, they
will normally advise if alterations are required to PCB layout for reliable
manufacturer.

Eg. When you are using SIOC and wave soldering, you would normally orientate all
the PCBs to suit the travel direction through the machine. You would also use
"solder robbers" to prevent build up on last pads through wave etc etc.

From my experience in larger volumes, 2.5% would seem to me maximum failure
sample.


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2001\05\18@094819 by Wynn Rostek

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I expect well under 1 percent.

Wynn Rostek

----- Original Message -----
From: "Scott Newell" <newellspamspam_OUTCEI.NET>
To: <@spam@PICLISTKILLspamspamMITVMA.MIT.EDU>
Sent: Friday, May 18, 2001 1:14 AM
Subject: [EE]: Solder bridging on low volume PCBs acceptable?


{Quote hidden}

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2001\05\18@095449 by t F. Touchton

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Your boards should be delivered functional, which means no bridges.  With
any machine squeegeed (?), placed, and reflowed parts there is a possibilty
of error that a visual inspection might detect.  A functional or flying
probe test (Genrad) could be used to spot problems, but this is usually
cost effective on high volume, or very high priced boards.

For 100 pieces, a visual should shake this out.  What you get, however,
ultimately depends on the service you paid for.  Could be you got a good
price because the house stated that they would simply run them and ship.

Scott F. Touchton
1550 Engineering Manager
JDS Uniphase



                   Wynn Rostek
                   <wynnr@GOLDEN        To:     KILLspamPICLISTKILLspamspamMITVMA.MIT.EDU
                   PRS.COM>             cc:     (bcc: Scott Touchton/US/UNIPHASE)
                                        Subject:     Re: [EE]: Solder bridging on low volume PCBs acceptable?
                   05/18/01
                   08:42 AM
                   Please
                   respond to
                   pic
                   microcontroll
                   er discussion
                   list







I expect well under 1 percent.

Wynn Rostek

{Original Message removed}
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part 3 144 bytes
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2001\05\18@102544 by Mark Skeels

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Hi, Scott.

I used to work for a circuit board house; I layed out  lot of PCB's.

In my experience, _no_ solder short was _ever_ acceptable. The customer
expected his/her PCB's to work out of the box.

Of course, if they did the layout, sometimes an *adjustment* had to be made
because of optimistic trace spacing, etc.

Mark


{Original Message removed}

2001\05\18@112502 by gacrowell

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Newell,

Some folks have mentioned wave soldering, I doubt you ran those packages
through a solder wave (i.e., TQFP-144, 48 pin TSOP-II, 28 pin SSOP), but be
aware that pad layouts for wave vs. reflow soldering are often different.
And, as someone mentioned, component orientation vs. travel through the wave
is important.

Someone mentioned solder mask; I think they meant solder stencil, that is
usually what controls the amount of solder paste put on the board in a
reflow process, and yes, too much can cause bridging.  Did you or the
assembly house provide the data for the stencil?

But solder mask is an important factor.  Did the solder mask data provide
for a web of mask between the pins of those devices?  What type of solder
mask?  LPI (Liquid Photo-Imageable) type is usually used on surface mount
boards.

Is a 4% fallout acceptable? No.  On a first run of a board that is the first
surface mount design done by a company?  Not surprising at all.  Work with
the assembler, they can probably point out exactly why the shorts happened.
It will be a matter of tweaking the process, solder stencil, mask, and very
likely the board layout.

Gary Crowell
Micron Technology



> {Original Message removed}

2001\05\18@114738 by Scott Newell

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Thanks guys!

Were the boards reflow or wave soldered?  Not sure.

It is my understand that the assembly house is to inspect the boards.

We did the board layout, but the assembly house generated the pick-n-place
data and the solder paste stencil.

From what I'm reading here, I've got good reason to be concerned.


newell

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2001\05\18@172910 by gacrowell

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And one other item, misplacement can cause bridging; you should have a
couple of feducials on your board layout.  Again, the assembly house will
tell you what they prefer as to placement, quantity, and style of feducials.

Gary Crowell
Micron Technology



> {Original Message removed}

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