Sorry for the delay in responding, but it may still be useful:
I read the following part of your message and "click", a lightbulb turned
on:
found out that the apertures on some components were 100% of the pad size
and another was a tiny bit bigger than pad size.
When you have Gerber data to produce a circuit board or a stencil it is
necessary to define where the decimal point goes into all those X and Y
location numbers. In Gerber language it is the "m.n" setting. If you send
the data out using the "2.3" setting it means that the location numbers are
going to have three digits behind the decimal point. If the data is in
metric units (specified as MOMM) 2.3 is not bad, the final digit represents
micrometers, small enough so that any rounding error in the last digit is
negligible. But if your data is in inch units (MOIN), it means that the
final digit represents 1 thousandth of an inch (or a mil). Most of the time
that is not a problem when everything is laid out on a grid and laid out
using whole mils. But when you use metric components, some of them have
pitches like 19.7 mil. And in Gerber data with the settings MOIN and 2.3,
the actual spacing of the apertures for those parts is going to be either 20
or 19 mil, because in 2.3 data you cannot get fractions of a mil!!!!!! With
apertures defined with a single fixed width of for example 10 mil, it means
that the space between apertures in the stencil is either 9 or 10 mil and
may not match the board, potentially giving you mismatches of up about 1.5
mil.
The Gerber language is great, but you have to know what you are doing when
speaking it!!!!
Regards, Ahne
A-Laser, Inc.
503-641-9428.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Trevor Goddard
Sent: Tuesday, February 06, 2001 05:03
To: [log in to unmask]
Subject: Re: [TN] QFP144 solder bridging
Nicolas
I have a couple of questions for you.
Is this a new board or a new Rev of an existing product?
Did you review the stencil design or did you actually have the stencil
measured?
I got caught one time when I was given a new Rev of an existing product. We
had a couple of QFPs .20mm and a .50mm pitch that were shorting out like
crazy, which was never a problem on the previous Rev. I questioned the guys
that ordered the stencil and the layout guys about what specs the stencil
was ordered to and if there had been a footprint changed. The answers to my
questions was that nothing had changed, but I could still not fix the
problem with process changes. Finally we were able to get the stencil
measured and the pad sizes measured and
found out that the apertures on some components were 100% of the pad size
and another was a tiny bit bigger than pad size.
My guess is, if nothing has changed in your manufacturing process i.e..
screen printing parameters, reflow profile and your placement is good than
something may have changed in the design side.
Trevor
-----Original Message-----
From: Nicolas Ortiz [mailto:[log in to unmask]]
Sent: Monday, February 05, 2001 3:45 PM
To: [log in to unmask]
Subject: [TN] QFP144 solder bridging
Currently we are experiencing solder bridging on a 0.5mm pitch, LQFP, 144
leads. I already reviewed the stencil design, screen printer parameters and
reflow profile. Everything is OK. We have HASL boards. Our current process
outputs 38 dpmo's. My concern is if I reduce more the apertures, I have the
risk to have solder open or insufficient solder, instead of bridging. Does
anybody has some input? What dpmo's do you have?
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