Rob,
If you spec anything lower than 6 mils you will most likely face Coplanarity
issues since you will have less than four mils of solder at reflow. I suggest 6
being your low and 7.2 the max. It has been my experience that if higher than
this, solder bridges are eminent.
Check your screen clean rates. My history shows Wet-Wet-Dry-Dry every 5 prints
for fine pitch and 10-20 for 25 mil and above. If you do not clean often
enough, the solder balls adhere to the stencil bottom side and give you a false
snap off resulting in high paste.
Squeegee speeds and decent rates should be as low as possible. This will ensure
the aperture is properly filled and that the aperture releases the entire
amount. Remember, "Speed Kills". Do not allow numbers to dictate your process.
As far as the volumes, that is design dependent. You need to calculate the
proper volumes so that the side, toe and heel fillets are met per your own
individual manufacturing requirements.
I do not now, nor have ever let the vendor spec this based on their experience.
Their experience usually makes the machine work best, not the process work
best. You are trying to control a process, not let it control you.
I pray the best for you.
Jim Keeton,
Senior SMT Process Engineer
EFW, Inc.
4700 Marine Creek Parkway,
Fort Worth, Texas 76136
[log in to unmask]
817-234-6718
-----Original Message-----
From: Rob Watson [SMTP:[log in to unmask]]
Sent: Monday, March 22, 1999 1:19 PM
To: [log in to unmask]
Subject: Re: [TN] Paste Height and Volume Tolerances
Hi Don,
Didn't I ask almost the exact same question this morning?
Anyways, here is what I have found out so far. I get this info from
CyberOptics who says that they have
done research with a lot of companies who use their equipment. The limits they
recommended are +/- 20% for
warning limits & +/- 40% for failure limits. These limits are only based on
the experience of the
companies that use their equipment. I would be interested if anyone knows of
any scientific research on
the solder strength and such. Would anyone be willing to share any technical
papers with the rest of us?
Rob
Hope this helps...
Don E Steffen wrote:
> Techno's
>
> We have a General Scanning GC8200 3D Laser Paste Inspection System for
> our SMT lines. We are trying to develope the methods and tolerances
> in order to establish warning and failure limits for each of our
> boards.
>
> I would like to know if anyone has any recommedations in percentage of
> the paste height, volume, and area for limits they set for the Warning
> and Failure limits. How were these limits derived? We started wide
> +/-30% and then started to narrow the limits!
>
> Has there been anyone then take the next step after establishing these
> limits and performed Vibration or Shock Test?
>
> Appreciate any help you can provide!
>
> Donn Steffen
> Quality Engineer
> VDO-CS
>
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