Two diffferent criteria.
1. How repeatably is the PBC located to the stencil?
       Mechanical locating stops?
       Camera locating fiducials on PCB and stencil?
       This is your off pad printing capability
       The product and paste dictates what the % off pad can be.

2. The volume of paste deposited PCB to PCB and over time?
        Does the stencil release the paste?
        Is the opening filled with paste?
        Do the blades scoop paste out?
        Do they drag excess paste over the aperatures?
        Some of the things that can happen during a run.
        The volume tolerance is determined by the product, materials and
process.

The other question becomes how is the paste measured, will it pass a gage R
and R?
Location much easier than volume.
Once there are numbers to what you can do, what can your process tolerate
;and meet the requirements?

All of these processes become "it depends" (Trademark Pending, used w/o
permission) on what the customer needs and what is willing to be risked by
you and your customer.



On 11/24/08, Bob Willis <[log in to unmask]> wrote:
>
> It's a fair comment but you have to set your own standard to be able to set
> any automated process?
>
> If a standard is required IPC, SMTA and Automated Learning offers a photo
> CDs that has lots of print defects and reference prints. There are also
> some
> reference standards for printing.
>
> I have recommended in the past that IPC have criteria for printing and
> adhesive deposits to make the us of the DPMO documents and data collection
> easier to use.
>
> Many thanks
>
> Bob Willis
> 2 Fourth Ave, Chelmsford, Essex, CM1 4HA England
> Tel: (44) 1245 351502
> Fax: (44) 1245 496123
> Mobile: 07860 775858
>
> www.ASKbobwillis.com
> New Bob Willis "Printed Board Inspection & Quality Control Workshop" 24th
> March www.ASKbobwillis.com/faworkshops.pdf
>
> "Troubleshooting Your Assembly Yields Workshop" 25th March
> www.ASKbobwillis.com/faworkshops.pdf
>
> Book Bob's "Step by Step Failure Analysis Workshop" 22nd April
> www.ASKbobwillis.com/faworkshops.pdf
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Andy Ng
> Sent: 24 November 2008 03:41
> To: [log in to unmask]
> Subject: Re: [TN] Screen printing
>
> There're no standards on misprinting as this is not a final
> workmanship/quality issue...rather a process issue...I'm sure
> engineers do set their own guidelines if you utilize an AOI or
> automated printing inspection system, depending on what type of
> component you're looking at...say, a 20-mil vs a 40-mil pitch QFP. But
> you're right, especially for high volume runs, controlling the
> printing can reduce a large portion of your defects and time lost.
>
> Andy
>
> Quoting John Goulet <[log in to unmask]>:
>
> > I've never heard of one. The IPC standard is for component
> > placement. Occassionally we may have the aperatures longer to print
> > off the pad on purpose to have more paste flow up the leads.
> > I put in a 1st peice inspection sheet for print, placement and the
> > paste used to assure at each changeover the process was good. This
> > prevents most of the misprints unless I get my finger in it during
> > inspection. Now that the operators know we are watching, the quality
> >  has gone up substantially. If the camera gets out of calibration
> > then the prints can go off. The manaul printers are doing better and
> >  stencil issues are caught much sooner before they cause a stop
> > process.
> >
> >
> > -------------- Original message --------------
> > From: Yannick Brisson <[log in to unmask]>
> >
> >> Hi everyone!!
> >>
> >> Is anybody have a chart or rule or standard for misprint. What I mean is
> if
> >> there is a way of saying if my print for some reason a switch 1%,
> >> 5%, 10%, etc,
> >> off the pad that is not acceptable.
> >>
> >> Thank you
> >> Yannick
> >>
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