To Larry
When the viscosity of the paste is to high then you will have all the problems listed in #2. The only thing we can do is increase the pressure or through it all out and try another tube however the next tube was no better.
From John G

-------------- Original message -------------- 
From: Larry Dzaugis <[log in to unmask]> 

> 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 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 : 
> > 
> > > 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 
> > > 
> > >> 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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