Charles Barker@I-O INC 01/12/98 10:59 AM Hi Steve, Andy et all, Good stuff! At another facility we were able to spec our 20 mil pitch devices' pads at .012" +.002/-.000. Somewhere, someone must compensate for the losses due to the etch factor to insure minimum pad width. The stencil opening was then spec'ed at .010 in .006" thick SS. If the operator(s) kept the stencil openings and the bottom of the stencil clean of paste residues, we had neither solder bridging nor solder starvation problems. We had just begun to experiment with Ni-Pd finishes when there was a change of ownership and our endeavors came to an end. Some folks like the rounded ends on pads, others will say it doesn't matter. Pad length, in your case will have to take into consideration the different packages the various vendors will be supplying you.. Good luck, Charlie B. [log in to unmask] on 01/12/98 10:40:32 AM Please respond to [log in to unmask]; Please respond to [log in to unmask] To: [log in to unmask] cc: (bcc: Charles Barker/US/I-O INC) Subject: Re: [TN] Landpatterns for TQFP devices Andy, Thanks for you response. It didn't sound flippant at all, I appreciate the time and thought you put into the message. In answer to some of your questions: 1. I'm in the manufacturing group and I was looking for help so I do not have to design land-patterns from scratch for every new TQFP device that gets designed in. I was hoping there were IPC guidelines (on TQFP's) that I did not know about to give our CAD guys a place to start so that I only need to review and approve TQFP land-patterns instead of designing them from scratch. We have found IPC-SM-782 to be an excellent guideline and starting point for landpattern design. We do reference the component manufacturers recommended footprint, but have found that they are better at designing and manufacturing components than they are at designing landpatterns and typically the parts are multi-sourced with slightly different dimensions from each manufacturer. The landpattern calculator on the IPC website should help me (thank you to those who responded with that info.) 2. This is not our first venture into 20 mil pitch (TQFP devices are not all 20 mil pitch, we use some that are >30 mil). 3. We have chosen immersion gold as the finish for all production volume pcb's with 20 mil pitch or boards with lots of 25 mil pitch devices. 4. We clean bad prints by hand right now. Causes a mess, so we are very careful to avoid bad prints. We have a stencil cleaner due in house in a couple weeks and will clean bad prints in it using a new cleaning agent called Zestron. (I would like some feedback on Zestron if anybody has had any experiences good or bad, our tests were positive but have not tried it in production yet). 5. We reduce 20 mil stencil apertures 10% in length & width. We use a 6 mil mono-thickness stencil. 6. Our solderpaste mesh is -325/+500. 7. Through experimentation at a previous employer, we have found that band-etch gives equally good results as laser etch at half the cost. (We use chem-etch for standard pitch stencils). 8. We require soldermask between the pads. Most fabricators balk at it on 20 mil devices and say they cannot do it, but they usually do it anyway. 9. We are using LPI soldermask. 10. We rework 20 mil pitch using our A.P.E. chipmaster (hot air) and Metcal irons. 11. Our designs are tight so sometimes other components (typically chip caps) must be removed prior to QFP removal and re-installed after replacement. This hasn't proven to be too much of a headache because the number of FP components that we rework is small and the time to remove and replace a chip cap is very small compared to the FP device and the component cost is also relatively small. Not desirable, but we live with it when real estate requires it. This is more info. than most people care to know about us, but I often find it interesting to see what others are doing in the industry so I thought I'd share... Steve From: Electr1998 <[log in to unmask]>