We wound up going 1 to 1 (of the part pad dimension) on the I/Os and a 50% reduction (of the PCB pad, which was smaller than the part pad) in a 4 x 4 grid on the thermal pad. The vias were closed on the thermal pad. 4mil stencil and type 4 paste, 63/37. We started with Actel's guidelines and got marginal results. Didn't want to go to a 4 mil stencil but the area ratio was better, tried an overprint with a 5 mil but got bridging...lots of fun. Virtually no standoff but the x-ray looked okay and boards passed test. Mike -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Tan Geok Ang Sent: Monday, August 10, 2009 7:43 PM To: [log in to unmask] Subject: Re: [TN] Actel QFN180 Assembly BTW. I am using Sn/Pb if using Pb-free the thermal pad opening has to be even lesser. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne - IIW Sent: Monday, 10 August 2009 9:49 PM To: [log in to unmask] Subject: Re: [TN] Actel QFN180 Assembly I haven't tried to put down this part, but I did want to share my simple rule of thumb for stencil design for QFN pads: Make the thermal pad look as much like the I/O pads as possible: Use a grid of deposits which are about the same size and overall density as the I/O pads. Probably for this part that will make the thermal significantly less than 50%, but often not much is needed. I usually end up potting almost everything we produce, so I can count on heat getting out through the potting material. Wayne Thayer -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of stephengregory5849 Sent: Monday, August 10, 2009 9:39 AM To: [log in to unmask] Subject: Re: [TN] Actel QFN180 Assembly I can tell you that the application note that Actel puts out for their QFN180 is worthless. The reduction for the thermal pad that GA Tan is in the ballpark, might even be a little too much. The problem with this part is that there is such a razor thin balance between figuring out how much paste you print for the thermal pad and getting decent I/O pad joints. The three row I/O pad pattern make things all the more difficult. We were dealing with single and double row QFN's without any problems until we ran across this beast. Oh, as far as trying to rework the part, good luck! It ain't happening. We even paid a company who does nothing but rework for a living to try and rework the part and it gave them fits. They were able to get the part back down after much difficulty, but the board never worked. Don't know if the part died during rework or if it was something else. Just know it wouldn't program when we got the boards back. Steve ----- Original Message ----- From: "Tan Geok Ang" <[log in to unmask]> To: <[log in to unmask]> Sent: Monday, August 10, 2009 2:24 AM Subject: Re: [TN] Actel QFN180 Assembly Hi Grivon, There is some guide from Actel : http://www.actel.com/documents/QFN_AN.pdf which I don't think it will work well. Go for a volume of 55% at the thermal pad (and a pad opening of 9mils (5mils stencil thickness) with slight rounded radius at the corner of the opening. What is your problem and stencil openings? With your result, you will able to know where gone wrong. FYI, we used only two rows of QFN as of now and lots of LGAs. Hope it help. Regards GA Tan ________________________________ From: TechNet on behalf of GRIVON Arnaud Sent: Fri 8/7/2009 6:04 PM To: [log in to unmask] Subject: Re: [TN] Actel QFN180 Assembly Coming back on an old post from Steve (at his former employer...). Does anybody have good stencil rules for the assembly of multi-row QFNs and especially triple-row such as ACTEL QFN180? Like Steve and probably many others, we tried to find the right aperture balance between I/Os and central thermal pad without success... A. 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