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QFN's and LGA's

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QFN's and LGA's | 12 May, 2009

Hi All,

I see more and more components like QFN or LGA with bottom-only terminations,with landpatterns getting finer all the time. I have no ppm-figures of our process, but based on a benchmark study by Agilent I'd say that 500 ppm is a decent value, especially for smaller series. Which means that if you assemble 100 boards each having 100 QFN-or BGA-terminations, 5 boards out of these 100 will be defective. With 1000 of such terminations half of the boards will not work! Repair is often more expensive than the value of the boards that wou want to save, in which case you could end up with piles of scrap. Sounds insane, doesn't it?!

Opinions, solutions, all comments are welcome!

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QFN's and LGA's | 20 May, 2009

We just placed a 56 pin QFN, 1 per board, on 10 boards and had zero defects. A good paste job, proper placement and a correct oven profile are the keys. If these are set up correctly, on larger runs your defects won't be zero but they should be less than 500 ppm.

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QFN's and LGA's | 20 May, 2009

Just out of curiosity, how are you or anyone else determining a defect? Last I checked, IPC had no criteria yet for solder defects for QFN components. Has this changed? Biggest problems we see are voids which our x-ray shows a lot of. Our problems is finding an acceptable standard to work from.

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QFN's and LGA's | 20 May, 2009

I have attached a pdf showing the dimensions for the QFN we placed.

First, The pins on the part are not only on the bottom side but also wrap around to come up each of the 4 sides of the part as shown in the side view in the attached pdf. This is simailar to a castellated termination which is covered by IPC-A-610 section 8.2.4 (January 2000).

Second, the assemblies passed full electrical test.

We did not use X-ray inspection on the QFNs so a void on the bottom side is a possibility. However, based on the 2 inspection/test criteria listed above I am claiming 0 defects.


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QFN's and LGA's | 22 May, 2009

Have any of you ever worked with Actel's QFN 180? A three row I/O QFN?

Actel has a published paper on design and assembly guidelines, but we're finding that they are pretty much generic, and don't really work as well as they lead you to believe.

We also found that rework is pretty much a non-option with these parts.


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QFN's and LGA's | 26 May, 2009

QFN or MLFs are mainstream now, QFNs already evloved to much more complex versions now like matrix pins, stacked dice and flipchip versions. IPC 610D already includes them, just make sure the pins have solder plating, the PWB pads have soldermask in between its pads and preferbaly NSMD. LGAs on the other hand are still categorized as an advanced package. IPC do not have clear guidelines yet on how to mount them. The best application guides are provided only by the top IC packaging contractors in the world, namely Amkor, ASE, SPIL or Chippac. Guides are available on this link for example.

contact me for more detailed instructions if you are having problems with them...

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QFN's and LGA's | 26 May, 2009

Amkor's MLF guidelines are very similar to Actels. 1 to 1 on the stencil to part pad on the I/O pads, 75% with a grid windowpane, 0.125 mm/5 mil stencil. For example, the Actel area ratio for a 0.3 mm sq. pad and 0.125 mm stencil is 0.6 which seems marginal...even with a type 4 paste.

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QFN's and LGA's | 3 June, 2009

Here is a little different spin on re-working/hand placing LGA's..

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