Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


QFN Packages

John S

#32883

QFN Packages | 25 February, 2005

We are developing a new product, and some of the designers are proposing QFN packages. These look like leadless QFP's. We don't currently use anything like this, but we operate in a high reliability automotive market. Can anyone provide advice/comments on the relative manufacturability and reliability of these components? Thanks John S

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RDR

#32884

QFN Packages | 25 February, 2005

Would these be the parts that have just pads located on the bottom of the part? If so, we place these every day in decent volumes and have never had any manufacturing problems with them. I cannot comment on the reliability since we are a contract Mfgr. and are not privy to that information. However, I have noticed that none of our customers have removed them from their Bill of Material. We place these components on pretty high end - hi speed assemblies for mostly medical applications.

I highly recommend that they come in tape and reel or a tray instead of tubes. They are very difficult to feed with any type of vibratory feeder that I have used.

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#32887

QFN Packages | 25 February, 2005

Land grid devices [ie, BCC�, LGA, QFN, MicroLeadFrame�, etc] are essentially BGA devices with the balls removed and with no solder on the pads. Search the fine SMTnet Archives for background discussion.

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#32899

QFN Packages | 26 February, 2005

Some of the QFN's have/has? (sorry for the grammatics but I live in Sweden) a "ground pad" underneath the center of the body. Some of the manufactories have made a specification of how much solder fillet that needs to be covered on the center pad [footprint versus /terminal/versus heat sink], usually 80%. This is because some of these devices operates with high efficiency = temperature.

Therefore; the stencil aperture is crucial for this type of package; to much solder in the center and you will have a risk; that one side of the component will be lifted during reflow = no solder joints on that side. There are some cases that the designer has chosen to put via's in the ground pad on the pcb; and that will cause the solder to fill the vias and you will find it hard to fulfil the manufact'rs spec. without to have solder joint issues to the terminals. We use 0.127mm stencil thickness and we have also found out, that the square center aperture in the stencil (ground pad); can be divided into 4 smaller quadrants; the life will then be easier...

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greg

#32906

QFN Packages | 27 February, 2005

#32908

QFN Packages | 27 February, 2005

Exactly.

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Bob R.

#32934

QFN Packages | 28 February, 2005

We use QFNs in automotive products and have done a good bit of experimenting with pad designs, paste patterns, stencil thickness, and thermal via patterns. All of them have an effect on assembly and thermal cycle reliability. There is not a yes or no answer to your question. Reliability is a function of the things I've mentioned above with the part itself being a major driver. All QFNs are not created equal. Pay attention to internal die size and epad size in relation to body size and your ability to make a toe fillet (which isn't easy).

To meet the reliability requirements of automotive you'll need to lay out some test boards and do some experimenting. Hopefully you've got a passenger compartment rather than an underhood application.

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ABHI

#32983

QFN Packages | 2 March, 2005

Are your packages mounted on boar using sockets or BGA balls? LGA's are usually no issue and are simple compared to BGA. No issue on missing balls or ball drop issues. However, LGA's need quite a bit of mounting / holding pressures during test and more over during ot tests. And also care must be taken to protect the LGA pads from moisture / atmosphere.

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