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How to improve the solder quality of QFN?

#35126

How to improve the solder quality of QFN? | 23 June, 2005

Hi, all. There is one 0.8mmX0.8mm QFN on almost of our PWBAs. But the quality report showed that we got bad solder quality for this component. My question is which elements are the root cause for this situation frequently? Thanks.

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#35142

How to improve the solder quality of QFN? | 23 June, 2005

Jason: We'd like to help, but we don't know how to diagnose "bad solder quality". It's too broad of a complaint. Please be more descriptive, like: * Too much solder * Too little solder * Solder has too sharp of wetting angle

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#35148

How to improve the solder quality of QFN? | 23 June, 2005

Hi, Davef. Thanks for your help. Most of the defective PWBAs, I can see solder shortage and bad connection between Pad and PCB. I have analysed them in the first time and the possible reasons maybe are > poor soldering of PCB. > unfit stencil hole of QFN > incoming problem for this component >......

What is your opinion? or what can I do for the next step? Thanks again.

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#35155

How to improve the solder quality of QFN? | 24 June, 2005

Problem 1: Solder shortage * Stencil aperatures [except the heat slug] should be 1:1 with the board. * Stencil should be 5 thou thick laser cut. * Board should be 1:1 with the component * Print should look good.

Problem #2: Bad connection between pad and PCB [by "pad" we assume you mean the termination of the QFN] * Component solderability. ENIG can have problems with the solderability of the nickel underplate. * Temperature of the solder should be sufficient to form intermetallic compound. With standard tin/lead solder, we'd expect you to be at 220*C for 5 to 10 seconds. * Paste is fresh and active enough for the application.

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Bob R.

#35181

How to improve the solder quality of QFN? | 27 June, 2005

What problems are the quality people seeing? I hope they aren't expecting a toe fillet. QFNs are typically cut from an array after plating. This leaves exposed copper on the sides of the part. We'll often see smearing from the sawing process that will deposit some finish on the sides, but it's hit or miss. You can't count on the exposed copper on the sides being solderable.

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RICHM

#35230

How to improve the solder quality of QFN? | 29 June, 2005

Most QFN's have a exposed die pad on the bottom for ground connection and or heat dissipation if you are print solder paste 1 to 1 on this pad it will lift the part to high and the solder printed 1 to 1 on the pads will not be able to reach up and connect to the QFN pads. Reduction of paste volume on center pad about of 40% to 60% depending on pad size Also check for capped/plugged via, silkscreen and thicker than normal solder mask >.0025" under the part, all should be avoided

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james

#35321

How to improve the solder quality of QFN? | 6 July, 2005

What I have seen too is that these parts normally come in a tube and from moving in the tube some of the copper starts to expose and solder will not adhere to these parts. Let me know if anyone else sees the same problem.

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sarag

#35334

How to improve the solder quality of QFN? | 6 July, 2005

Jason, If you have any pictures of this phenomena that might be helpful for the forum to see in helping diagnose your problem, you can send them to me and I can post them on my website to view. My email is sg_foresite@residues.com

Sara Gorcos Foresite Inc. www.residues.com

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HOSS

#35335

How to improve the solder quality of QFN? | 6 July, 2005

Bob,

We have seen this same issue and haven't found a solution other than touching up to create a toe fillet. Luckily, it's a low volume product, for now.

What have you done to combat this?

Thanks.

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Bob R.

#35339

How to improve the solder quality of QFN? | 6 July, 2005

The only way we can consistently get a toe fillet is to use a paste with an extremely active flux. We allow that paste on certain products, but discourage it's use because it's on the hairy edge of not meeting our lower limit in surface insulation resistance testing.

Whether or not you need a toe fillet depends on your application. We have high reliability products where we can't use a QFN over a certain size, regardless of toe fillet. We have other applications where we meet the thermal cycle requirements for all QFN sizes, but only if we have a toe fillet. We don't have many consumer electronics products, so I haven't done "easier" thermal cycling of QFNs such as 0 to 100C so I really don't know if a toe fillet is required for those applications. I'm guessing they aren't.

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Jason Fullerton

#35406

How to improve the solder quality of QFN? | 8 July, 2005

"Whether or not you need a toe fillet depends on your application. We have high reliability products where we can't use a QFN over a certain size, regardless of toe fillet. We have other applications where we meet the thermal cycle requirements for all QFN sizes, but only if we have a toe fillet."

Page 8-84 of IPC-A-610D gives the criteria for sodlering to a QFN. Basically, it says toe fillet is only required as determined by design (Class 1, 2, and 3). So, in the above case, if there is a specific reason for requiring toe fillets (i.e. can't pass thermal cycle testing) then that requirement it is defined by the design and/or end use of the assembly in question.

Otherwise, since the ends of the lands on the QFN part are not required to be solderable (as mentioned above), there is no standard requirement for toe fillets at all.

In my experience, side/toe fillets can be used as an indicator of the quality of the solder joint: a good toe fillet means you can be assured that you have a good solder joint underneath where it counts, BUT lack of a toe fillet doesn't mean you necessarily have a bad solder joint.

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seaK

#35569

How to improve the solder quality of QFN? | 15 July, 2005

Our production is putting QFN40 and QFN56 package on board. With 80% opening on terminal, 40% on thermal pad, we found QFN56 100% forming toe fillet, but it does not work out the same to QFN40. We suspect it's because of the weight of component..... We use solder paste OM-338, 5-mil stencil.

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