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0402 tombstoning and dewetting on QFP256 trade-off

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0402 tombstoning and dewetting on QFP256 trade-off | 5 March, 2009

Hello to all.

for certain time, we have been struggling with some issues on a PCB where have tombstoning on 0402 components (due to pad design) and dewetting in qfp256 0.5 mil pitch (discussed on another threads on this forum) component, we have proven that wetting is improved on this component applying nitrogen on our process (I know, is only a patch, but so far is the unique working solution...), BUT on the other hand we have tombstoning issues on our 0402 components while running on nitrogen, this is my scenario:

Solder paste: Indium NC-SMQ92J 0402 components: Stackpole electronics (several P/N) Stencil: Stainless steel laser-electropolish (Interlatin) Pad/Stencil (see attached) QFP: Renesas 256 QFP (90Sn10Pb) Reflow oven: Heller 1809 MK III (9 zones, 3 cooling) Nitrogen parameters: Exit flow: 650 SCFH air Entrance flow: 300 SCFH Cooling flow: 300 SCFH Fan Seal: 300 SCFH

We have been using both RSS & RTS profile, RSS with a soak between 170 & 180 to minimize temp imbalance to avoid tombstoning (recommended by Indium) and a conventional RTS profile (Attached screenshoots), the deal is if I cut down N2 tombstoning is almost eliminated (less than 1%) but we increase wetting issues on QFP and viceversa. I also include the pad design and stencil design to avoid this issue, a new design is coming on but while is implemented we need to live with the issue.

My biggest issue is that I have no idea on N2 settings, heller only toldme "use settings what works for you, i could recommend something but you need to try" in fact there's not a "one size fits all" solution but they didn't mention initian settings, have any of you experienced or do you know about a tutorial where explain how N2 works? I will like to try a optimum point where tombstoning and dewetting is minimum and I will like to consider the following parameters:

1.- Stencil design actual vs "other" 2.- Profile (RSS VS RTS) 3.- N2 settings 4.- Printer stroke speed 5.- Printer system pressure




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0402 tombstoning and dewetting on QFP256 trade-off | 6 March, 2009

Have you tried higher peaktemp? I saw the QFP is sn90pb10, This has a liquidus of 268 - 302 C. A good paste should be able to handle 230C (or more), of course you would need to check your other components peak temp.

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0402 tombstoning and dewetting on QFP256 trade-off | 6 March, 2009

First, we agree with Loco that your QFP is nonwetting, rather than dewetting. Your sceen-shots look like ol' timey SnPb paste recipes. Turn-up the heaterometer dial.

Second, on nitrogen: * We kinda think your O2 needs to be around 100ppm with 99.99% N2 purity. * So, what does you N2 use cost per year? * Here's an article on inerting the solder process:

Third, on setting up a printer: * Set the down-stop to default * Start with pressure of about 1 pound/inch of blade. * Adjust the pressure up/down to leave a very thin film on the stencil. * Speed should match your paste, (slower speeds wipe cleaner)

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0402 tombstoning and dewetting on QFP256 trade-off | 6 March, 2009

Just my opinion but it look's like your doing a great job. i would try another paste. I'm somewhat lucky that we get to use OA's. If your stuck using the no-cleans they are so tempermental. (The nitrogen is making up for the poor paste) Your de-wetting is due to the flux burning off to soon due to your soak to fix the 0402's. your paste is just not cutting it. be open minded to the idea that one paste for everything is not going to cut it. Also be open minded to the idea of a different paste manufacture. you'll be amased at the difference. i have nothing against Indium. I've used their product too. But for certain problems every paste manufacture might have a different solution. you need to find the one that works for you. Order some samples from some other manufactures and give them a try. Good Luck

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0402 tombstoning and dewetting on QFP256 trade-off | 6 March, 2009

I have attached a pdf version of an article that explains the basics of a nitrogen atmosphere when soldering. It gives some basic guidlines as to when one may want to use nitrogen and what it can and can not accomplish.

FYI- I did not write that article. I found it on the internet.


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0402 tombstoning and dewetting on QFP256 trade-off | 6 March, 2009

whats your board finish?

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0402 tombstoning and dewetting on QFP256 trade-off | 12 March, 2009

I agree you are too cold for the profile get it up to 230C and possibly allow it to dwell a little longer in preheat as opposed to pushing up the reflow zone only. Sn90/Pb10 melts at 220C so 2C hotter than your profile so you are reliant on it alloying together over a short profile length. This will help also with Tombstoning issue getting everything evenly heated. At 218c if you have Lead Free devices unknowingly on the PCB then they will struggle to wet well Hope it works for you Cheers Greg

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