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Flux appearance

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Would i be able to judge how good is a reflow profile from t... - Mar 15, 2006 by Vick  

#40422

Flux appearance | 15 March, 2006

Would i be able to judge how good is a reflow profile from the appearance of the flux residue? Often times i come across flux (not sure if residual) between a non-wet QFP lead and the reflowed solder paste on the pad (it's sandwiched between the bottom foot of the lead and solder paste). Does this mean there was a gap between the bottom of the QFP lead and solder paste during the reflow process? Can this is also point towards unoptimized reflow profile? No-clean solder paste is used. Looking forward to replies from experts in this field. Thanks.

Rgds

Vick

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RDR

#40425

Flux appearance | 15 March, 2006

Best way to judge how good a profile is by measuring it. It is not real accurate to determine by the amount of residue left. You should be able to tell if the lead is bent causing the open.

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Vick

#40426

Flux appearance | 15 March, 2006

The lead was inspected for coplanarity and it's within the specification it's designed for and doesn't show any variation in terms of height as compared to it's adjacent lead that has good wetting

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#40427

Flux appearance | 15 March, 2006

Vick Russ is very correct. Get the paste material specificatioin. Please get your favorite data logger (ECD, KIC, DatPaq, etc..), attach some TC's and take a profile. This is the ONLY way to know what your reflow system is doing and how to alter. Once you have a known good profile, perodicaly check this with your profile board to ensure your process is in check. thanks marc

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#40428

Flux appearance | 15 March, 2006

Thanks for the input. Just one question. Is what i am seeing normal? I am refering to the flux being between the lead and molten paste after the reflow process. This only found on a non-wetting lead.

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#40429

Flux appearance | 15 March, 2006

Vick I would not think what your seeing is normal. Just guessing without a profile but.... - not reaching peak temperature - not enough soak time or mild ramp to burn away the flux - shortened TAL, not allowing proper time for the solder to wet - longshot... contaminated leads on component

All paste will leave a mild residue on the board after reflow. No clean is just "less residue." thanks mac

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#40430

Flux appearance | 15 March, 2006

Thanks Marc. I will try the things that u have mentioned. I am guessing if the flux can find its way between the bottom foot of the lead and the paste, there has to be a gap between the solder paste and the lead when reflow takes place. As u have said, I guess the soak time was not long enough for the flux to evaporate and thus it gets trapped between the lead and the paste and solidified. Can that actually happen?

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#40431

Flux appearance | 15 March, 2006

Is the lead leaving a depression in the solder that wetted the pad or is there maybe just not enough solder to fill the gap?

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#40433

Flux appearance | 15 March, 2006

It does leave a saddle like appearance on the solder

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#40440

Flux appearance | 15 March, 2006

Based on that criteria I would also guess profile but as has been stated, you don't really know w/o profiling.

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#40441

Flux appearance | 15 March, 2006

By any chance is it eutectic paste and pallidium leads? Also how tight are you checking coplanarity? At least with the MPA-G3 you can check so tight that any IC will fail in the real world. Or you can check so loose that you might as well not check.

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RDR

#40445

Flux appearance | 15 March, 2006

Your profile is definitely off. It could be bad leads but it sounds like your max temp is too low and the TAL is too short. By how much? Don't know.

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#40466

Flux appearance | 16 March, 2006

Paste is eutectic and it's copper based parts. Just another observation, i notice that the "flux" between the bottom foot and solidified paste is still in gluey state. Observation on good reflowed leads showed brittle type of flux residue. Can anyone comment on this?

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#40475

Flux appearance | 16 March, 2006

I would again suggest that you REALLY need to profile that board.

It does sound like you're not getting enough heat to that component, but if you just crank up your reflow zone how will you know you're not overheating something smaller unless you bite the bullet, hang a thermocouple on there and check it?

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RDR

#40487

Flux appearance | 16 March, 2006

The copper based part is it copper that you are soldering to or did you tell us what the base material was for the lead. Generally all leads are plated with something such as Tin/Lead, palladium over nickel, tin, etc... You cannot solder to bare copper without some type of precleaning first. So have you run a profile yet? Just do this and tell us what you got and we can help.

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#40497

Flux appearance | 17 March, 2006

Copper is the base material for the lead. It's tin/lead plated. I finally got profile. Here are the parameters"

Peak : 219 Max (+) Slope : 2.29 Max (-) Slope : -1.94 Time above 200 deg. C : 50s Time 140C - 160C : 126s

Additional info : Stencil thickness : 0.15 mm Solder paste : Koki

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RDR

#40500

Flux appearance | 17 March, 2006

nothing appears to be out of line. Is this KOKI a no-clean or water soluble?

You may want to try to hit 225 at the problem lead with a TAL of 70 sec. You may also want to remove the soak or at least shorten if you can.

We were talking about a single lead on a part right? or is it the whole part?

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#40501

Flux appearance | 17 March, 2006

It's a no clean paste. Yes it's a single lead (sometimes a few at random locations) and i do notice that corner leads (or close to it) fails more.

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