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TAL during reflow

Views: 1544

Mike

#39562

TAL during reflow | 7 February, 2006

Can someone point out what sort of defects at the joint I can expect if solder is above liquidus for 3-4 minutes rather than the typical 1 - 1.5 minutes. Voids? Grainy joints? Poor wetting?

Thanks

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#39563

TAL during reflow | 7 February, 2006

Hi,

I would say the most worrying defect would be damaged parts due to exceeding their temp specifications.

Regards,

Grant

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GS

#39566

TAL during reflow | 7 February, 2006

Hi,

other effect could be the extra inter-metallic thickness that could compromise the solder joint reliability.

just my 0.00001 Doblon

Best regards.........GS

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#39574

TAL during reflow | 8 February, 2006

'TAL' is a poor measure of solder connection heating. In a reflow process, what you need is about 5 seconds at liquidus for the specific solder plus: * 20*C when soldering to Cu. * 35*C when soldering to Ni. This time over temperature is measured at the slowest heating connection on the whole assembly.

Too long above liquidus can cause a range of problems, including those mentioned earlier: * Charing * Solder balls * Damaged components and boards

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Cal Kolokoy

#39614

TAL during reflow | 10 February, 2006

DaveF is correct in saying that TAL is not the best metric to use in reflow soldering. That is why paste manufacturers give such a wide range; typically 30 to 120 seconds.

The peak temperature is more important, especially in the mixed Pb and Pb-Free environments of today. It goes without saying, however, you as the Engineer will need to establish your "process windows" for TAL and Peaks and base it of course on solder paste used, board types and product mix.

I've always treat reflow soldering as this: what you do in your preheat zones, and weather you choose to ramp & soak, or straight linear ramp, will affect the behaviour of your flux, but it's the reflow zones (TAL's and Peak temps) where the actual soldering takes place. Essentially, what you did earlier in your profile to optimize your flux activity, will affect what happens when "the moment of truth", the solder joints forming, takes place. And again, profile strategy depends a lot on your paste and product mix.

Dave is also correct that excessive time above liquidous AND peak will lead to problems like flux burnoff, charring, and excessive intermetallic layer.

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Chunks

#39620

TAL during reflow | 10 February, 2006

Wow, how people get off track. You'll probably see OK joints, but the flux will be burt. Your boards will prob. discolor and possibly delaminate. Some of your parts amy discolor too. Depending on your paste, you will probably see some balling/beading. Anyone that's ever been around an oven will know it's a burnt baord.

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#39657

TAL during reflow | 13 February, 2006

You will begin to de-wet at a high TAL and actually likely see all of the conditions you list.

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