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Reflow cooling rates

Rich Taylor

#11534

Reflow cooling rates | 10 May, 1999

Can anyone tell me what the recommended "typical" cooling rates are after reflow? Are the cooling rates considered as critical to control as the heating rates and why/why not?

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DNC

#11535

Re: Reflow cooling rates | 10 May, 1999

| Can anyone tell me what the recommended "typical" cooling rates are after reflow? Are the cooling rates considered as critical to control as the heating rates and why/why not? | I do not think there is a "typical" post reflow cooling rate out there. But as far as the importance is concerned, it is a critical phase of reflow process in general. I think a lot of us do not realize or forget the importance that the cooling phase plays in the thermal process. We I know myself get so caught up in the actual heating process of things and forget about the cooling phase. The cooling phase of the thermal process will actually determine the solder joint grain structure. Thus affecting solder joint strenght and reliabilty. The way this works is that the faster you cool down or solidify your solder the tighter your finished grain structure will be. You might want to check the following publications for more in depth information on this subject: MIL-STD-883D,JEDEC STD's, and IPC-SM-785.

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Steve Gregory

#11536

Re: Reflow cooling rates | 10 May, 1999

| | Can anyone tell me what the recommended "typical" cooling rates are after reflow? Are the cooling rates considered as critical to control as the heating rates and why/why not? | | I do not think there is a "typical" post reflow cooling rate out there. But as far as the importance is concerned, it is a critical phase of reflow process in general. I think a lot of us do not realize or forget the importance that the cooling phase plays in the thermal process. We I know myself get so caught up in the actual heating process of things and forget about the cooling phase. | The cooling phase of the thermal process will actually determine the solder joint grain structure. Thus affecting solder joint strenght and reliabilty. The way this works is that the faster you cool down or solidify your solder the tighter your finished grain structure will be. You might want to check the following publications for more in depth information on this subject: MIL-STD-883D,JEDEC STD's, and IPC-SM-785. |

Hi Rich!

Deon pretty much hit the key point about cooldown, the quicker the cooldown, the tighter the grain stucture of the solder when it solidifies. (Say Deon, your nickname wouldn't be "Neon" would it? Just kidding! Spellings not right anyway...hehehe) thus a stronger joint.

But there's a few other reasons why a quickly cooled joint is stronger; 1.) You're limiting the time above liquidous to the minimum which stops the formation of the Tin/Copper intermetallic to what's only needed for a good joint (remember, a thick intermetallic layer is brittle), and 2.) Because Lead is heavier than everything else in the joint, with a slow cooldown could cause the Lead to "pool" at the metallurgical interface giving you a weaker joint....plus the longer it takes to cool down, the more of chance there is for a component to become misaligned. A "typical" cooldown rate (from my limited experience) is around 10-degrees C. per second...

But as to why it's more critical going up than going down, is because of moisture...ol' h2O. If there is any entrapped moisture in any of the components, you'll stand more of a chance fracturing them or popcorning something because of the fast ramp rate. You quickly turn the moisture inside to steam and quickly build up the pressure inside, then BLAMMO! Also, in the first part of the profile you're driving the volatiles in the paste off...you want them to leave the paste with sort of a "sigh"...a-h-h-h-h-h-h. Not heat them so fast that they start to boil (the boiling points of most volatile compounds and solvents are pretty dad-blame low) that's where you get your solderballs and spatters from...plus tombstones and other neat stuff...

Hope this helps a bit...

-Steve Gregory-

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Ruby Zheng

#11537

material with thermal coeffecient 16 ppm/C | 10 May, 1999

| Can anyone tell me what the recommended material for molding with thermal expension coeffecient around 20ppm and transparent for lightwave 850nm. Thanks |

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#11538

Re: Reflow cooling rates | 12 May, 1999

| | | Can anyone tell me what the recommended "typical" cooling rates are after reflow? Are the cooling rates considered as critical to control as the heating rates and why/why not? | | | I do not think there is a "typical" post reflow cooling rate out there. But as far as the importance is concerned, it is a critical phase of reflow process in general. I think a lot of us do not realize or forget the importance that the cooling phase plays in the thermal process. We I know myself get so caught up in the actual heating process of things and forget about the cooling phase. | | The cooling phase of the thermal process will actually determine the solder joint grain structure. Thus affecting solder joint strenght and reliabilty. The way this works is that the faster you cool down or solidify your solder the tighter your finished grain structure will be. You might want to check the following publications for more in depth information on this subject: MIL-STD-883D,JEDEC STD's, and IPC-SM-785. | | | | Hi Rich! | | Deon pretty much hit the key point about cooldown, the quicker the cooldown, the tighter the grain stucture of the solder when it solidifies. (Say Deon, your nickname wouldn't be "Neon" would it? Just kidding! Spellings not right anyway...hehehe) thus a stronger joint. | | But there's a few other reasons why a quickly cooled joint is stronger; 1.) You're limiting the time above liquidous to the minimum which stops the formation of the Tin/Copper intermetallic to what's only needed for a good joint (remember, a thick intermetallic layer is brittle), and 2.) Because Lead is heavier than everything else in the joint, with a slow cooldown could cause the Lead to "pool" at the metallurgical interface giving you a weaker joint....plus the longer it takes to cool down, the more of chance there is for a component to become misaligned. A "typical" cooldown rate (from my limited experience) is around 10-degrees C. per second... | | But as to why it's more critical going up than going down, is because of moisture...ol' h2O. If there is any entrapped moisture in any of the components, you'll stand more of a chance fracturing them or popcorning something because of the fast ramp rate. You quickly turn the moisture inside to steam and quickly build up the pressure inside, then BLAMMO! Also, in the first part of the profile you're driving the volatiles in the paste off...you want them to leave the paste with sort of a "sigh"...a-h-h-h-h-h-h. Not heat them so fast that they start to boil (the boiling points of most volatile compounds and solvents are pretty dad-blame low) that's where you get your solderballs and spatters from...plus tombstones and other neat stuff... | | Hope this helps a bit... | | -Steve Gregory- | | All the above it correct. Faster cool-down rates are better for the solder joint (Tighter grain structure - less intermetallics etc.)However (seems there is always a 'however' in these responses), there may be issues with your components if you exceed 3 deg.C/sec cool-down. Many will not guarantee their products should you exceed this number. I suggest you contact your component manufacturer to learn which components you have which are most thermally sensitive. You have piqued my curiousity as to why this became an issue. Let me know if I can offer further assistance. -Tim O'Neill-

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