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Reflow Oven Profiling

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TMC

#41165

Reflow Oven Profiling | 25 April, 2006

I am in the process of profiling some 8"x10"x0.063" double sided boards in our 4 zone oven(4 top and 4 bottom heaters). Was wondering if I can create a profile so that the components on bottom side of a board do not reflow while the componets on top reflow at a peak temperature of around 215C?

One of my colleagues seems to think that by setting the bottom heaters to a low temperature this can be accomplished. My thinking is that on a 0.063 thick board, once the top components/pads reach melting point (183C), the bottom components will do the same in a couple of seconds later. Any truth to either hypothesis?

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#41166

Reflow Oven Profiling | 25 April, 2006

What kind of parts are on the bottom? Small passives are very unlikely (won't unless other abnormal factors come into play) to fall off when reflowed a second time regardless of temperature differential.

There is a simple weight/soldered surface area formula that's a pretty good place to start as far as what you can get away with on double sided reflow without chip bonder. I think it's on Bob Willis' site but I'm not sure. I'm sure it's on this forum too now that I think about it. Give it a search and see what you come up with.

Re: your assessment of the situation in your last paragraph I think you're probably right as far as passives go, but higher mass parts might fall more in line with your colleague's way of thinking.

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TMC

#41169

Reflow Oven Profiling | 25 April, 2006

Almost all of the components on the bottom side are passives however, there are a couple of mBGA's and qfp's. I am aware that even if the components on the bottom do get reflowed, surface tension of solder will hold them in place. As far as IC's, we may apply an epoxy at the edges to hold these components assuming the pad to surface area ration is off the mark and components will fall off/sift.

I guess my question is more equipment/material related. Since the thermal mass of components on the top side is considerably more than on the bottom (more and larger IC's) to reach my peak of 215C, the whole board would need to be around 215C (top and bottom). In other words, if there is variation in temperature on the board, cooler section will act as a heat sink drawing heat away from the hotter section until, temperature across all of the board stabilizes. If I am correct up to this point, it is hard to imagine that I can have a profile where top pads of a board reach 215C while pads on the bottom are below 183C (even that of the mBGA's, qfp's).

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Rob

#41173

Reflow Oven Profiling | 25 April, 2006

Hi TMC,

It's very unlikely that you'll get a temperature variation of 32 degrees by varying the top and bottom heaters - especially if it's forced air convection, as if it's a half decent system the hot air will be circulating in the zone, pretty quickly raising the temperature of the lower zone.

Cheers,

Rob.

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Hoss

#41175

Reflow Oven Profiling | 25 April, 2006

TMC,

Are you interested in this approach because you only have a flat belt conveyor and want to keep the components from being moved by the belt on the second pass?

If so, you might try adding some stiffeners to the board edges to stand the board off the belt. Re-profiling for the extra mass would be in order.

Good luck.

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#41176

Reflow Oven Profiling | 25 April, 2006

Hoss, that's what we do here. Slotted Durastone bars is all it takes unless the board is big enough to require a center support

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RDR

#41180

Reflow Oven Profiling | 25 April, 2006

Its not going to work.

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#41187

Reflow Oven Profiling | 25 April, 2006

TMC: What are you aiming to accomplish in doing this?

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TMC

#41188

Reflow Oven Profiling | 25 April, 2006

Thanks for all the replies. Rob . . . that's what I was looking to get feedback on; to see if others can program their reflow ovens so that double sided boards only reflow on top during the second pass. Our oven does have an edge belt conveyer so there is no problem of transporting double sided boards.

The thinking here is that although surface tension should hold components on the bottom side from falling off/shifting (assuming they reach and pass the meting point); if there is a way we can program our oven to eliminate bottom side reflow alogether, we will eliminate this variable as well as the possibility of having solder joint defects on bottom side. I was just looking for some feedback to see if it is common practice to program reflow ovens where bottom of the board does not reach an alloys melting point.

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KEN

#41192

Reflow Oven Profiling | 25 April, 2006

I agree with Russ.

The problem is your thermal isolation is so small 1/16 inch. Also, most ovens measure temperaure in the plenum. You don't solder in the plenum. Therefore, you have a intermidiate "zone" temp. mixing between the top and bottom (which coincidentally) is where you do solder at. Both sides are exposed to virtually the same temperaure. You need top / bottom thermal isolation.

I've worked on several different (pronounced "Modern") furnaces experimenting with this method. Best results were achieved when there is zero gap between the boards in the furnace (end-to-end processing). However, I could not achieve enough process margin to deem a reliable process.

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#41193

Reflow Oven Profiling | 25 April, 2006

We've talked about this previously on SMTnet. Search the fine SMTnet Archives. For instance: http://www.smtnet.com/forums/Index.cfm?CFApp=1&Message_ID=17463

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RDR

#41212

Reflow Oven Profiling | 26 April, 2006

Sorry, If you need to do this, I would run a high temp alloy in the first side and then a lower temp on the second side. Components must be able to withstand the high temp process for the first side.

Russ

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JW

#41437

Reflow Oven Profiling | 9 May, 2006

An option to consider is leave top and bottom heaters settings the same; to get correct liquidus time and temperature on one side, you will have some amount of reflow on the second side. If you have BGA's, QFP's on the bottom side yet they are smaller than the IC's on the top side, run the bottom side first then the top side. If both side have components exceeding the formula given earlier, add adhesives under the larger IC's. It will help hold the part and cure before reflow.

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Chris

#41452

Reflow Oven Profiling | 10 May, 2006

I agree with all of you. If it is a good forced convection oven, the bottom side of the PCB will get very close to reflow temp and will probably reach reflow temp. Plus your oven may fight your settings. The lower heater temp settings will never be reached and will never stabilize because the top heaters will heat the bottom heaters. In 1995 when I first got a forced convection oven I was a little upset. Having used IR oven for several years, I could reflow the top side only. Probably the one and only good thing about IR ovens. Wanting to only reflow the top side, I had to flip off the circuit breakers on the lower heaters and disable them. Still the bottom side of the board got to reflow temp.

You can still buy an IR oven. I ended up using a pallet to shield the bottom side from heat.

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