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New Oven Now Getting PCB Twist

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New Oven Now Getting PCB Twist | 24 February, 2009

Hi All,

Pretty new here, and to the industry and doing my best to learn what I can. Sorry if this topic has been discussed extensively, but I couldn't seem to find it via a forum search.

Our company recently purchased a 7-zone reflow oven to replace our older 5-zone. Based on profiling our boards, our profile is falling within the limits of the solder paste and for the most part we appear to be getting good solder joints. However, the operator and supervisor believe there is significantly more bend/twist in our larger single-sided boards exiting the reflow oven than we had on the 5-zone.

One thing that stands out is the slope in the pre-heat stages is much higher now (pushing upper limits). Besides that, every other aspect seems to be about the same (Peak temp, time above 217, time between 150-217, etc). Would a higher slope during preheat be a factor contributing to excessive bend/twist of the PCB?

Also, they are telling me the amount of bend on double-sided boards is more than it used to be on the 2nd pass through reflow (board on rails), so I'm wondering what might be causing this as well.

I greatly appreciate any input/feedback I can get on this. Or, if anyone has a relevant link discussing this topic that would be great. Just for the heck of it, below is our basic profile on the 5 zone vs what we're using now on the 7 zone. Thanks again!

5-zone: Speed - 17cm/min Zone1 - 130C Zone2 - 170C Zone3 - 190 Zone4 - 240 Zone5 - 270

7-zone: Speed - 60cm/min Zone1 - 185 Zone2 - 190 Zone3 - 195 Zone4 - 200 Zone5 - 245 Zone6 - 255 Zone7 - 253

The technician helped us get the profile while doing the install of the 7zone, and at the time the bend/twist issue wasn't noticed and now we're trying to correct it. Oh, and this is a lead-free solder-paste. Thanks!

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New Oven Now Getting PCB Twist | 24 February, 2009

I think you are too hot out of the gate as well as too slow on conveyor. I think settings similar to 130, 140, 150, 160, 230, 240, 260 and conveyor speed at about 28 or 29 IPM would be more realistic but it depends on your paste and oven config/zone length. If you have an unusually thick or dense board you might have to crank it some. I think your TAL would be way too high based on the fact that you are using zones 5, 6 and 7 at high temps that are so close in value.

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New Oven Now Getting PCB Twist | 24 February, 2009

Make sure the rail is'nt set to tight on some ovens the rail may get a little closer in the middle of the oven

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New Oven Now Getting PCB Twist | 24 February, 2009

Wrongway - Interesting about the rails. I'll check to make sure operators are leaving a little 'wriggle room' on the rails in case they are in fact getting narrower in the middle.

SWAG - I suspected the initial temp. might be a bit hot, thus causing a fast preheat rate. I'll try getting a profile with the numbers you posted and see what happens. The profile we've been getting has been putting our time above 217C around 65 to 75 seconds. According to the recommended reflow profile, we want 60-75 sec typical, 90 sec max.

The oven is 134" long, 7 heat zones and 1 cooling zone.

Thanks for the tips guys! Hoping to get this figured out soon.

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New Oven Now Getting PCB Twist | 24 February, 2009

We have to be careful with the cooling zone in our oven (especially lead-free recipes) as the cooling rate is much too high with the fan above its lowest setting. Depending on the design of your oven, it's also possible to pull a lot of heat out of zone 7 when cooling fans are too high = lower TAL. Just something else to watch when you mess around with profiles...

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New Oven Now Getting PCB Twist | 24 February, 2009

Hmm, I'll check the cooling-fan setting too. And thanks for the head's up on how this might effect TAL. Actually, thanks for all the help! :)

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