Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


double sided reflow

Jason

#3729

double sided reflow | 21 June, 2000

We are going to try double sided reflow for the first time. Can we use the same 63/37 paste on both sides? The board also has bare copper pads i'm pretty sure. will this present any challenges? The bottom side has a QFP on it. Should it stay on? I am unsure of it's exact size yet. Last but not least. Anyone have a rule of thumb or just a suggestion on how much we will have to lower the heat on the lower zones of our oven and can we leave the upper zones set as though we were on soldering a single side. I read the archives on this as much as i could. Any info is welcome. also can anyone tell me where i can get "Soldering in Electronics by R.J. Klein Wassink" in the USA? thanks all.....

reply »

#3730

Re: double sided reflow | 22 June, 2000

Jason,

Double sided reflow should be no problem. People have been doing it for years. The surface tension of the molten solder will hold the components on the bottom side during your second pass. This is almost always the case, unless you have really, really heavy components. A QFP should hold on no problem. If you want to run calculations beforehand to make sure the parts stay on, there is a rule of thumb - 30 g/in2. Calulate the pad area and the mass of the part. If the ratio is less than 30 g/in2, you're in fine shape. Another calculation is .375 g/in of joint length, which means calculate the perimeter of all the pads, and use the mass of the part in the calc. Either way, it's pretty simple.

I would not suggest using lower bottomside temps than topside temps. Your oven will not be very accomodating in terms of temp control, and even if it were, you would be heating one side of the board more than the other and asking for warpage.

The only real consideration in the oven is the conveyor. If it tends to vibrate or jerk, you can shake the parts off by disrupting teh surface tension that's holding them on. If it runs smoothly, you'll be in fine shape.

Chrys

reply »

#3731

Re: double sided reflow | 22 June, 2000

Jason,

Double sided reflow should be no problem. People have been doing it for years. The surface tension of the molten solder will hold the components on the bottom side during your second pass. This is almost always the case, unless you have really, really heavy components. A QFP should hold on no problem. If you want to run calculations beforehand to make sure the parts stay on, there is a rule of thumb - 30 g/in2. Calulate the pad area and the mass of the part. If the ratio is less than 30 g/in2, you're in fine shape. Another calculation is .375 g/in of joint length, which means calculate the perimeter of all the pads, and use the mass of the part in the calc. Either way, it's pretty simple.

I would not suggest using lower bottomside temps than topside temps. Your oven will not be very accomodating in terms of temp control, and even if it were, you would be heating one side of the board more than the other and asking for warpage.

The only real consideration in the oven is the conveyor. If it tends to vibrate or jerk, you can shake the parts off by disrupting teh surface tension that's holding them on. If it runs smoothly, you'll be in fine shape.

Chrys

reply »

Boca

#3732

Re: double sided reflow | 22 June, 2000

Jason,

What Chrys said! Don't try to run the bottom cooler, tried it in the mid 80's, don't work and don't want it to work. If one side of a fab is maintained cooler than liquidus and the other side into reflow it would have an easy 40C temp difference from one side to the other, think of the stress on the fab and inner layers... If reflow temperature is approched then intermetalic growth is accelerated, the intermetalic region is often the weakest part of the joint. If reflow is achieved then it appears that the many of the intemetalic compounds are 'sloughed' off into the body of the joint instead of at the pad to joint junction. The reduced intermetalic's at the pad to joint interface increases the overall strengh of the solder joint.

If the fab is copper and has an OSP on it, pay attention to the number of washes the fab goes thru. The OSP's we've seen are removed in the wash cycles, and the number increase with the number of solder steps the fab goes thru. As the OSP's are removed, oxidation sets in, and you lose solderablity. You may be able to to 1st and 2nd reflow before any washing to preserve more of the OSP.

HIH (hope it helps)

Boca

reply »

#3733

Re: double sided reflow | 22 June, 2000

Hi Jason - I'll second (third?) everything said. Wassink's bible is available in the US from the SMTA bookstore with free shipping. It's pricey but worth every penny. 612/920-7682 www.smta.org jesse@smta.org John Thorup

reply »

Bob Willis

#3734

Re: double sided reflow | 23 June, 2000

Also I suggest you read through the Charity Report I produced on the specifi process of Double Sided Reflow Assembly which you can also get from the SMTA. All the money from the sale of the report goes to the Charles Hutchins Scholarship Fund which The SMTA run.

Bob Willis Technical Director SMART Group

reply »

reflow oven profiler

Reflow Oven