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Reflow profiling of double sided assemblies

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#69045

Reflow profiling of double sided assemblies | 23 May, 2013

How many of you run a different oven recipe for the secondary side of a double sided reflow assembly.

In other words, would you use the same recipe for both sides ?

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#69046

Reflow profiling of double sided assemblies | 23 May, 2013

We would usually have the bottom zones at the reflow with 10 degrees less than the top ones for the secondary side. This we proved helps to keep the parts of the primary side on the board.

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#69047

Reflow profiling of double sided assemblies | 23 May, 2013

We run the same profile for both sides. We also run the bottom temp 10C cooler in the final 2 heat zones.

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#69059

Reflow profiling of double sided assemblies | 24 May, 2013

I run the same profiles for Top and Bottomside assemblies for all of my products. Only in rare instances have I been forced to run a profile with cooler bottom zone settings. This was obviously driven by assemblies with heavier components on the bottom during secondary reflow, or with assemblies with large Electrolytic Caps. Although not very heavy, these electrolytic caps do not have enough surface area contact between lead and pad to allow for surface tension to hold the parts in place.

There is a formula that you can do where you input the weight in grams of the part, the surface area of the leads and pads, and it will give you the probability of the component falling off during reflow. Unfortunately I'm at home today, and do not have access to that formula.

Possibly one of these other fine folks could chime in with the formula?

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#69060

Reflow profiling of double sided assemblies | 24 May, 2013

What you are saying is very interesting and it would be great if you later post this formula.

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#69062

Reflow profiling of double sided assemblies | 24 May, 2013

Google for this: "weight_limit_qfn_smta.pdf" Although it is focused on QFN, formula can be used for most parts...

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#69065

Reflow profiling of double sided assemblies | 24 May, 2013

The formula is a great help, where was it 20 years ago? LOL

After running a few thousand different products over the years, you start to get a feel for what is going to be trouble with regards to falling off the bottomside during the secondary reflow. When we see the potential, we advise the customer through "Mfg Feedback" and ask for permission to provide a bonding agent - usually Loctite Chipbonder to those parts that are likely to be an issue.

I find that funny enough, over time, we have reverse engineered ourselves just about into that formula, in that whenever something fell off, we would find the weight, then the surface pad area plus the solder volume, and put it on the FAIL side of the line. Over time you knew what not to do by seeing your failures. Many times subtle failures like "stretched" solder joints from heavy backside components do not manifest themselves until a higher assembly is done, where a loss of vertical clearance is noted. But all things being said, we still target the same profile for both sides of the boards. 'hege

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Reflow Oven

reflow oven profiler