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 Reflowing misconceptions 
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Penny Fixer

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Post Reflowing misconceptions
What does reflowing refer to? In the electronics manufacturing industry, the boards are manufactured using solder paste reflowing, or wave soldering, or vapour phase soldering. Solder paste and flux manufacturers will provide time/temperature profiles for their products and studying these profiles will make it clear as to the temperatures and times that is used in the manufacturing industry.

Looking at a typical profile from any of the established solder paste/flux manufacturer and also studying the Jedec profiles, will show that a profile is made up of the preheat, soak, reflow and cooling stages that make up a profile.

In the industry, the boards will move on a conveyor belt and will travel at a rate in and out of the ovens to experience the different temperatures and times before it goes into the cooling zone. Typically the PCB will be heated from room temperature to 150C in about 90 seconds in the preheat oven. This temperature also happens to be the activation temperature of modern fluxes used in LF soldering. The flux acids are inactive until they are heated to their activation temperature.

The soak phase follows the preheat phase and is typically 90 seconds or even 120 seconds.
The flux is responsible for the removal of oxidation from the solderable copper surfaces. Once the flux becomes active, then the rate of temperature increase is slowed down so that the flux can do its job removing the oxidation and also the volatiles can evaporate off at the same time. The flux also helps distribute the heat evenly at this time. This is of course the soak phase and 180 to 200C is acceptable. The fact that the temperature only rises by a small amount, the ramp rate is slowed down during this phase.

The reflow phase is where the maximum temperatures will be reached and the recommended time is 60 to 90 seconds. In order for the intermetallic layer to be formed, it is necessary to raise the reflow temperature by 15-40 deg C above the melting point of the solder alloy. In the case of LF solder, manufacturers will go as high as 260C. The metallic layer is the dissolution of the tin in the solder and copper pads, which forms a micron thick layer and is a function of the time and temperature. Too long a time will produce thicker intermetallic layers which are brittle and likely to form unreliable solder joints. The ideal in my view is to use a maximum of between 235 and 245C and no more than 60 seconds for the whole phase. Let's say at the end of the soak phase the temperature was 185C, then entering the relfow phase, the temperature would be ramped up relatively quickly to say 217C, at which point the TAL (time above liquidus) would start. Following on the temperature would be allowed to reach the desired maximum, say 235C, dwell at this temperature for 10-20 seconds, and then start the cool down stage. The TAL refers to all the times and temperatures above the melting point of the solder, (in this case 217C), and ends as the temperature reduces below 217C. This time between the two points on the curve shoudl be 60 seconds.

The cooling down should be no faster than 6 deg C per second. The profile described above would be considered as a full thermal cycle and shows that very long profiles would be considered multi thermal cycles that is harmful to the BGA. Manufacturer's recommended profiles are the most desirable to use rather than using profiles that someone has arbitrarily chosen to use, with no scientific value. Even though a bad profile may fix a problem, only time will show how much more damage may have been done to the board.

So, in summary the following is sound advice.
Starting from room temperature 25C to140C in 90 seconds.
Carrying on to 180C and the time so far is 180 seconds.
Ramping up to 235 or 245C in the time as the peak is reached is now 210 seconds.
Dwell at the max temperature for 10 seconds and switch off heating. As the temperature starts to fall, and is 217C, the time should be 240 or 270 seconds.

Now allow the board to cool down to room temperature.

Not many machines are capable of producing an ideal profile like above. If you are interested in a machine that can, then PM me.

 
 


Mon Apr 15, 2013 9:09 pm
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Professional

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Post Re: Reflowing misconceptions
Thankyou for the comprehensive description of lead free reflow profiles. It proves that people need to follow industry profiles and not individual trial and error. Too many people have posted profiles in the past based on the temps on their smt work stations withiout even using TCs, then wondering why they get a 20% fail rate within the first few months.

With regards to xbox repair, do u also agree with a low temp (75-80c) pre-heater bake for a few hours to remove flex and moisture in the pcb from years of flexing or hack 'screw/washer fixes'. Do u also wash under the chips with isopropyl alcohol to remove dust/oxides prior to reflow?

Any thoughts on different workstations would greatly appreciated.

 
 


Mon Apr 15, 2013 11:16 pm
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Penny Fixer

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Post Re: Reflowing misconceptions
taijigamer wrote:
Thankyou for the comprehensive description of lead free reflow profiles. It proves that people need to follow industry profiles and not individual trial and error. Too many people have posted profiles in the past based on the temps on their smt work stations withiout even using TCs, then wondering why they get a 20% fail rate within the first few months.

With regards to xbox repair, do u also agree with a low temp (75-80c) pre-heater bake for a few hours to remove flex and moisture in the pcb from years of flexing or hack 'screw/washer fixes'. Do u also wash under the chips with isopropyl alcohol to remove dust/oxides prior to reflow?

Any thoughts on different workstations would greatly appreciated.


Baking a PCB to remove moisture has to be done at a temperature well above the boiling point of water. This is typically 125°C and the time should be about 4 hours at least if you can afford the electricity bill.

I have never cleaned the underside of a BGA. I leave that to the Insat RMA Super Flux which is preferred by many who choose to reflow. While on the subject, I'd like to add a few points regarding Jedec compliant profiles. Personally, I don't go much above 245-250°C as my maximum reflow temperature and usually try and stay at around 235°C. The recommended maximum is 260°C but this would imply perfectly dried boards having been baked for 24 hours or more. It is therefore better to stick to lower maximums when uncertain about moisture levels in the BGA/PCB

Room temperature = 25°C
Minimum preheat temperature = 150°C (at end of preheat, time typically 60-90 seconds)
Maximum preheat temperature = 200°C (at end of soak, typically 90 - 120 secs)
Maximum reflow temperature = 260°C +0/-5°C (stick to 235°C unless board is bone dry)
Solder melting temperature (LF) = 219°C

Average ramp up rate 3°C per second.
Time from end of preheat temp to end of soak stage = 60 - 180 seconds
Time within 5°C of reflow maximum (dwell at max reflow temp) = 20 - 40 seconds
Total time maintained above liquidus temperature of solder TAL = 60 - 150 seconds
Ramp down rate (cooling down) = 6°C per second max.
Time to Peak temperature (from 25°C) = 8 minutes max

Goog luck with your reflows.

 
 


Tue Apr 16, 2013 9:39 pm
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Joined: Fri Jan 27, 2017 9:50 am
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Post Re: Reflowing misconceptions
Hi there! Thank you so much for the comprehensive description! More exclusive information about misconceptions can be found at resume-service.org/blog/5-misconception ... job-search!

 
 


Fri Jan 27, 2017 9:53 am
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Heatgunner

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Post Re: Reflowing misconceptions
Nice post! I love visiting in this thread, thanks for sharing.
goldenslot

 
 


Sat Jun 10, 2017 9:43 am
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