Help me with SMT resoldering using hakko style hot air tool

[jhallows]

SMT Resoldering Revisted

Well I decided to take a stab at resoldering SMT's but it didn't go as plan. (See pictures).

I am using a hakko style hot air soldering tool.

I apply flux to the SMT then use the hot air tool to heat the SMT then pry out the smt. Some of the traces on the board get's damaged. Is not preheating the board causing the problems?

 

[fuzzzy]

Re: SMT Resoldering Revisted

I think you didn't wait enough. solder was not completely melted, perhaps you used a low temperature and it hadn't time enough. When i do something similar with ho air I always use relatively high temperature and i don't take off chip strongly and quickly. I've never had problems always without preheating board.

 

[pisoiu]

SMT Resoldering Revisted

I also use one similar tool, and it works perfectly. All you need is some experience. When desoldering such ICs with small pads, you have to pull it very, I mean VERY gentle. If you pull it before the soldering material is completely melted on all pads, you will damage the pads, because their adhesion to substrate is much lower due to temperature. Also, you have to take care not to overheat the board, because you will fry it, and if it is a 4 or more layer board, you can delaminate the layers or damage the vias in the heated areas. Damaging the vias is the worst thing, because it will not be visible. Take some unused boards with such ICs on them and practice. I reccomend to set the air speed at minimum, because a high air speed can blow small components away (like R and C in small forms - 0603, 0805 or SOT-23).

/pisoiu

 

[jhallows]

Re: SMT Resoldering Revisted

Is preheater really necessary? I was thinking I overheated the board. I didn't have a maual for this thing, (it is in chinese), but I found out there is a chart to set the tempertaure and the air speed for each nozzle. I will take your advice and be more patient and practice with some old boards.

 

[johnyaya]

Re: SMT Resoldering Revisted

For rework, I don't use a preheater a lot of the times. However, I do set the air flow higher than for production (original) soldering. The surface tension of the already melted solder will keep parts on the board. For original soldering, higher air flow will blow the small parts around because the solder isn't melted yet, and the surface tension from the hot solder paste is nill. The higher air flow helps heat the part and the pcb up faster. Larger parts take longer to heat all leads, and a preheater definately makes removal of large parts faster. When all leads are hot enough (all solder in liquid state) the part can be simply lifted off with very little effort. Avoid making the air stream too hot as it will weaken the adhesive between copper and board. I set my unit for 750F (400C). When teaching others SMT techniques I usually need to remind them that their through-hole soldering wasn't perfect from the start either. Just takes practice. If you have some worthless PCBs laying around, practice on them first.

 

[jhallows]

Re: SMT Resoldering Revisted

Avoid making the air stream too hot as it will weaken the adhesive between copper and board.

Okay so what is the recommended temperature? You said 400C and that was the temperture I was using. Is too much flux bad? Roughly how long to to leave the hot air above the component and how far above the compenent?

I was trying 100 Pin QFP without a preheater and the solder wasn't melting until after 4 minutes. Was that too long?

 

[johnyaya]

Re: SMT Resoldering Revisted

I don't use flux for desoldering. I definately use flux for soldering though. I could see that an excessive amount of flux could delay the solder from reaching melting point, kinda like heating a cup of water versus a pot. The package itself doesn't need to be at the solder melting temperature, just the feet and pads. Your picture of the hot air wand shows the correct kind of tip for the package, although I cannot see the exact dimensions. I usually use a single flat tip on mine for everything. When I use the single flat tip I slowly work around the part until all leads have the solder in a liquid state. I prefer the single flat tip because I can angle the air stream towards the package, not just straight at the PCB. I would check a couple things. You want the temperature at the part to be 400C not just somewhere in the wand. I'm not sure where the unit you have measures the temperature, but if you have access to a thermocouple probe for a DVM you can adjust the temperature so that the air temperature is 400C at the distance you are holding the wand from the part. I hold the tip 10 to 15 mm from the part. A higher air flow will heat the part up faster. All of the hot air soldering stations I've used can have the air flow turned up too much. The air then goes through the wand too fast and can't heat up to the desired temperature. Again, the thermocouple probe will help you find where that point is. The initial soldering profile takes 2 to 3 minutes. Four minutes is a little long, but not bad for the size of the part. It shouldn't take more than 5 to 6 minutes on a part.

 

[jhallows]

Re: SMT Resoldering Revisted

I could see that an excessive amount of flux could delay the solder from reaching melting point, kinda like heating a cup of water versus a pot.

I think I put too much flux.

Your picture of the hot air wand shows the correct kind of tip for the package, although I cannot see the exact dimensions.

Here is the tips I have for my unit.

**broken link removed**

Thanks for the tips. I am going to try this.