Author Topic: Basics of hot-air rework repairs  (Read 2571 times)

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Offline sokoloff

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Basics of hot-air rework repairs
« on: April 24, 2016, 03:32:59 pm »
A couple years ago, I bought a hot air rework station and gave myself some practice by stripping components off a bunch of old PCBs I had lying around the junk bin. That gave me confidence to start some repair work (broken connectors, etc) and have had mixed success with surface mount components. I've watched a bunch of youtube videos on the subject and done some practice hand-assembly and repairs/rework on those simple circuits. When it comes to replacing broken connectors, I have a much harder time as the plastics involved aren't as robust as typical SMD passives or chips.

I think part of the problem is that I'm unsure of what temperatures I should be using and the role of flux during removal operations.

Could anyone give some guidance on temperatures and techniques they use for component removal for eventual replacement, and whether that varies for chips or plastic-containing parts? Or recommend some videos with explanations (as opposed to just demonstrations of technique, which I've watched a lot of).

Thanks!
 

Online Fraser

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Re: Basics of hot-air rework repairs
« Reply #1 on: April 24, 2016, 04:08:31 pm »
Watch some of the videos by Louis Rossmann on You Tube as he is often removing the LCD panel connectors.

As a general rule, it is OK to use hot air in the removal of a plastic connector provided you do not intend to reuse it. They normally get burnt and/or melted at solder melting temperatures, especially now that lead free solder is used.

I have specialist solder bar heads for my PACE soldering irons and tweezers. These can melt the solder on a row of connector pins, or in the case of the tweezers, two rows of pins. The connector is often removable without damage and may be reused if needed. You would normally be removing an already faulty connector though !

Fitting a new socket is best achieved using a soldering iron as Louis regularly demonstrates. Plenty of liquid flux and drag soldering works. Tidy up with good solder wick and more flux. Clean flux away with IPA or flux remover.

Some SMT IC sockets are made of high temperature thermoset plastics or bakalite like material. These can often be soldered into place with hot air just like a normal SMT IC as they do not melt.

As Louis has said in his videos, for many situations exact hot air temperatures are not necessary. Set a temperature above solder melting point and try it. If needed, increase the temperature a little at a time. I regularly use 350C without problems. It's about technique more than temperature. Start with the nozzle about 4 inches from the component. Keep the nozzle moving in circles around the item to be soldered or desoldered to warm the PCB. Then move in closer to the component and keep the nozzle moving over the pins until the flux flows and the solder melts. Then gently pull away whilst still circling the component to let the solder cool and harden before touching the PCB. I normally end up with the nozzle circling about 1" from the component when wanting to melt the solder quickly. With desoldering you would of course remove the component when the solder melts.

Also set the air flow such that it is not blowing components off the PCB ! I tend to run at quite high air flows but the mid range point is often fine.

For really fine work I use a hot air pencil and you can solder small plastic connectors with that due to its very fine hot air jet of less than 2mm diameter. It is still risky though.

Most ICs are made of materials that will not melt easily. They can be scorched but then you need to look at technique rather than temperature.

Fraser
« Last Edit: April 24, 2016, 06:35:36 pm by Fraser »
 

Offline nanofrog

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Re: Basics of hot-air rework repairs
« Reply #2 on: April 24, 2016, 04:39:11 pm »
Based on my experience, Fraser is spot on. :-+

If you're worried about temp, start with say 300C, and go up in 15C - 25C increments until it melts in a reasonable amount of time (stay on one spot too long, and you run the risk of scorching the PCB as well as components).

As mentioned, scorching components isn't a big deal if defective; just don't scorch the board in the process. But if you intend to recycle components, you will definitely have to get your technique down. SMD IC's & passives use packaging that can take the heat as a general rule (read the data sheet for any soldering info if you're unsure), but expect to have to replace nylon connectors as they tend to deform/melt at the temps you'll end up using.
 

Offline Raj

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Re: Basics of hot-air rework repairs
« Reply #3 on: May 21, 2016, 03:39:23 pm »
the size of nozzle should be according to size of component to make it easy

don't pull the components with high force, handle them gently

heat evenly around all pins of components

wear eye protection to boost confidence

use flux, lots of it
 


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