Electronics > Beginners

Trying to solder with a cheapo hot air gun.

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mikerj:

--- Quote from: sourcecharge on July 30, 2018, 06:29:46 am ---I was under the impression that in order to have good solder joints, you have to heat the part up first, then bring the solder to the heat, not the other way around.

--- End quote ---

For items with larger thermal mass (including through hole parts) this is the correct way to solder.  For SMD parts that have tiny thermal mass it's easy to make a good joint by reflowing solder with an iron.


--- Quote from: sourcecharge on July 30, 2018, 06:29:46 am ---I've seen the videos, and heard the comments about putting the solder down on the pads, or on the tip first, but I've been told by an experienced electronics engineer, that this will make bad solder joints.

--- End quote ---

This is simply not true.  That said the solder must be able to wet both parts of the joint, and use of a suitable liquid flux in a pen style dispenser will ensure this.  With just a little practice soldering SMD parts like this becomes second nature, and far easier and quicker than putting paste down and using hot air.  If you have steady hands and a good magnifier (or preferably stereo microscope) then even soldering 0201 parts is not too bad, though I do struggle with 01005.

JS:
The reason of the bad joints when reflowing the solder on the pad is the lack of flux, if you tin half of your pads for SMD parts (on of each component, don't be stupid and tin both pads of half the components) apply flux in excess to the board and solder all those joints, then solder all the remaining pads your joints will be good. Bad solders are usually due to oxidation and that's because of the lack of flux, working under a tab of flux would be nice... they do it in wave soldering where the atmosphere is oxygen free, usually nitrogen only, that way the solder can't get any oxygen on them.

JS

rstofer:

--- Quote from: janoc on July 30, 2018, 03:23:57 pm ---
You don't put solder on the tip of the iron (except to initially tin it) but you do need to put solder on the pads! Unless you have 3+ hands you will have hard time to hold the part & and the solder and the iron at the same time if you don't put solder on the pad first.


--- End quote ---


Actually, I do!  I put flux on the pads, place the part and then put a dab of solder on the tip and apply it to the component/pad interface.  The flux makes sure the solder flows quickly and neatly.  Kind of like drag soldering without the drag.  I prefer the method to tinning the pad because the component starts out laying flat to the board.

If I hold the part down with the tweezers perpendicular to the axis of the part, I can solder both ends without moving the tweezers.

I have also done the same kind of thing with solder paste.  Put a dab of solder paste on each pad, place the part, hold in place with tweezers and apply the iron tip to the paste.  Works well...  But not as well as bedding the component in flux.  Flux is the key to SMD.

For a board with a lot of SMD components, a hot plate works well if the board is small.  I haven't tried a frying pan but it may work well if it is flat enough to heat the entire board and I built up a reflow oven from a Black & Decker InfraWave Toaster Oven.  It does a fine job.

This SMD soldering thing just isn't any big deal.  Drag soldering and a little SolderWick to fix the inevitable bridges and it's done.


KL27x:
^Yep.

There are a lot of NASA and telecom rules which are meant for thru hole parts and for splicing wires, using flux core solderwire, specifically. NASA probably doesn't do a lot of hand-soldering of SMD parts. Their PCB's are mostly going to be oven reflowed, just like every other industry.

The thing about loading the tip with solder is you need to consider the type of tip to use for this. Really fine chisels have a section of tinned conical shaft right next to the tip, similar to fine pointed conical tips. Surface tension can suck the bead up the shaft, away from the tip. These fine, conical shaft tips are generally going to be very clumsy and messy if you try to load them with a solder bead.

If you want to paint the solder on (to prefluxed pads, of course), you need so choose the right tip for the job.


--- Quote --- took my time to align the resistor to the pad, I tried using tape, I tried holding it down with tweezers, but what felt like forever, it kept moving off of the pads.
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I have done production SMD soldering for a spell, now. What you're describing will come to pass; that's just you not knowing what you're capable of and what you can get away with. What you are describing, you are totally not taking advantage of several things, not least of which is the soldermask layer and extra flux :). When repeatedly doing something that bores you out of your skull, this is where you start learning tricks. If my soldering iron happens to be closer to the part than my tweezers, I will often grab the passives/caps with my solder bead and drag it to the pad. Then by the time my tweezers catch up, they just hold the part while I remove the iron. (I mostly use a huge CF tip for soldering tiny passives. Turned sideways at the right angle, the resistor/cap will align to the tip, pretty straight and flat to the board.)

When you know how to do SMD, it is overall much faster and easier than thru hole. The most tedious part is getting the part from the package to the board. This is another reason it helps to flux the pads. When you drop the parts on there, they are glued down; they won't disappear when you nudge the board. If you are placing one part on the board, then soldering it, then going back and getting another part, you will not realize the speed/ease benefit over thru hole.

Another thing is learning how trivial it is to fix most mistakes. There are a ton of mistakes I was initially afraid to make. And I end up spending time to avoid making them. When it is faster to occasionally make one and fix it when it happens. 
 

Nerull:

--- Quote from: sourcecharge on July 21, 2018, 10:56:05 am ---
--- Quote from: janoc on July 21, 2018, 10:09:57 am ---I don't understand why are you messing with hot air and paste on the uCurrent boards. All those components are perfectly solderable by hand and it would likely be also faster and less messy than dealing with paste.

Keep the paste for boards with hundreds of parts or stuff that doesn't have accessible pins.

Hot air tools are primarily used for rework, not general soldering. They can be used to solder in a pinch but there are better tools/methods for that where you don't risk blowing the components all over the place, melting plastic and delaminating the PCB.

Your original Harbor Freight "gun" is literally meant for paint stripping. That should give you a hint that it probably is not an appropriate tool to use for electronic soldering. That's kinda like trying to solder with a propane torch. Possible but the results will match the tool used ...

If you want a hot air tool, you need to have one that has both the temperature and the air flow adjustable. It also needs swappable nozzles so that you don't "cook" half of the board when you are trying to (de)solder one resistor.

That Circuit Specialist one is a cheap junk, don't expert any miracles from that especially if you plan to desolder/rework larger components. But you don't need any of this for the uCurrent boards. Just use a normal iron and normal solder, add some flux and you will be fine.

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My FG died on me once and I tried to fix it unsuccessfully.  It had SMD parts.

The biggest problem I had before using a soldering iron was to keep the part aligned to the pads.  Is there something I can use that will solve this problem?

--- End quote ---

A pair of tweezers? It's really not hard. Soldering 0603 takes about 5 seconds.

Using a hot air gun, especially one not built for soldering, will take far longer and is likely to just scatter your parts across your desk.

I've seen people with essentially zero soldering experience learn to hand solder SMDs with just a little bit of practice, and i've seen the same people completely destroy a board the first time they tried a hot air station. It is not the magic solution you think it will be.

Get a good pair of tweezers, some paste flux (liquid works, but spreads too far and dries up too quickly for my taste), and a small chisel tip. Tin one of the pads, put the part down, touch the pad you tinned for a second. Part is now attached to the board. Add solder to other pad. If the first pad needs a little more solder, you can add it now. Otherwise, you're done. All of this before your hot air station warms up.

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