Inexpensive Sot-23 hot air setup

[John Coloccia]

I'm looking to put together a simple and inexpensive setup for soldering Sot-23. I have two components per board that I need to install, so it's not worth having a board house run it through.

I'm not well versed in hot air stations. Are the cheap stations OK to use? It's not the same sort of thermal mass/quick cycling that irons need to deal with so I'm hoping a cheaper station would suffice.

Also, I'd like to have some sort of solder paste and dispensing solution. Something that won't break the bank and doesn't require cleaning. I'm thinking something like Kester R276, along with some sort of dispensing apparatus.

Anyhow, I realize I can just solder them with an iron, and I do that now, but I'm looking for something a little faster.

Any help is appreciated!

[John Coloccia]

I should also say that I don't have my heart set on a solder paste dispenser. If it would work just as well getting a stencil style solder paste, and just dispensing it with a toothpick or something like that, I'd be open to that if that works. I just need a solution that's quick, easy and works.

[nctnico]

I strongly doubt hot air will be faster. You'll need to heat up much more mass and paste is messy.
What might do you more good is glueing the parts to the board prior to soldering. That way you don't need to hold the component in place during soldering with an iron so you can work quicker.

[Howardlong]

I don't bother with hot air for packages like sot-23 where you can solder pin by pin.

For manual low volume assembly I use exactly the same method as I do for most leaded SMD parts, i.e. wet one pad in one corner, hold the part with tweezers and solder that corner pin. Adjust fine orientation as necessary: for sot-23 just an illuminated magnifier is enough, but for smaller pitches a microscope becomes more of a requirement. Then once one corner's anchored down, do the other pins, starting with the opposite corner. With a fine tip you don't need to resort to drag soldering at sot-23 pitch.

For desoldering, I find soldering tweezers the fastest, together with a generous helping of solder on each side, but you could use air. I usually save the air for larger SMD parts.

[Fraser]

+1 for hand soldering with a fine wedge tip.

Hot air is really only justified for more demanding components and easier component removal.

Fraser

[Monkeh]

You're not going to save time by using hot air for simple parts like that.

Get some paste in a syringe. Paste pads, place part, hold with tweezers and touch with the iron.

[Howardlong]

You're not going to save time by using hot air for simple parts like that.

Get some paste in a syringe. Paste pads, place part, hold with tweezers and touch with the iron.

Happy to be educated, why is paste easier than the wetting the pad method? I find getting paste reasonably distributed in any way other than with a stencil to be akin to pinning a tail on a donkey.

[Monkeh]

You're not going to save time by using hot air for simple parts like that.

Get some paste in a syringe. Paste pads, place part, hold with tweezers and touch with the iron.

Happy to be educated, why is paste easier than the wetting the pad method? I find getting paste reasonably distributed in any way other than with a stencil to be akin to pinning a tail on a donkey.

If you wet the pad you still need flux for the component. Paste will deal with both.

[Howardlong]

You're not going to save time by using hot air for simple parts like that.

Get some paste in a syringe. Paste pads, place part, hold with tweezers and touch with the iron.

Happy to be educated, why is paste easier than the wetting the pad method? I find getting paste reasonably distributed in any way other than with a stencil to be akin to pinning a tail on a donkey.

If you wet the pad you still need flux for the component. Paste will deal with both.

There's flux in the solder, you retouch the anchor point if necessary, you don't need to apply extra flux. What I still don't get is how you dispense the right amount of paste in about the right place, hence my tail/donkey comment. My attempts at doing what you suggest have generally resulted in too much or too little paste on the pads, and little consistency, that's why I'm trying to figure out how this method works.

[Monkeh]

You're not going to save time by using hot air for simple parts like that.

Get some paste in a syringe. Paste pads, place part, hold with tweezers and touch with the iron.

Happy to be educated, why is paste easier than the wetting the pad method? I find getting paste reasonably distributed in any way other than with a stencil to be akin to pinning a tail on a donkey.

If you wet the pad you still need flux for the component. Paste will deal with both.

There's flux in the solder, you retouch the anchor point if necessary, you don't need to apply extra flux.

Not once you've burnt it off with the iron. Sure, you can just tack one then feed solder in at each and every pad, but that's boring and means juggling wire too.

What I still don't get is how you dispense the right amount of paste in about the right place, hence my tail/donkey comment. My attempts at doing what you suggest have generally resulted in too much or too little paste on the pads, and little consistency, that's why I'm trying to figure out how this method works.

Practice and the right size needle. Once you get it down you paste all the boards in one go, then place the chips in one go, then just gently hold in place while touching with the iron. Just like hot air, except you can do all the joints in about five seconds total instead of having to heat the whole area.

[John Coloccia]

Yeah, Monkeh's nailed the issue. I don't want to have to apply additional flux because then I have to clean, and I obviously don't want to reflow with more solder and leave big solder glob there.

I like the idea of paste with an iron. I hadn't really thought of that. Honestly, I really don't want anymore equipment on my bench so that sounds like an excellent solution. And his suggestion is exactly what I had in mind. Apply paste on 20 or 30 boards, and then slam through soldering them.

It doesn't seem like tacking them down and then soldering pin by pin takes a long time, but when you have anywhere from 200 to 400 of them to do, it's really quite tedious and slow, especially because there's no way to really batch the steps.

Could you recommend a dispenser, Monkeh?

[Howardlong]

I don't need to apply additional flux. What's in the solder is plenty.

Whether you apply additional flux or not, you may well want to clean anyway.

The problem is I've never managed to find a reasonable way of applying the right amount of paste in the right place unless I use the stencil method, which is not really appropriate to this. Because of that, you're more likely to need to clean the board after using solder paste than just with good old fashioned solder wire.

Practice and the right size needle.

What is your recommended dispenser and needle?

Once you get it down you paste all the boards in one go, then place the chips in one go, then just gently hold in place while touching with the iron. Just like hot air, except you can do all the joints in about five seconds total instead of having to heat the whole area.

Hmm, I re-read the OP. It seems as though there's two SOT-23s per board on many boards. I can't see that worth a stencil, so if you can dispense the solder reasonably I could see it working, but I'm just not convinced it's any faster than the old fashioned way!

[Monkeh]

I don't need to apply additional flux. What's in the solder is plenty.

Only when applying wire. Which means grab wire, tin pad, grab tweezers, align component, roughly tack, drop tweezers, grab wire, solder the other pads, reflow the tacked pad.. You spend all day picking up and putting down tools.

What is your recommended dispenser and needle?

A syringe (small quantities of paste come in one) and a needle slightly smaller than the pad width.

[Howardlong]

A syringe (small quantities of paste come in one) and a needle slightly smaller than the pad width.

I have tried a number of syringes and needles of various sizes, and can dispense manually or with a pump, but I have never had any luck on pining the tail on the donkey reasonably well. What is your needle size and syringe? Is it manual or with a pump mechanism?

[Monkeh]

The syringe is whatever the paste comes in, I don't have a specific brand for a tube with a plunger..

I'm running down a box of apparently 25 gauge dispensing needles currently. A little small for most paste, but it works. I just use the syringe, no fancy pump, hence the need for practice.

[ebclr]

This don't work



This work



https://pt.aliexpress.com/item/on-sale-TT-Tapered-Tips-14G-25G-dispensing-needles-dispensing-tip-glue-dispensing-needle/32288087823.html?spm=2114.02010208.3.28.t3ZVGK&ws_ab_test=searchweb201556_0,searchweb201602_3_10057_10065_10056_10068_10037_10055_10054_10069_301_10059_10033_10058_10032_418_10073_10017_10070_10060_10061_10052_10062_10053_10050_10051,searchweb201603_2&btsid=acf721e2-7d8c-48a1-9f83-b37860b37254

[Monkeh]

This don't work

This work fine.

[ebclr]

Try the other one, and you will change your concept of work

[John Coloccia]

I don't need to apply additional flux. What's in the solder is plenty.

Monkeh nailed it again. When you tack down that first pad (tin pad, heat, move on), then you need to come back and reflow it, and for that you really should either add flux, or add more solder which includes flux, and either way makes a mess. If I had a couple more hands, I could skip tacking it down and just solder it properly from the outset. With paste, I have one additional hand free to hold the part in place because I don't need to feed wire.

Definitely not worth a stencil. The reason it's much faster is that I can do it in batches. As Monkeh said, doing them with just an iron and solder has you forever picking up and putting down different tools and things. With the paste, it's

1) goop a batch of boards
2) place parts on a bunch of boards
3) hold down with tweezer, and touch with soldering iron

It doesn't seem like it would save much time but it does.

One day, I'll convert most of the components over to surface mount and just have them populated the modern way, but I don't like doing that until a design has been stable for a while. I'm forever tweaking things for customers, and using old boards to prototype new designs. That's much harder to do surface mount.

[nanofrog]

Given it's only two SOT-23's per board and you're trying to avoid cleaning, I'd go with hand applied paste using a syringe on a reasonable number of boards (i.e. 20 at a time). Then come back and set the components on the pasted boards, then hit the pins with an iron.

Doesn't require any additional tools than you already have, and will get the job done without swapping tools so much. 

As per paste, if leaded isn't an issue, I'd go with Kester EP256 (63/37 no-clean) as it flows very nicely, has a good shelf life, and is rather forgiving on time IME. CML Supply sells it in syringes suitable for hobbyists/prototyping/repair.

[John Coloccia]

Given it's only two SOT-23's per board and you're trying to avoid cleaning, I'd go with hand applied paste using a syringe on a reasonable number of boards (i.e. 20 at a time). Then come back and set the components on the pasted boards, then hit the pins with an iron.

Doesn't require any additional tools than you already have, and will get the job done without swapping tools so much. 

As per paste, if leaded isn't an issue, I'd go with Kester EP256 (63/37 no-clean) as it flows very nicely, has a good shelf life, and is rather forgiving on time IME. CML Supply sells it in syringes suitable for hobbyists/prototyping/repair.

I didn't realize you can dispense it by hand with a syringe. I thought you would need a dispensing pump. Cool! Sounds like I don't really need to buy anything, which is great. It's not like I don't like new tools, but the thought of cluttering up my already crowded workshop with boring stuff like solder paste dispensers/hot air is really not too attractive.

Thanks everyone for the help. I'll pick up some of that paste and give it a try.

[Monkeh]

Just a syringe is fine, just watch out for the pressure needed.

Make sure you let it warm to room temperature before using or it'll be like trying to extrude solid lead.

[John Coloccia]

Given it's only two SOT-23's per board and you're trying to avoid cleaning, I'd go with hand applied paste using a syringe on a reasonable number of boards (i.e. 20 at a time). Then come back and set the components on the pasted boards, then hit the pins with an iron.

Doesn't require any additional tools than you already have, and will get the job done without swapping tools so much. 

As per paste, if leaded isn't an issue, I'd go with Kester EP256 (63/37 no-clean) as it flows very nicely, has a good shelf life, and is rather forgiving on time IME. CML Supply sells it in syringes suitable for hobbyists/prototyping/repair.

Just ordered some from CML. We'll see how it goes. Thanks again!

[Kjelt]

Make sure you have paste suitable for syringe (often sold in a big syringe) instead of paste for stencils. The latter is much cheaper but the solderballs are much bigger so you can not use it in the smallest needles.

[nanofrog]

Just ordered some from CML. We'll see how it goes. Thanks again!

You're welcome. 

Flows pretty good with the needles that come with it. Just make sure it's at room temp before using it as Monkeh mentioned. It'll take a little practice to get the pressure right to dispense the right amount, but you should be just fine after the first few boards (fortunately a cotton or foam swab fixes mistakes very easily).