Advice on SMD Soldering

[Supercap2F]

Hey Everyone!

I have never done any SMD soldering before, but I'm thinking of learning the skill. Is there any special tools that would be good to have? Like special tweezers? Can you share your best techniques for SMD soldering? Like how you hold your components down, how you apply the solder, etc.

Any other general advice on SMD soldering is appreciated.

Thanks

Dan

 

[Gryd3]

Depending on answers, I can see this being a common reference.

 

[(*steve*)]

Ideally, you need:

• liquid flux
• solder paste
• metal (preferably curved tip) tweezers
• rework tool or soldering iron with a fine tip.

There is typically a lot of liquid flux in solder paste, but I find that a little bit extra helps.

Solder paste will save you a hand. You can use normal solder, but holding the iron in one hand, the tweezers in another, and the solder in your remaining hand can be a problem for some.

Ideally with solder paste you use a hot air rework tool, but a fine tipped soldering iron will also work. Just be careful of all the solder wicking up onto the tip!

1. Place paste on the board (a very small amount is needed for hot air). Generally speaking if you cover the entire pad when you apply it with a syringe, you may have applied too much.
2. place the component onto the solder using tweezers.
3. Use the tweezers to hold the component down. Don't worry if solder paste squishes off the pad.
4. If using hot air, you may be able to get away with removing the tweezers (but be careful you don't blow components off the board. Moving the jet of air in a circular manner, lower the end of the hot air tool toward the component. The solder will melt, if not using tweezers the component will either pull itself into place or blow off the board (you're aiming for the former). Once all the joints have melted the solder pull the rework tool away in a manner opposite to how you moved it in.
5. If using a soldering iron, hold the device in place until you solder one leg, then check that everything is aligned. If not, nudge it with the tweezers as you melt that leg again. On large components then tack down a leg on the opposite corner. If you're not using solder paste, tin the two pads first. This normally provides enough "stick" to hold the component in place. If using solder paste you often need to get it right first try. When using paste, solder each leg by placing your soldering iron on top of the pin or at the junction of the body and the board (for resistors and similar components). If using solder, solder exactly the same way you would solder anything else. Each joint should only take a fraction of a second using paste and under a second using *fine* solder. The tacked joints will need to be properly soldered after you have done some other pins.
   
6. Don't worry about solder bridges until you're finished. In most cases you can remove solder bridges by either wiping a soldering iron tip through the bridge, or for larger bridges (too much solder) using solder wick will be effective.

Solder paste, if not used excessively will tend not to form bridges but may leave small balls of solder. It is very much worth your time to clean the board afterwards as these balls, although small, may be large enough to bridge very fine pitch devices.

Whilst I suggest a very fine tip, there are other techniques which allow you to use a tip that is large in comparison to the leads. You-tube will probably have examples of "drag soldering".

with hot air, if your pins are a little bent it is possible that not all pins will be soldered even though the solder has reflowed. Check joints under magnification and fix anything that looks dodgy.

Perhaps the most important thing is: find a source of cheap SMD parts and practice soldering them. You can even find dead boards and practice removing and reapplying components (although this won't give you the practice of soldering onto "naked" copper.

 

[davenn]

And I strongly suggest ... BEFORE you start working on SMD components that have cost you money

retrieve some old SMD boards out of old gear and practice lots with removing and resoldering various parts


oops ... sorts repeated Steve's last comment
cheers
Dave

 

[shumifan50]

I find, with components like resistors and caps, that tinning the one pad, tacking the component to that pad while holding with tweezers and positioning it correctly, to be the easiest. This leaves the component sitting nice and flat against the PCB.

Add de-soldering braid (solder wick) as an essential tool.

Start with 1206 components as they are a bit more forgiving, or if you're brave start with 0804.
I find SOT23 transistors are easy to work with.
SOIC chips are also easy components to use.

AT ALL TIMES KEEP YOUR SOLDERING IRON TIP CLEAN and use a temperature regulated soldering iron - preferably one that you can select the temperature.

The 'drag soldering' that Steve mentions takes a bit of practice, but works well. Solder wick is a must when using this technique.

When buying components, be sure to buy exactly the right package as many components come in a selection of packages that are not that easy to distinguish in the component pictures as they look remarkably similar but their lead spacing is different(burnt my fingers a few times before learning).

 

[Supercap2F]

Lots of great advice here!

Solder paste costs about US$15 for 15g. That seams kind of pricey, how fast would I use that up (like how many pins do you think I could do with that much)? Is This what you mean by "liquid flux"? How long would that last? I have some tweezers that I think will work, I will post a photo of them later and see what you guys think. I am thinking of making a PCB with only SMD pads on it, and practicing with that.

Thanks Guys

Dan

 

[(*steve*)]

Solder paste tends to go off before you use it all unless you're using it all the time.

think volume. A ml is a cubic cm. There are 1000 ul in a ml. One ul is a cubic mm. One cubic mm of solder paste is more than you'll use on a so8 pin, and maybe a little less than you'd use on each end of a 1206 resistor.

for longest life, freeze the solder paste for long term storage, and refrigerate it between uses.

The flux in the paste tends to be volatile and the paste will dry out if left at room temperature.

Better solder paste is better, but you can get very cheap solder paste from China which is ok for non critical stuff.

mixing liquid flux with the paste might seem like a good idea, but manufacturers warn that if the fluxes are not compatible bad things will happen. I'm not sure it ca be worse than having to throw away dried solder paste.

dispensing solder paste is an art of itself. Using a syringe can result in you dispensing uneven quantities of solder until you're well practiced. More elaborate dispensers may cause you to waste more solder than you use. A colleague of mine has experimented with just dipping the legs of chips into the solder paste before placing them on the board.

for really small jobs I've been known to apply paste with a toothpick.

 

[Supercap2F]

How do you think these will work? (sorry for the crummy photo, but I didn't feel like getting out my dslr) How would I apply the flux (assuming that the link in post #6 is what you meant by liquid flux)? Put in on the pad, and then put the solder paste on top of it?

Thanks

Dan

 

[shumifan50]

I have never used solder paste and do all my SMT work with a roll of solder(22SWG 60/40 tin/lead alloy from Rapid in the UK) and a flux pen. The roll of solder has an infinite shelf life. I don't have a reflow tool, so soldering iron it is for me.
I tend to use tweezers that require pinching to hold the part, not spring loaded like the one in the picture, although that might work as well.
Removing parts:
A. 2 pad parts you can normally heat both sides in time to remove the part.
B. Multi-leg ICs I cut the legs against the body and remove them one at a time(remember no reflow tool).

 

[davenn]

How do you think these will work? (sorry for the crummy photo, but I didn't feel like getting out my dslr) How would I apply the flux (assuming that the link in post #6 is what you meant by liquid flux)? Put in on the pad, and then put the solder paste on top of it?

Thanks

Dan

Looks good. I have some like them myself. Doing SMD would be almost impossible without them

For my home projects I have always used roll solder the really fine ( thin) stuff. Will put a little solder on ONE pad only, bring the component to it and solder just that one end, making sure the component is sitting flat. Then solder the other end ( great for anything other than close spaced pins of a micro processor chip)

I did use solder paste at another place I worked where I was regularly changing ATMega uPC and other multi pinned chips. In those cases the solder paste excelled

cheers
Dave

 

[(*steve*)]

I use tweezers that look very much like the ones on this page. I prefer those with very fine tips as they can slip under a chip to allow me to turn it over (SMD components always land upside down) without having to grab a pin -- the pins are very easy to bend, especially on 0.5mm pitch devices. This is less of an issue with 1.27mm pitch.

In addition to bending pins, it's just a good idea from an ESD perspective not to touch the pins.

I mostly agree with the tinning one pad method, except for devices with lots of pins. You really need to tack down opposite corners as the leads on these are often pretty flimsy.

Without a hot air rework tool, solder paste is sometimes useful as it can free up one hand. For soldering components using hot air, it's pretty much a requirement.

I mentioned liquid flux (a flux pen is an option) because when using solder you may use so little that the flux contained within it is insufficient (another big plus for solder paste -- it has lots of flux in it).

Has anyone mentioned lots of light and a magnifier? Some people do this work under a binocular microscope.

 

[davenn]

Has anyone mentioned lots of light and a magnifier?

haha at my age, they are a permanent feature in both my workshops ... I cannot even start any work without them, not even on normal through hole components

The one at work is a fluoro tube and the one at home is a ring of LED's

 

[Supercap2F]

Well I got out a old SMD component that I got in a electronics lot, and a PCB with a IC missing. Here how it turned out:

Pin three and four didn't turn out so good. I used those tweezers, and they didn't work so well. I used a soldering iron with about a 1.5mm tip, 1mm dia 60/40 solder. I think I'm going to invest in a finer tip for my soldering iron, and some thinner solder!

Regards

Dan

 

[(*steve*)]

First tip, use about 1/4 of the amount of solder (or less!). The amount of solder you need is *tiny*. You only need to fill the gap between the bottom of the pin and the pad.

The solder should be on the pad and up the sides of the leg, not on top of the leg.

Place the (almost dry) soldering iron tip on top of the leg, then feed a *tiny* length of solder to the joint between the leg and the pad.

If you place flux on the pad first, you won't need so much solder (because thin solder has only a tiny amount of flux in it).

Finally slide the tip off the leg in the direction away from the body of the IC. If the tip was dry then some excess solder will flow up to the tip and be carried away.

1mm solder is a little too large, you need about 1/2 a mm of it! Try to get some 0.3 mm to 0.5 mm solder. I use 0.7 mm solder, but you still need a fine touch.

A finer tip is not necessarily required (but will likely be helpful. I have a 3mm tip on one of my irons and it can do this sort of work. Remember that you can use the edge of the tip, you don't have to use the flat part (assuming it's not a conical tip).

Employing some solder wick to these joints will probably clean them up and remove the bridge, but due to the amount of heat required, I don't recommend using this method as a matter of course.

 

[Supercap2F]

Thanks Steve! Great detailed advice on how to apply solder to a components pins! Yeah, I'm using a conical tip soldering iron. I think another reason it turned out so bad was I applied solder to some old solder that was on the pad already (you can see the pads with the old solder behind the IC). Heres a test I did awhile back on a PCB with no solder on the unused pads.



All the best!

Dan

 

[(*steve*)]

OK, that's looking a little better, but notice that your network resistor is not quite aligned with the pads.

That's not a huge problem, but it does appear that the leads are too far back. The entire flat section of the component's lead is normally on the pad.

I'll try to draw it.



The grey is the pin, the orange is the copper pad, and the red is the solder.

The pad normally extends back behind the pin.

In your case, it looks like you're getting this:



The important part about having the lead positioned correctly is that it makes it easier to get a good joint.

Looking at the leftmost pin in your image above, it appears that the connection at the front of the pin is imperfect. Whether or not this is actually soldered, or soldered well is hard to tell. The 5th pin is also suspect.

What you're ideally looking for is a small fillet of solder up the edges of the pin.


The solder should flow up the edges like this. It's pretty much the same shape as you'd see on a soldered joint for wire coming through a hole in a pad, just smaller.

 

[Supercap2F]

Heres the last test I can do: (I ran out of SMD ICs)



I tried to do what you said.

Regards

Dan

 

[BobK]

Halve the amount of solder once more and you will be there!

Bob

 

[(*steve*)]

Whilst I agree with Bob about the quantity of solder, I think I would classify these as perfectly serviceable joints. With practice you should improve heaps.

Using a soldering iron you're always going to get solder on the top of the pin. Often you'll get a small fillet of solder in the bend of the leg of the chip. It's best if you can still make out the shape of the leg a little. You should aim to have some smooth concave parts of the joint. Something that is convex everywhere can hide a dry joint underneath. It's a lot harder for one to hide under less solder and a concave surface is a fairly good indication of two wetted surfaces.

You might like to consider a different tip on your iron. I prefer flt tips because they are more versatile. You can use a corner, and edge, or a flat face to give different amounts of contact area and hence heat transfer.

Also look at how you clean your tip. I use a "dry brass sponge". There's not much in it, bit I think they have an edge over a wet sponge. I think it's been mentioned that a clean tip is very important. I find that there is less solder on the tip after using a dry brass sponge than a traditional wet sponge.

What sort of soldering iron do you have? If there are replacement tips available you might like to purchase a smaller

Here are some tips you can get in a pack for an iron similar to mine:

For general soldering I use one like the top left one. It's about 3mm wide at the tip. You might want to consider something like that, or maybe a smaller version like the 6th one along. Bit you'd have to see what's available for your iron.

The next thing you should try soldering is a component like a 1206 resistor or capacitor. These are pretty big (they might not look like that at first, but trust me).

These components require a slightly different technique. For these, you need the component to overlap the pad at least as far as the tinned contact on the end, but you ideally want some pad visible beyond the component. The idea is to get a nice fillet of solder between the pad and the end of the component. Beware that the surface tension of the solder may be sufficient for the component to be lifted up onto the tip of your iron if you don't hold the component down or have one end already tacked down.

These small passive components are named by their size. 1206 means 12/100 of an inch by 6/100 of an inch. That's almost 1/8 of an inch long! Hand soldering is reasonably easy for 1206 and larger, Of the smaller components, you'll get used to 0805 pretty quickly, 0603 will become pretty easy with practice, but I find 0402 to be too small to be comfortably worked on using a soldering iron. 0201 and 01005 are getting into the region of fantasy for hand soldering, although I will admit to a successful resoldering of one end of an 0201 component on a USB stick.

1206 and 0805 components can even be used on veroboard. They'll span two adjacent tracks. Possibly not a routine thing to do, but possible.

 

[Rleo6965]

I usually put small amount of solder flux to the smd ic bottom part of pins before soldering it to pcb. Solder flow easily and good connection. But my problem was to clean the small amount of solder flux on under the smd components on pcb. Does solder flux contains acid that may cause corrosion or affect high frequency signals? Sometimes I use Contact cleaner to clean it. Maybe you guys can suggest better liquid cleaner.