SMD soldering

[muddyfox]

Hey folks! Lemme pick your collective brains for a minute here...

Waiting for my first ever smd board to arrive from ITead, I was reading up on soldering techniques. Needless to say, a hundred people will have two hundred opinions which continues to hold true when investigating the best soldering tip size.

What do you guys use for 0805 and 0603 and sot23 and such? Basically for anything one could possibly find in a pedal, I'm not talking gaming console rework and whatnot? Conical, chisel, straight, bent? Diameter?

[rullywowr]

There are many ways to do SMT soldering, however the following two are perhaps the most popular in a DIY home workshop (at least from my limited experience).

1.  Iron.  First flux your pads with liquid flux or a flux pen (I like the pen).  Basically, you will put a small blob of solder on one of your SMT pads on the PCB.  Then carefully grab the component with tweezers and melt it down to the blob.  This will allow you to place the part so it doesn't move.  Then flow the other side, and perhaps touch up the first joint.  Make sure you get good "fillets" on each side of the component.  A smaller size tip would be helpful but it doesn't really need to be micro sized at all.  It is more helpful to use a smaller gauge solder wire.  I use small weller tips like the 1/16" ETA screwdriver or conical.  It really doesn't make or break any SMT project...personal preference.

2.  Solder paste/reflow.  This is quite cool but there is a small learning curve.  Essentially, you get a syringe of solder paste and put a dab on each pad.  It takes a bit of practice to know when is too much/too little.  If you are doing medium scale production, you could also use a stencil, but this is more complicated and costly for the actual stencil. 

After the paste is in place, put each component with a set of tweezers on the place where it should be.  Orientation is important but doesn't need to be exact.  After this, you will gently lower your board onto a skillet (hot plate) and turn up the temp.  In about 3-4 minutes, the solder paste will turn from grey into silver and magically conform to the pads and even move some of your components around.  Once everything has reflowed properly, you can wait about 30 seconds and remove the heat.  Shutting off the heat is one way to do this, or if you are careful you can pick up the PCB and remove it from the skillet surface.  I like doing SMT this way as it is faster for me and not as tedious as an iron.

You will have some joints which may not have flowed properly or perhaps a solder bridge (especially on ICs) after the reflow.  You can touch these up with a bit of #2 desoldering braid and an iron.  Check out Sparkfun's Reflow Skillet tutorial for the deets:
https://www.sparkfun.com/tutorials/59 

[muddyfox]

Thanks Rully!

I'm gonna go with the soldering iron for now, as fun as barbecuing boards may be. 
I only build a few boards (at best) a month and time-saving is not a concern, especially if it necessitates another (rather large) piece of hardware.

I think I'm gonna go with 1/32 chisel to start things off with and change if I don't end up feeling it.

[wgc]

I wrote up some tips about a year ago, pickdropper has some great comments too

You may find it helpful, but maybe not

Good luck, and enjoy

[Govmnt_Lacky]

Couldn't a heat gun be used with the solder paste idea? Granted, it would probably have to be one piece at a time (in order to hold it in place so the heat gun doesn't blow it away) but, at least you wouldn't run the risk of frying the PCB.

In addition to this.... don't most skillets only go up to about 400F? Is that hot enough to melt the paste?

[pickdropper]

This is copied from an old post I wrote on the subject:

1.)  Apply flux to pads

2.)  Grab component with tweezer and hold it in place on the board

3.)  Put a small amount of solder on the soldering iron tip

4.)  Touch the tip of the soldering iron to a component/pad joint

5.)  Repeat for the rest of the connections.

After everything is soldered, I often clean up the joints by adding flux to the component pads and reflowing each one very quickly.  For this step I do not add any solder to the iron tip, I just make sure the tip is clean.

If you are using a type RMA flux (like I do), you are going to want to clean (wash) the PCB when you are done.  There are two good ways of doing this:

1.)  Flux Remover - This is the best method, although more expensive.  If you have a Fry's nearby, they have good prices on Flux remover.  It's handy for a lot of things.

2.)  High-Purity Isopropyl Alcohol - This is much cheaper (you can get a bottle at Target or Walgreens for about $1).  I have a bottle of this only for cleaning boards.  I use a clean soft-bristle toothbrush (not previous used for brushing teeth) that I dip in the alcohol and then scrub the board with.  It will take longer with alcohol; you'll likely need to brush it off in 2-3 passes to get rid of the residual stickiness the flux leaves behind, but you can get it.

It sounds like a lot of work, but it's really not.  It's just a different process.  A cleaned SMT board can look really cool, too (at least I think so).  Here is a close-up of a PCB after cleaning:

[pickdropper]

I have a couple of different tips that I use.  For SMT work, I generally stick to round tips.  There is one that I have that is very very small and thin, but doesn't work as well if the pad is connected to the ground plane.  I nave others that are a bit larger in geometry and transfer heat better.

[muddyfox]

What sort of iron tip do you find you use most?

EDIT: hah! wrote at the same time... is there a way to delete a post? if there is, I can't find it...

[pickdropper]

Mostly the smaller conical one, but I don't always change the tip if I have the bigger one on.  It really depends on the amount of soldering I have to do.

As I mentioned above, I rarely ever use chisel on a SMT board, although I have on rare occasions.

[muddyfox]

Interesting. Having no smd experience and seeing how the end of a smd part is perpendicular to the pad, one would (well, I would) think that a chisel tip would fit perfectly at the contact point and certainly transfer heat better to both surfaces than a conical tip?

[croquet hoop]

That's what I thought before I tried to solder small 0805 stuff, but, at least with this size, a small conical tip works exactly as you figure a chisel tip would. You need to heat the pad and the end of the component, but it's not necessary for the tip to fit like a Lego.

[muddyfox]

OK thanks guys... you just convinced me to go with a small conical tip (I don't have anything smaller than 1.8mm chisel right now). Would 1mm be enough? That's smack in the middle between 0603 and 0805 so I'm guessing I should be fine with that? Or should I go smaller, like 0.8mm (0603 width) or even 0.6?

[Thomas_H]

I just soldered my first QFN and TSSOP devices and it works fine.
Funny thing is that the QFN soldered way easier than the other which is the opposite I thought it would be.

Be not afraid - try it.

[muddyfox]

Oh I will... i have f(x) board coming in and also my own two smd layouts getting shipped from itead and seeed right about now. Kits bought off ebay, tweezers and flux pen ready to go. Now all I need is a smaller tip and the boards to get here and I'll get started losing parts all over the room.   

[wgc]

Interesting. Having no smd experience and seeing how the end of a smd part is perpendicular to the pad, one would (well, I would) think that a chisel tip would fit perfectly at the contact point and certainly transfer heat better to both surfaces than a conical tip?

This is what I do.  Helps align the part to the pad.