Sizing SMD components for Hobbyist Kits

Question

More components are only available in SMD packages. For hobbyist assembly the options are to buy breakout boards or solder SMD.

Since components are usually packaged in a couple of SMD package types I am trying to put together a set of guidelines for choosing packages that are compatible with hobbyist skills and tools. I would consider hobbyist level tools for SMD assembly as -- soldering iron in the $50-$100 range (new), for magnification a $40 visor (like the B&L) and tweezers.

For the kits I make now I use the following guidelines --

• Passives 0805 or larger
• Min Lead Pitch for SOIC or QFP -- 0.5mm
• No QFN, LGA or BGA
• Prefered package for gates, BJT, FET --- SOT23
• Diodes SOD123 (or larger)

I am interested in recommendations on component selection, minimum tool requirements and assembly issues. Specific tool changes (like solder tip size) that enabled you to do SMD assembly with your existing tools would be useful too.

Thanks.

Answer 1

0603 isn't too bad to solder by hand (I won't do 0402 or smaller though).

SOT23 is probably a good guideline (for diodes too, not just transistors); there are some SOT323s that are smaller that are a pain.

I would avoid certain SOT23-6 parts because it can be very difficult to determine which way the package is supposed to go. (For some dual MOSFET packages it doesn't matter.) We had one where there was a slight bevel along one edge. Grrr.

I would also avoid SOD123 because of the backwards nature, if possible. SMA/SMB/SMC aren't as much of a problem.

And avoid those cylindrical diodes (LL-34 / MELF) like the plague! they will roll off the board.

Answer 2

I would recommend using a SMD rework tool for small projects, and a reflow oven (you can make one from a toaster oven) for larger boards.

Reflow makes sense because you have far less problems with solder bridges and it's actually harder to destroy components. Components tend to pull themselves into position, so placement of smaller components becomes less critical (than with soldering by hand). 0805 and 0603 are a breeze.

For reflow it makes sense if you have a tool to deposit accurate amounts of solder. Using a syringe by hand and really a bad idea. The smaller the component the more critical this is.

Answer 3

As far as my skills, to add a point of data. Using a $40 soldering iron, and lots of flux (I have a 'pen' with the liquid kind inside), and the occasional desoldering braid.

Easy: 0805 passives , 0.7mm pitch ICs

Doable if careful, but have ruined a couple: 0603, 0.5mm pitch ICs

Haven't tried smaller than those yet, I think that's about my limit.

Answer 4

about packages
What you want and don't want is very much a personal preference, and I can't say much about it. Just one thought, though. With time you'll become more confident, and maybe find some good tricks to work with packages you always regarded as impossible. Equipment like a reflow oven also opens opportunities, like for "bottom packages" (QFN, DFN, and maybe even BGA). This is just as well, because manufacturers don't give a damn about us DIYers, and the market wants ever smaller packages, no leads to start with.

I posted the following in comments to another answer, but I think it might be interesting enough to be an answer.

tweezers
The wrong tweezers can be very frustrating. Rounded tips are definitely out. Good tweezers should open and close(*), and not allow any movement whatsoever in the perpendicular direction. I use Erem 102ACA tweezers, and they never let me down.

The tip shape makes working with 0402s doable. The tips are also very thin, so that they allow you to place components very close together.

component storage
You can store your SMD MLCC capacitors (unmarked!) in compartmented boxes, but the Law of Conservation of Misery says that you'll accidentally drop that part you just picked in one of the other compartments. Unmarked MLCC, great!
These Licefa boxes are a solution.

They contain 60 phials (there's also a box with 130) sized 1cm x 1cm x 2cm high. If you need a part you can take out the phial, so that different parts don't get mixed up. A phial can contain dozens of passives. I find them useful for packages up to SOT-23.
One minor point is that they're not antistatic.

---

(*)Yeah, obviously. I don't remember what I was about to write about opening or closing when I posted this ;-)

Answer 5

If PCB area is not an issue I would prefer 1206, but 0805 is doable. I don't like smaller sizes, they are difficult to grab and hold with my tweezers. 1206 resistors have a value printed on top, do 0805's have a value?

Answer 6

SOD323 diodes are sometimes doable. They're good for those token diodes you sometimes need like 1N4148.

DFN, QFN, power-pad SOP, and LGA (and maybe BGA) can be done using a trick a friend of mine shown me, as long as all parts are on one side of the board.

1. Tin all the pads that are under components.
2. Place the board in a skillet.
3. Flood the footprints with flux.
4. Place the components on the pads, carefully aligning them.
5. Heat the skillet to 400 F or so (reflow temperature). You might want to get a surface thermometer to keep track.
6. When the solder reflows, quickly make sure all the parts line up with where they need to be. If anything is off, quickly realign or remove the part. (My friend didn't tell me where to put a removed part. >_<)
7. Remove the assembly from the skillet and shut off the heat.
8. Assemble the rest of the parts with a soldering iron as normal.

There's probably a few things that can be done better, but that's the basic plan. He used this with an ITT "capstone" class (basically, a senior lab) he was teaching, because the necessary motor controllers and switching converter chips were only available in DFN.

Another thing to remember about DFN and QFN parts is that they are singulated (cut from the lead frame and mold) after the leads are plated. This means that the ends of the leads, if exposed on the sides of the package, may be oxidized, exposed copper and might not reflow solder (i.e. form a fillet). This is perfectly normal; only the bottom surface is expected to wet to the solder.

Answer 7

Passives 0805 or larger

IMO Less important than the size of the packet is the ammount of space arround it. Given the choice I would rather have an 0603 with plenty of space arround it than 0805 parts crammed up next to each other.

I would also suggest generous pad sizing. It's much easier to hand solder if the pad goes beyond the end of the component.

Min Lead Pitch for SOIC or QFP -- 0.5mm

This is IMO getting to the point where it is too small to reasonablly solder pin by pin. Blob dragging or flood and wick techniques can work but require a fair bit of practice.

Sometimes you may have no choice but if a larger pitch is available I would suggest choosing it.

I would also suggest lengthening the pads so they stick out further beyond the chip than commercial pads do. This will make the pads more robust and make it easier to heat pad and leg at the same time with an iron.

Answer 8

I'd recommend having a hot air tool (I have a cheap gas one) if you need to, e.g., desolder bigger components. You can heat a large component or area without touching it pretty easily. On the other hand, there's always some more effort/risk in avoiding burning any adjacent stuff.

Answer 9

For me the key item is the soldering tool. An iron with a good control over temperature good selection of tips is essential. If you can, make sure that the iron has a miniwave tip option.

With a good iron, good set of tweezers and magnifier, I could easily work 0603 components, SOT23, fine pitch (down to 0.5mm pitch).

Solder wicks of different width should also be included.

I seconded @Steve recommended of a cheap reflow oven. It saves a hell-a-lot of time.