Chip Mounting

SMT soldering isn't that hard, even with a standard size iron tip. Google around a bit, you'll find plenty of tutorials. My way: apply solder paste to the board (I have a small syringe of it), place part, heat the paste to solder it, use copper braid to remove any shorts. It's quick and clean that way.

Once you get used to it, SMD parts are easier to solder than through-hole parts.

I do tssops (0.5mm pitch) and us-8 (0.4mm pitch) chips by hand

you can too. 0402s are easy, 0201s are harder, but even 01005's can be done by hand

For initial soldering, you're best off with a "hoof" tip, one that can touch 2-3 pins at once, so you can just swipe it over all the pins. I use a regular chisel tip for this, though.

Besides that, I use a 0.020" conical (pointy) tip, a temp-controlled (600F) iron, and a 3.5x magnifying visor.

It's not hard to make those at home with single-sided copper clad boards and some FeCl.

Yep. We have these SMT's where I currently work that we call dust specks. Takes a 20 power microscope to solder them. Their so small when you use your tweezers to pick them up you don't have to squeeze. They are so light just the capacitance of your body will pick em up.

We've named them "quantum capacitors" (the 01005's). One second they're there, the next second they've vanished. I usually hold my breath when I'm working with them. I use the 3.5x visor to place them, then a 60x scope to adjust and/or verify the placement. With care, though, they can still be soldered by hand.

You buy an eval board with the CPLD/FPGA or whatever on it first to develop your firmware. try

for some reasonable priced development boards.

You must not be wearing an approved esd wrist-strap. Isn't that a firing offense at motorola? ;)

More likely, skin oils make your fingers sticky enough (or surface tension in action) to adhere the parts. The recommended way of picking up some small parts, for example, is to dip a toothpick in alcohol to make it just sticky enough to pick and place the parts.

I've found that even the tiniest resdue of flux on my tweezers can make it very difficult to place the tiny parts, because they don't weigh enough to stay put when you let go of them - they'd rather stick to the tweezers.

Soldering a TSOP or QFP to an adapter isn't actually as bad as you might think. You'll need ultra fine solder and an ultra fine tip but it's certainly within the capability of a competent tech.

Graham

Don't invest in too much solder paste though. It 'goes off' very quickly. I used barely a fraction of the syringe I once bought.

It's worh adding that it doesn't matter if a few pins are 'shorted together' at this stage. You can clean up later with solder wick. The wick *with flux* works best ! I find the 'no-clean' wick utterly useless.

I prefer to use HCl and H2O2 to etch actually. FeCl is nasty stuff.

Graham

Don't sneeze even with 0603's !

Graham

Mine is SynTECH from stencilsunlimited. The 35g syringe was only $20 complete and it doesn't need to be refrigerated to get a reasonable shelf life. So far I'm happy with it, and if it goes bad, I don't have that much invested in it.

Yup, I mentioned that. In fact, it's *easier* to solder if you let it bridge the pins occasionally - it takes less time to fix with wick than to avoid the bridges in the first place, and the extra solder ensures a good joint.

Wick with flux?

Personal preference. I've never had a problem with FeCl, it's cheap and locally available, and only a single chemical is needed. If I stumble upon a local source of HCl and H2O2 I'll try them.

Any modern CPLD is programmable in-circuit, and anything available in PC44 is so simple and cheap that I wouldn't bother socketing it. A PC44 socket that brings the pins out to a PGA is more of a pain to work with than a straight PC package.

I just made a CPLD board for a friend as a trial of gEDA, and I wrote it up here:

I've hand-soldered kynar (30ga solid) to adjacent pins on a QFP using a

1/32" tip and no magnification. Of course it helps that I've done similar things under a stereo microscope so I knew what was happening without having to see it.

Forget adapters. The key is flux. With enough flux, the solder wants to stick to the metal so badly that it's hard to bridge pins. Actually PLCC might be one of the worst SMT packages to deal with, just because the 'hook' part under the chip can bridge and you have to suck that solder out. I'd rather do a TQFP given the choice.

--
Ben Jackson AD7GD

http://www.ben.com/

On Wed, 21 Feb 2007 01:06:19 -0600, Ben Jackson wrote: ...

^^^^

A socket is only $1.85 or so:

:-)

Cheers! Rich

Hey all,

Thanks for the comments on the Chip Mounting, it sounds like I have a lot of options to explore.

FYI, the reason I was talking about adapters to bring it out to DIP form was so I can stick it in a breadboard and play with my circuit. Sure, if you had a "real" circuit board, you could do it that way right away, but I'm not sure how well it lends itself to experimenting (Unless you had it soldered to a board, and then brought wires off that board to your circuit...)

After reading all this, a question or two comes to mind:

Let's say I'm not going to use solder paste (which it sounds like most of you don't go for) but was going to use conventional solder with flux. If I were going to "slobber" solder all over the pins and then wick it up, wouldn't I first also have to tin the pads on the board (otherwise it sounds as if I've got metal pins to metal pads, with slobber solder all over, but none between the pins and pads!)

Or the other option - conical tip and then carefully soldering each pin - still, you'd have to tin each pad, correct?

Sorry for the ultra-basic questions, but I guess I've come to the right group for it :-)

Done that:

Although that was sdip to dip adapter, the theory is similar. I used female headers as a "breadboard" (22ga wire sticks into them just fine) and male headers for my logic analyzer.

If you've put flux on the pins and pads, the solder wicks between them. Still, if you're making your own board, it's a good idea to tin the exposed copper with solder (wiping off the excess with desoldering braid) to make it more solderable and to avoid corrosion.

No, but you should flux each pad.

Keep in mind that a PC44 adapter is very wide (will span multiple breadboards) and not cheap. For the same price (but a bit of a wait) you could order a pcb from batchpcb with your own design.

I know you are concerned about re-use and flexibility, but if you want to play with a CPLD, just make one board with some lights and buttons and play with it.

No. If you're starting with a homemade PCB that's bare copper, it's easier to solder if you do wet all the pads first. You can do that with an iron or with tinning solution.

--
Ben Jackson AD7GD

http://www.ben.com/

Assuming your pads are pre-tinned, put down a layer of flux. Too much is better than too little, assuming you can clean the board later. Tack down the chip, put a tiny bit of solder on the iron (use flat edged tip, NOT a tiny conical) and wipe across the pin-row semi- quickly near the bottom. The idea is to not try to solder each individually, not to depend on the flux in the solder rather than pre- added flux, and not to try to get a "good" visual looking solder joint. By this I mean some people see a lot of solder and think that's good. That's excessive, there should barely be any solder visible as it's mostly inbetween the lead and pad where it isn't seen. If there was plenty of flux and the pins weren't horribly dirty, they will all get coated with solder. If the part was tacked down flush before soldering you barely need any solder at all, having too high a solder-added-flux ratio then having to remove excess solder later is one of the most common mistakes, IMO.

If it doesn't turn out good, put down more flux and reflow with just the tinned iron tip. Adding more solder is a last resort. A ball of solder on the iron is too much solder unless the board is pretty rough/ crude/homemade/etc with large gaps. If the gaps are large you might just take a cold tool and press down the stay pins a little before soldering. Most people starting out may use 4X as much solder as needed, roughly. Ideally you get the technique down on scrap chips 'n boards beforehand.

Yep. We wear them faithfully. It doesn't matter with these things. They are really sensitive to body capacitance. I wish I worked for Motorola. That would be sweet.