soldering small surface mount discretes

How many caps are you using? Which ADC is it?

John

Yup.

What I do now (for 0603s) is manually put down solder paste, place the parts, and use the iron to reflow the paste while holding the parts down with tweezers or a toothpick. You don't have to get the iron as close as with wire solder; you only have to touch the copper trace and the heat is conducted under the part to melt the paste.

If you can etch a paste stencil, you can put the paste down with that and use either a hotplate or iron to reflow it. Otherwise, it's still possible to manually paint the paste if the syringe is small enough.

Note that with the really small parts, both pads heat up at pretty much the same time even if you only touch the iron to one side. This means the "solder one pad to hold it down" trick doesn't usually work; the first pad re-melts when you solder the second pad.

With hand ironing, it's not just tombstoning you have to worry about - it's bumping the parts and messing up the alignment. The surface tension of the solder on one side can easily move these parts, and jiggling the solder with the iron moves them too. Even with parts as big as 0603, I have to hold the parts in place while soldering.

So you don't have any of those boards left, right? It sounds like a good idea to make a trial board like that.

So with 0201s, is there still enough space to hold on to them with tweezers while soldering them down?

Interesting - those buggers sure are teeny. At that size is there any worry about them getting damaged from the heat? I mean with larger components there is a whole lot more surface are to dissipate the the heat.

I do the same - I always put a dot of solder on one side of the PCB, pick up the part with tweezers in my left hand, then heat up the dot again with my iron in my right hand, and place the part at the same time. Easy as can be.

Thanks,

-Michael

Currently 23 capacitors, 8 of which are fairly large tantalums. The rest are either 100nf or 1uf, so I think could be pretty easily replaced with much smaller parts. The ADC is an Analog AD7656.

-Michael

How come you're not using a hot plate? D from BC

Along the same lines as my first question: anybody know of any small, reasonably priced large capacitors? My standard decoupling on a chip is a .1uf on every Vcc/Vdd and a 10uf cap on each supply line/chip. I've been using Digi-Key # 478-1751-1-ND, which is a 10uf 10V in a

But I want to go smaller. I was just looking through Digi-Key and they have some 0402 and 0603 tantalums with fairly good looking specs, but they cost $3 or more each! Ouch. Any options? The M size parts (1.6x0.85mm) seem like the best alternative, like the 511-1494-1-ND:

Suggestions?

Thanks!

-Michael

Actually I'm going to have to use a hot plate to do one of the components on this board (a BGA).

To replace tantalums and small electrolytics, I use a lot of the 10 uF 10% 25 volt 1206 X5R Panasonic ECJ-3YB1E106K. These cost $0.55 each in quantity 10 from Digikey and have about a milliohm of ESR. I've never had to parallel them with a smaller value, yet.

Those decoupling requirements are insane. I doubt that the chip actually needs all these caps.

John

We've sworn off tants for cost and reliability reasons, and off aluminums because of their low-temperature issues. So we mostly use ceramics. I think we're using mostly 0.33 uf 0603 for general bypassing, and for the big stuff we have

or something like that.

John

That's quite possible. Then again, I'm not going to argue with the guys who designed the part. I need every ounce of accuracy that I can get out of this thing.

-Michael

Do you know of some good 20uF or so at 75V? They seem to be just above what's available at Digikey etc. (especially the 75V requirement). I can put lots in parallel, of course, but it starts to get silly.

Best regards, Spehro Pefhany

They're too small to make - practically - in small quantities. I originally made a batch of 100 kits (so, 100 of each part and shared FAB costs), and sold them at cost to folks on the gEDA mailing list, so that we could get some experience with the smaller parts. Each kit cost $2 plus S&H, but it would have been much more if I had made less. Even then, I added it to another board I made, to share the FAB costs across projects.

I put the design files online in case anyone wants to add a dozen or so to their existing two-layer projects, for their own use. They're only 0.5" square.

I've thought about doing another run, but I keep mentioning the project and people keep not asking me to make more ;-)

Not really. The tweezers I have, have needle points. I use one point to hold the part down (i.e. press down on top, not hold from sides) while soldering the two ends. Even toothpicks need to be sharpened with sandpaper to be small enough.

Here's a comparason of 0805, 0603, 0402, 0201, and 01005 parts:

Close-up of 0201s:

My first hand-soldered 01005 (right side is 0.4mm pitch IC):

Some microscope pix of various parts on laser printed footprints:

I've found the real problem is mechanical damage. It's really easy to try to nudge a part into position after soldering one end (i.e. while melting that end's solder), only to rip the entire cap off that end because the solder wasn't quite melted enough.

Desoldering pretty much ruins the parts too, assuming you can find them afterwards.

Take advantage of it, and reflow all the other SMT parts at the same time.

hotplate experiments:

sample hotplate-reflowed boards:

I have a friend that sells educational electronics kits for hobbyists. I'm going to try to convince him to make something similar.

If you do another run let me know, I'll take a couple off of your hands.

Ah - that'll be interesting. So with both pads clean and free of solder, you place the part and hold it down, then solder both sides? How do you get solder on to it? It seems to me that both of your hands would be busy. Just put a little on the tip of your iron maybe?

I pretty much toss all surface mount components that I desolder - life's too short to try to keep track of them all.

Well - I guess I'll making the big switch from 0603 to 0201 for now. Hopefully I'll survive. Do the electrical characteristics of the parts change much from 0603 to 0201?

Thanks,

-Michael

Caps do - the dielectrics are often very voltage dependant. High k dielectrics are the worst. Going from a 0603 to a 0402 cap with almost identical specs can show over a 40% - 60% reduction in capacitance at high percentages of working voltages. With 0402's, you are usually working at high percentages of the working voltages.

of

that

There are some that come with a double stacked cap with leads soldered on the side with high temp solder to hold them together. They were 100v and around the range in capacitance you were looking for. I can't remember the brand, but they were larger caps, may be 2010's, I think they were bought at Digikey.

There is nothing wrong with placing several caps in parallel.

Honestly, I stuggle with 0603's. I suspect I don't have the right equipment and/or technique.

I just wanted to add (for us Engineers anyway..) that maybe the Eye Doctors could update their charts. Instead of 20/20 eyesight, we could have 06/03, or 02/01.?

Just a thought.

-mpm

Do you need a very low wideband noise floor? If it's accuracy you need, oversample and average or lowpass filter. Any real-life sampling

That adc scares me. Some of the nodes are bypasses of active circuits, and whenever I see strange, unexplained bypassing requirements, I wonder if the datasheet is hiding ldo-type stability problems, or if the applications engineer was just being too enthusiastic.

John