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Old 22nd April 2002, 11:16 PM   #1
dorkus is offline dorkus  United States
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Default is SMT practical for DIY?

was wondering how many of you out there have built your gear using SMT (surface mount) devices - opamps, transistors, passive components. i am about to begin work on a multi-channel preamp and would like to use all-SMT devices for my circuit but is it practical? i've modified SMT circuits before and have to admit, it is a huge pain compared to traditional PCBs.

cheers,
marc
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Old 22nd April 2002, 11:42 PM   #2
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Hi Marc,

I've built one project that was approx 50% SMT. I had made the PCB myself, and it was not tinned or pre-soldered. I found placing and soldering SMT devices with nothing more than a standard iron, thin solder and tweezers a fraction more tedious than thru-hole, but not drastically so (just think of all the holes you don't have to drill)

I must admit SMT devices makes for a very nice looking PCB, very low profile, and with the added flexibility of being able to place components on both sides of the PCB. I can't comment on sound quality, as 1) It was all digital circuitry (in a DAC), and 2) I didn't build two DAC's one with and one without SMT. Bottom line is it worked.

De-soldering them is a different story, that is very annoying and difficult, although I guess if I had the correct tools it would have been better. All I had was solder wick, and I was lucky I didn't mess up anything more involved than resistors and caps (of which I had spares thankfully).

I'm sure others will chime in on the sound quality of SMT resistors and caps. It has been discussed before on this site, so do a search. I think it would be advisable to avoid SMT caps anywhere near the signal path.

Good luck, it should be a fun project...
Adrian
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Old 23rd April 2002, 12:02 AM   #3
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Default SMT

the current issue of Circuit Cellar has a "smt-it-yourself" guide. it's easier than cutting a lot of holes and there are a lot of new more efficient devices which are only available in SMT -- such as the low noise switchers for power supplies (although there are very good through-hole devices available.) My advice to you,young man: get used to using an illuminated magnifier.
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Old 23rd April 2002, 12:07 AM   #4
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Hello Marc,


I replaced some opamps (smd) and I must agree with Adrian, the soldering isnīt that difficult (when youīve got a nice magnifying glas), the desoldering of the old devices is. Resistors and caps are fairly easy but opamps defenitly arenīt. I removed about 20 opamps and had to replace 3 or 4 tracks that just hung on to the opamps.
So if you know what youīre building and you donīt have to swap devices I think itīs possible to build a project using smdīs

william
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Old 23rd April 2002, 03:18 AM   #5
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Default Desoldering SMT

Removing SMD is a breeze if you do it right.
I have removed/refitted many 100 pin postage stamp size processors in my servicing work, without damaging the pcb.

Two methods -

1 - With soldering iron, add plenty of solder (and new flux), so that all pins are shorted together.
With quick movements of the solder iron, keep all solder molten and the device will lift off with tweezers.

2 - With a Weller pyropen with hot air attachment, or paint stripper heat gun, heat the device so that all solder is molten, and with a fine probe lift one side of the device and flip it over.

With a solder iron and braid the board can be perfectly cleaned of solder. Isopropyl and a cotton bud will remove the flux.

With a solder iron, tweezers and plenty of new solder, all the old solder can be removed from the device, ready for reuse.

I have been using these methods for 15 years - Works For Me !.

Regards, Eric.

PS - MUST be varitemp soldering iron.
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Old 23rd April 2002, 07:25 AM   #6
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One advantage of SMT that I am starting to like is you can design a really tight layout. An 0402 chip resistor, for example, is not significantly wider than the traces. You can jam an 0402 bypass capacitor between two pins on SOIC packages, and that also saves space. On my latest layout I think I will be able to place the DIR, DF, four DACs, and all the supporting power, clock, and interface circuitry onto a board about 3"x4".

0402 is a pretty small package, though. 0805 packages are common and pretty easy to deal with. 1206 are also common but now your talking about pretty big discreets. A 1206 might even feel big and clumsy once you get used to SMT construction.

The most useful tool for SMT work is a stereo microscope with 10x or 20x eyepieces.
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Old 23rd April 2002, 07:45 AM   #7
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cool, thanks for the tip guys. i'm feeling considerably braver about the whole SMT thing now.

jwb, the smaller package components definitely let you pack things tight, but they limit the types of components you can use. e.g. if i want to use a SMT resistor with at least a .25W rating, i would need to go with 0805 or larger probably. i think resistors with higher power ratings will perform better in an audio circuit, particularly in high bias preamp/bufffer circuits where the currents going around are not insignificant. e.g. IRC tantalum film chip resistors, .25W is in 0805 package but i can get a .35W part in a 1505 package, which is the same width but almost twice as long. but i wonder if the extra .1W will buy me better sonics? hmm...

cheers,
marc
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Old 23rd April 2002, 08:42 AM   #8
dorkus is offline dorkus  United States
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p.s. what kind of soldering irons do you guys use? i need to get a real soldering station to do more serious PCB work, and relegate the old $40 Weller to making cables and stuff like that. what type should i get and how much should i spend?
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Old 23rd April 2002, 11:41 AM   #9
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Default soldering iron

I just use a 50 watt weller station with the finest tip I could get with 270°C

william
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Old 23rd April 2002, 12:15 PM   #10
ALW is offline ALW  United Kingdom
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Default SMT DIY

I use SMT all day at work, and have no problems with it, but you need good eyesight!

You may wish to take a look at www.roth-elektronik.de fore their range of SMT prototyping board.

I've even used humble vero (strip) board, but it requires some careful planning.

Andy.
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