Combining SMT and through hole

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JohnRoberts

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Back last century while at Peavey we mixed SMD and TH technology in value (low cost) mixers.

TH jacks, pots, and switches were combined with SMD passive and active components on the solder side of single sided PCBs.

Yes vision is an issue for elder solderers. I sometimes combine reading glasses with a head band magnifier for the extra magnification.

JR
 

Matador

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You could always tweak the annular ring width rules in your layout program if you didn't want to make an explicitly 'combo' layout that has both SMD and PTH pads. I don't know what layout program you use, but it's typical to specify a ratio between drill diameter and ring width, so you could just increase that ratio.

For example, if you increased the minimum annular ring up to 20 mils (or 0.5mm), that would give you approximately 1mm of extra solderable area around the holes for SMD pads to be stuck.
 

ruffrecords

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Attached is a first stab at what I mean. I have just taken the footprint for a standard vertically mounted 1/8W resistor and changed the pads from round to square.

The pads are 56 mil square
The hole diameter is 28 mil
The hole spacing centre to centre is 200 mil
Hence the gap between the pads is 144 mils.

The standard width of a 1206 resistor is 122 mil so the pads would need extending to accommodate it. On the other hand a 2010 is 197 mil wide so it would straddle the holes.

Cheers

Ian
 

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Matador

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If you make your pads big enough it should work. Here is a resistor on a 0.2in pitch, with 75 mil PTH's with a 28 mill drill, with a 1206 inside of it. I would think this should work fine in practice.

If this is on a new board, you could also just make a combo layout that includes the SMD pads as well.
 

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ruffrecords

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If you make your pads big enough it should work. Here is a resistor on a 0.2in pitch, with 75 mil PTH's with a 28 mill drill, with a 1206 inside of it. I would think this should work fine in practice.

If this is on a new board, you could also just make a combo layout that includes the SMD pads as well.
Oops, just make a big blunder. The lead spacing on the footprint I showed is only 100 mil so the gap is 44 mil not 144 mil. So a 122 mil wide 1206 almost completely covers the drill holes (see pic). I think will just by a bunch of 1206 resistors and have a go.

Cheers

Ian
 

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ccaudle

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Oops, just make a big blunder. The lead spacing on the footprint I showed is only 100 mil so the gap is 44 mil not 144 mil. So a 122 mil wide 1206 almost completely covers the drill holes (see pic). I think will just by a bunch of 1206 resistors and have a go.

Cheers

Ian
I guess I forgot to ask in my early reply, is this for hand assembly only, or do you need it to work for automated assembly? You can get away with some things when hand soldering that wouldn't necessarily be reliable for volume automated production.
If you mark out on the picture where the solder pads are on the SMD resistor, you can see that you lose a lot of surface area under the contact area to the holes.
Do have room to put the SMD pads beside the through hole pads? Or to increase the spacing between the through hole pads? That would give you solid pads under the SMD resistor, it would just take a little additional space compared to your current spacing. I guess that basically describes the picture that Matador put up.
By "beside" I mean move the SMD pads downward in the orientation of the picture you drew, so that the total component footprint on the PCB would be more of a square rather than rectangle. Might help if you are more constrained in one axis than the other.
 

john12ax7

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Ian, you might consider making the pads a little bigger in the y-direction so the 1206 sits better. Could do something like.0.050"x0.060" rectangular, or just 0.060" square.

Typically for a 1206 you would have 0.080" gap between pads. Right now you have 0.044". Think this would work fine for hand soldering, but just something to keep in mind.

Also for smt resistors, avoid thick film, you want to get the thin film ones.
 

Dan Kennedy

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I've added more than one 0805 33pF cap across the pins of a dip dual opamp for stability in a modification
of a terrible board layout in commercial products. I love the old Steve Dove quote, paraphrased here, but "it's hard
to make a circuit misbehave when you're using an opamp that's bound and gagged", mostly referring to TL-072's and 741's if I remember correctly.
 

ruffrecords

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Thanks for the excellent tips. Yes, it is for hand assembly. I didn't realise thin film are better than thick film. I bought a kit off eBay. It does not say whether they are tick or thin film. I will just use them for test on some spare PCBs and get soem proper thin film ones if I decide to go this route.

One question. how do you hold the part in place, hold a soldering iron and some solder with only two hands?

Cheers

Ian
 

matriachamplification

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Newmarket

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Thanks for the excellent tips. Yes, it is for hand assembly. I didn't realise thin film are better than thick film. I bought a kit off eBay. It does not say whether they are tick or thin film. I will just use them for test on some spare PCBs and get soem proper thin film ones if I decide to go this route.

One question. how do you hold the part in place, hold a soldering iron and some solder with only two hands?

Cheers

Ian

Possible technique for soldering iron (that I use):
Wet one pad with solder.
SMT chip held with tweezers flat to pcb.
Soldering iron to wetted pad to melt solder.
'Slide' SMT Chip on to wetted pad and remove soldering iron to to let it set.

SDMT chip is now tacked in place.
Solder other pad.
Go back to solder the first pad (the 'tack' would probably be fine but 'belt and braces' plus you can then have each solder joint look visually similar.

It's quicker than it sounds once you're 'in the swing' of it.

If doing much of it you might want to consider a hot air station and solder paste.
That way you can flow both sides at once and surface tension effects will tend to pull the SMT chip 'square' wrt the pads.
Beware that SMT paste doesn't have much of a shelf life before it dries out so don't buy a big tub of it initially !
 

ruffrecords

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Good Luck. I reckon you'll eventually appreciate the time you save snipping off the component legs !
The crazy thing is, 40 years ago i was a consultant advising companies on this new fangled thing called SMT. We even set up a small batch build line with pick and place etc to show them what could be done, but I have never used SMT at home.

Cheers

Ian
 
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