Combining SMT and through hole

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JohnRoberts

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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.
I am not one to argue with Steve Dove, but I never heard him characterize TL07x that way. In my experience when used within their capabilities, TL07x performance is respectable. I can't imagine grouping them together with 741s, a very early and quite slow op amp (0.5V/uSec).

JR
 

ruffrecords

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I am having difficulty in finding a 1206 size thin film resistor kit. Loads of thick film ones and plenty of individual 1206 thin film resistors but no thin film kits.

Cheers

Ian
 

ccaudle

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I am having difficulty in finding a 1206 size thin film resistor kit
I guess there is not as much demand for 1206 size these days. I don't know about the UK suppliers, but here on the western side of the Atlantic I can find 0805 kits in stock, but also no 1206 kits here.
Google found this place, which I have not heard of before. Seems to specialize in part kits, and does have 1206, but only Yageo thick film:
SMT resistor kits from SMT Zone
They are based in California, and from the description page it appears they specialized in making the containers for part kits, and also sell their containers pre-filled with parts.

Depending on what you are doing thick film might be OK, but they have more excess noise than thin film. Not sure about any voltage coefficient of resistance, so I don't know if they have slight distortion compared to thin film, or just a little noisier.
I don't remember a lot of classic tube equipment being noted for only using metal film resistors, so if this is for tube designs the noise difference between thick and thin film may not matter much. What type of construction through-hole resistors are you using?
 

JohnRoberts

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I still have a wide assortment of TH resistors from the 70s, but when designing for SMD production I am motivated to reduce the number of values I deal with. To get best prices you generally want to buy 5K piece SMD reels. Less different parts reduce set up labor. Machines don't care as long as there are plenty of empty stations on the automated pic and place machine. Unless you are running thousands of PCBs, some value standardization much be useful.

For non-critical applications I used a couple small quad smd resistor packs. I tried x8 resistor packs and their pin pitch was unwieldy for hand soldering. In manufacturing a quad only counts as one pop (component placement). I found the x4 a good compromise for GP resistors, x8 makes layout harder, and rework difficult.

For high quality audio you may want to experiment but I would avoid using too small SMD parts, as they misbehave in sundry ways. As I think I shared before when designing my outlet tester I had to go up to a 1206 SMD resistor to get one that would actually deliver 100M @ 500V. :cool: Since you are a vacuum tube guy, high voltage behavior might matter for your application too.

JR
 

Newmarket

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I can't recall seeing any ThinF / MELF kits. I don't think lack of demand for 1206 is that much of a factor. Rather, ThinF is significantly more expensive than Thick Film so the kits would cost correspondingly more expensive. And ime the reality is that you end up with values that just sit in the folder for years and never get used while you need to order refills for the values you use as standard. So really just focus on those values.
Order alternative values small qty - eg strip of 50 - or solder in parallel to achieve value.
For TF you will probably need 1206 (or the MELF equivalent - the naming gets confusing but not going into that now 🙄) anyway so that seems to work out okay with your PCB footprint idea.
 

ruffrecords

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Thanks for the pragmatic advice. It is funny how in engineering, good ideas sometimes do not work. I could reasonably rationalise resistor values on my tube amplifier boards - in fact I already have. But these do not require the space saving properties of SMD parts. Where I do need it is around small size 12 way PCB mounting rotary switches which are used to set gain or, in EQs, to set attenuation. A such, nearly every one of the 12 values is different and it is one set of 12 for the gain switch and a different 12 for the attenuators. Even so, I might just buy 10 off each of the 24 values I need and be done with it.

Edit: It turns out I only need 19 different values.

Cheers

Ian
 

ruffrecords

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Thanks for the pragmatic advice. It is funny how in engineering, good ideas sometimes do not work. I could reasonably rationalise resistor values on my tube amplifier boards - in fact I already have. But these do not require the space saving properties of SMD parts. Where I do need it is around small size 12 way PCB mounting rotary switches which are used to set gain or, in EQs, to set attenuation. A such, nearly every one of the 12 values is different and it is one set of 12 for the gain switch and a different 12 for the attenuators. Even so, I might just buy 10 off each of the 24 values I need and be done with it.

Edit: It turns out I only need 19 different values.

Cheers

Ian
Edit again. It turns out RS and Farnell only do thin film resistors in a MELF package. A 1206 is 3.2mm long = 126 mil. A miniMELF is 3.6mm long =142 mil. Total width of the current vertical leaded resistor footprint is 156 mil so at first glance it looks like a miniMELF will do.

Cheers

Ian
 

ruffrecords

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Funny. My son was going to ask you about using Temp-Resistant Tape to hold Harwin 100mm pins onto the boards while soldering.

He found this https://www.amazon.ca/Rolls-100ft-T...rds=heat+resistant+tape&qid=1642150389&sr=8-5
If the Harwin pins are loose in the PCB what I do is get a new 12AX& or 6922 tube and push it into all 9 pins. I find that usually holds them all in place whicle you solder them and has the added benefit that you can be confident that when you finally plug in the tube you are going to use, it will fit.

Cheer

Ian
 

ruffrecords

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OK, after further investigation, both Farnell and RS do both thin film and metal film MELF resistors. However, they only seems to do E12 values in metal film 1% tolerance but they have 24 in thin film 1%. (Of course you can get metal film 0.1% E24 in metal film but the prices are ridiculous).

Which is better for audio - thin film or metal film?

Cheers

Ian
 
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Newmarket

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OK, after further investigation, both Farnell and RS do both thin film and metal film MELF resistors. However, they only seems to do E12 values in metal film 1% tolerance but they have 24 in thin film 1%. (Of course you can get metal film 0.1% E24 in metal film but the prices are ridiculous).

Which is better for audio - thin film or metal film?

Cheers

IAn
Yes - that's what I've done on the PCB footprint. 1206 allows 1206 chip or MiniMelf.

For small signal audio there's nothing in it for Thin Film Vs MELF. They are both thin film types. MELF has some advantages but these are related to power / heat / long term drift. Basically MELF has greater surface area / volume.
You can dig into the data sheets and technical literature, or keep it light and watch this:
Vishay - Vishay MELF Resistors Take on Thin and Thick Film Technologies in Head to Head Competition

And always use a tray for MELF - they love to roll away.
 

Andy Peters

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

Put a tiny dab of solder on one of the pads. Hold the part on the pads with standard fine tweezers, not heated; I like the sort with serrated jaws. Heat the pad that has the solder dab until it flows and holds the part. Then heat the other pad and apply solder. If you put down too much solder, clean it up with fine solder wick.

Remember to not press down on the part as you solder it in place. Let the solder melt. Pressing down on an MLCC cap will break the terminals off the cap.
 

Andy Peters

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I am having difficulty in finding a 1206 size thin film resistor kit. Loads of thick film ones and plenty of individual 1206 thin film resistors but no thin film kits.

Cheers

Ian
It’s not worth making up kits. I use 0805 for my projects. If I need an odd part value, I buy a small strip of a hundred parts. For common part values (1k ohms, you know, also 100 nF caps), buy a reel. The parts will cost more to ship.
 

Andy Peters

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Having a 2nd iron would be very helpful. I tend to use Newmarket's approach for install, and then Dan's two iron approach for removal or repositioning
Get a cheap hot air rework tool with an assortment of nozzles. I think I got mine from SparkFun for $80. Use a narrow nozzle, just about the size of the parts. Turn it on, put the nozzle right on the part, and in a few moments the solder will melt and you can lift the part off the board with tweezers. Clean the pads with solder wick.
I use this thing for removing everything from small passives to large QFP chips.
You can also use the hot air to solder parts to the board. Dab a bit of solder paste onto the pads. Position the part on the pads. The paste will hold it in place. Alignment isn’t even critical. Put the hot nozzle over the part. Make sure the air flow isn’t gale force. After a moment or two, the paste will melt and the part will just settle into place, and it will self-align.
 

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JohnRoberts

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Put a tiny dab of solder on one of the pads. Hold the part on the pads with standard fine tweezers, not heated; I like the sort with serrated jaws. Heat the pad that has the solder dab until it flows and holds the part. Then heat the other pad and apply solder. If you put down too much solder, clean it up with fine solder wick.
+1 I use small tweezers to hold the smd part, then tack one end first. That will hold it in place so you can solder the other end. If you are worried about it not sitting square, reflow with a hot air wand will generally cause it to self align.
Remember to not press down on the part as you solder it in place. Let the solder melt. Pressing down on an MLCC cap will break the terminals off the cap.
+1... small Rs and Cs are not robust, I recall some early film SMD caps that didn't even survive our contract manufacturer.

JR
 

pahstah

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For non-critical applications I used a couple small quad smd resistor packs. I tried x8 resistor packs and their pin pitch was unwieldy for hand soldering. In manufacturing a quad only counts as one pop (component placement). I found the x4 a good compromise for GP resistors, x8 makes layout harder, and rework difficult.

Hi John, would you suggest small quad resistors pack as bus resistor?

thank you

paolo
 

JohnRoberts

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Hi John, would you suggest small quad resistors pack as bus resistor?
no... for LEDs and non-critical audio path,,, while I never put one on a distortion analyzer. This is perhaps over engineering but I know one console designer (now RIP) who did rigorous bench testing and settled on larger SMD resistors for linearity performance.

I used the quad resistor packs in my drum tuner for digital glue circuitry. The 8-packs were used in a friend's DSP processor. He probably used them in the audio path and I can't say I actually heard them, I do recall they were a royal PIA to do rework on.
thank you

paolo
Thank you for spelling bus correctly... :cool:

JR
 

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