soldering parts to PCB

[spendorspain]

Hi, Diyers

I'm afraid this is a silly question, but I'd like to know your opinions.

It is said that hard-wiring components (soldering directly one to each other via its leads) is the best way to connect them and that using printed circuit boards is more for practical reasons (it's easier to place and solder the parts, can be automated...).

I'm wondering if soldering the parts to the PCB but also soldering the leads, when possible, is worth the extra work (of course in a simple circuit with a few components). I know this would be very time-consuming, but diyers have all the time to do our hobby ;-) (moreover, this double soldering would be done only in critical signal paths).

For instance, solder a resistor "R1" (or any long-leaded component like inductors, axial capacitors, transistors, voltage regs, etc) in its PCB solder pads will connect it, through the PCB traces, to other resistor "R2" also soldered in its PCB pads. If R1 and R2 are close, it is possible to solder directly their leads (only several mm long) under the PCB. The "normal" method uses two solder joints (R1-PCB and PCB-R2) and a copper trace sometimes very thin. The second method uses only one solder joint (R1 to R2 leads) and copper leads (even OFC in good
components), thus "bypassing" the two solder joints and the PCB trace.

My questions are:

Is this double soldering worthwhile, from a sound quality point of view?

Is there any problem in having these two parallel signal paths (one via PCB traces and other direct lead to lead)?

If the leads are soldered directly under the PCB (if possible), is it needed to cut the duplicated PCB traces? (in this case, the solder pads on the PCB would be used only to fix securely the components to it, like a breadboard with tinned copper holes).

Thanks in advance for your comments.

Regards
Jose

[poobah]

There is no advantage; unless your copper traces are designed too small in the first place

In fact, in certain types of circuits this would create problems. It doesn't arise much in audio; but small thermocouples are developed at solder joints. These in turn create small voltages. when the circuit is constructed:

lead - solder - pcb || pcb - solder - lead
+++ --- || --- +++


These small voltages cancel themselves out... provided that both solder joints are at the same temperature.

[anatech]

Hi Jose,
Poobah is right, but this is generally not too much of a problem unless your levels are way down there. In industrial temperature measurement would matter a great deal for example. Test insterments is another really good example.

If the PCB is designed properly you should use the traces. Layout can be everything to the operation of many fast circuits. Like some op amps these days. This will ensure consistant performance from sample to sample of the circuit. It also makes component replacement much easier. I guess you weren't around in the days of point to point wiring.

High current traces may have provision for heavier conductors to be mounted in parallel. But, I wouldn't second guess the designer unless you have a lot of experience. That will take years of practice.

-Chris

[AndrewT]

Hi,
the tube guys reckon that hard wired sounds better than PCB mounted.
Much of their current equipment is still hard wired.

[poobah]

There was a man that 1/2 Irish and 1/2 Scottish. He wanted a drink, but couldn't bring himself to buy one.

[anatech]

Hi Andrew,
One wonders how much of that is due to high impedances and possible leakage from PCB material. That and PCB's tend to pack components more tightly, so capacitance between parts is higher.

Higher temperatures and high voltages would push me away from PCB's with tube circuits. Ever notice the holes/slots between parts in a well designed PCB at higher voltages?

-Chris
Edit: Poobah - now that's torture!

[electronjohn]

The main problem with tubes and PC boards is heat. If you look at a board from an old tube color TV (or even a cheap clock radio), you'll see severe discolorization around the tube sockets. The heat eventually causes the PC board traces to lift off the board, solder connections start to go bad, everything goes downhill. Looking at the chassis of tube gear, you'll often see holes punched in a ring around tube sockets...particularly power tubes. A little convection goes a long way. Tubes and PC boards just don't mix...unless significant forced-air cooling is provided.

[peranders]

Originally posted by spendorspain
It is said that hard-wiring components (soldering directly one to each other via its leads) is the best way to connect them and that using printed circuit boards is more for practical reasons (it's easier to place and solder the parts, can be automated...).

I'm afraid I think you have picked up a rumour. I can't think of many circuits that requires hardwiring.

[johnthedoctor]

When I last repaired TVs many of them had tubes. A popular model in the UK was the Decca 30. 'Set dead' was 90% down to its sound tube burning a hole in the pcb and cutting the tubes heater supply. It was such a common fault I had lengths of wire pre cut ready to bridge the pcb. I'd done the repair so often that from walking into the customers home to leaving them with a working TV was less than 10 minutes. The Phillips G6 was point to point but it was less reliable and a pure swearword to repair.

So yes, point to point if there is heat involved. IF and only IF the design is good to start with. For low temperature high frequency designs it's a whole new ball game. From what I've learned so far the unit stands or falls by the quality of the pcb design.