Brass ground plate. Necessary or not??

[glivo]

I'm hoping someone here can advise me or at least reinforce my thoughts.

I have just made a 5F2 chassis from an old shop front steel sign I had lying around. To my pleasure once I'd removed the sign sticker and sanded pack the undercoating I discovered that the material is actually Tin plated steel. Of course this means that I can solder to it quite easily and I have done a couple of test runs with perfect continuity and zero resistance measurements.

Would this mean I can ground straight to the chassis and thus remove the need to have the Brass grounding plate that is normally used behind the input jack and volume / tone control pots? My understanding is that it's only there to facilitate soldering.

If not why not?

Thanks
Greg

[nuckollsr]

One could probably turn this into a real science fair experiment . . . but it would take years to deduce the outcome. If you can get a wire or other conductor to make a well-wetted bond with your chassis by soldering, I would suggest that the connection is sufficient for most purposes. The arguments that favor a common ground plate of copper or brass can be supported by caveats on dissimilar metals reactions and ground loops. All of which are exceedingly weak effects.

I've had airplane builders ask about the corrosion that appears over soldered together copper wires exposed to weather. This is a surface-only condition. Where the "electrons hit the road" is inside the joint. When you twist two nice and shiny copper wires together, you have at that instant about as low a resistance joint as you can get with those materials. The addition of solder over the twists has more to do with encapsulating the copper to the exclusion of moisture than it does with making the joint "electrically more pure".

Soldering directly to your steel chassis will be electrically adequate years after evidence of surface corrosion show up . . . which is probably years after you've lost the ability to enjoy the amplifier.

[c2cthomas]

The life of the solder joint will last even longer if you take the time to properly clean off the solder flux. Juz say'n.....

[Andersonix]

It doesn't make sense to me that with a >typical< soldering iron one can get enough heat into a large piece of metal to make a reliable solder joint. Sure, the solder might stick and be 'zero-resistance' today, but I suspect it's actually a 'cold joint.'

[glivo]

Thanks guys. I'm not sure how I'll proceed at this point and I take all of you points on board. I believe the grounding direct to chassis should be effective but you just never know.
@andersonix - I agree completely and we all know how difficult it can be to just solder onto the back of a potentiometer with a weak iron. The test pieces I did using a pencil butane torch for fast pinpoint heat. Once I have that initial good bond I can then use a decent iron to wet other components or wires into the blob.

A little googleing provided another solution which is a length of heavy gauge copper wire attached for grounding purposes rather than a plate. There is also some argument out there in favor of just using toothed washers between components and chassis.

Once again thanks for the input.

[nigelwright7557]

Originally Posted by Andersonix

It doesn't make sense to me that with a >typical< soldering iron one can get enough heat into a large piece of metal to make a reliable solder joint. Sure, the solder might stick and be 'zero-resistance' today, but I suspect it's actually a 'cold joint.'

I would agree, I sometimes have enough trouble soldering to large PCB pads never mind a metal plate. You would have to use a very high power iron to make a good joint.

[Johnny2Bad]

Originally Posted by nigelwright7557

I would agree, I sometimes have enough trouble soldering to large PCB pads never mind a metal plate. You would have to use a very high power iron to make a good joint.

With big metal I've preheated the target with a hot air gun and then applied the iron, solder and pre-tinned wire to finish the connection. Works well and is easy on your soldering iron which doesn't have to work so hard to create the temperature rise all day long.

There are also some putty-like materials that can heatsink (maybe not the right word ... it's a barrier of some kind, anyway) the work area, preventing the temp from going elsewhere in the metal. Places like Eastwood sell it. You remove it after you're done. Never tried it (didn't have to) but for what it's worth you could look into it, especially if a heat gun is too broad and might loosen other soldered connections in your work area. Micro torches with open flame are a bit unwieldy in some cases, burning instead of heating, but if you can work it without damage, it might also work.

Also, although it doesn't sound like it from your results, be sure it's tin-coated and not galvanized steel. Not healthy.

[nuckollsr]

It doesn't make sense to me that with a >typical< soldering iron one can get enough heat into a large piece of metal to make a reliable solder joint. Sure, the solder might stick and be 'zero-resistance' today, but I suspect it's actually a 'cold joint.'

A cold joint is one where the joint is disturbed while the solder is cooling from a liquid to plastic to solid phase. A eutectic alloy of solder has very close to zero plastic phase and is relatively incapable of a "cold joint".

I would agree, I sometimes have enough trouble soldering to large PCB pads never mind a metal plate. You would have to use a very high power iron to make a good joint.

It doesn't take a lot of power, just get it all into the right place. For example, I have Metcal solder stations good for not more than 30w of power . . . but it's nearly ALL at the tip. See:

metcal | eBay

With the largest tip in this tool I've soldered solid copper lugs to 2AWG wire using 63/37 solder and got good flow. It's not FAST but it will do it. Similarly, the old 47W, Ungar 120 volt elements with solid tips (no tiny screw-ins!) put a gratifying concentration of heat into tough jobs.

UNGAR MODEL 363 SOLDER IRON ELEMENT 45W NEW W/ TIP | eBay

The secret is not a secret. The right concentration of heat, GOOD solder, and clean parts will make some pretty amazing joints with ease.. I do a lot of prototype and even some one-of production products in enclosures made of copper-clad ECB material. You can shear the sides, drill and punch holes, and solder it together with a 30w Metcal.



You end up with a box that is already finished in a nice blue color without painting it. If the material is double sided, you get a copper exterior.

http://aeroelectric.com/Pictures/Too...tech_DAS_2.jpg

Solder is not a glue. It forms a new alloy of molten solder and base metal in the few molecules thickness of the solder/conductor interface. Copper readily dissolves into molten tin/lead acting as a as a solvent . . . just like salt dissolves in water. The thing about 63/37 tin lead is that the melting point of eutectic solders rises sharply as the alloy mix shifts. This means that a wire bathed in solder is shedding molecules of copper into the pool . . . but as more the more molecules join the mix, the temperature for keeping the "pool" liquid rises and the 'swimmers' find themselves in an ever thickening raceway. The end result is that only a few molecules thickness of copper have joined the solder in the well-wetted joint.

When you 'tin' a wire, you find that you cannot reheat the wire and wipe the tin-lead coating off . . . because it is no longer tin-lead. It is now tin-lead-copper at some melting temperature much higher than your iron delivers. Hence, tinned forever.

[glivo]

Originally Posted by Johnny2Bad

k.

Also, although it doesn't sound like it from your results, be sure it's tin-coated and not galvanized steel. Not healthy.

Definately tinplate, not Gal.

[marce]

If during the soldering process the correct intermetalic layers are not formed the joint may become open at a later date due to thermal cycling and other envoironement issues.
Or as being said it is a dry joint, or will be in a very short time.
But as siad, pre heating the base will help the joint formation, but there is also the risk of to much heat being applied or applied for two long again this may cause joint failure.
A good solid ait tight mechanical join can be as good.