Board-mounted 3PDT's: any real life evidence against them?

[Taylor]

I've heard many people say that mounting the bypass switch to the board is bad, because it can stress the board and crack traces. While I can definitely imagine that to be true, I'm wondering how much of a problem it really is. For example, MXR does this on a number of their pedals, and people generally rate them as a very reliable pedal company. The time-savings in building is obvious when board-mounting, but if the failure rate is high, this would negate the initial savings by having to pay to fix lots of pedals.

Does anyone have real-world experience of cracked boards because of board-mounted switches?

[JKowalski]

How would it stress the board?

[Naz Nomad]

That would only be evident on a poorly designed, poorly built, poorly supported PCB. No problem using them if it's done properly.

[Taylor]

That would only be evident on a poorly designed, poorly built, poorly supported PCB. No problem using them if it's done properly.

Could you elaborate on that a little? What sort of design considerations would make this a problem? So many boutique builders don't use board-mounted switches, even though it would obviously save time and be neater. The only downside other than the stresses to the board would be slightly higher board costs.

[Naz Nomad]

... for instance, if you had the switch mounted on one corner of a large PCB with no other standoffs or support. That would cause a lot of unnecessary stress on the area around the switch.

[skiraly017]

My two cents for what it's worth...

Board mounted jacks and switches do cut down assembly time and while I personally have not come across any that have failed because they were board mounted, I did find out the hard way that having to replace a faulty jack or switch that's been board mounted may be difficult to remove cleanly, resulting in a broken trace/pad or having to replace the entire board (if you're doing this on a commercial level).

I'm all for the uber-clean look internally but if a freestanding switch goes bad the replacement cost is about $4. If the switch is board mounted and can't be removed cleanly, you're out the cost of the switch, PCB and components.

[jacobyjd]

I don't have a real-world failure story

However, I have dealt with at least one project that was entirely board-mounted, and the main difficulty I ran into was that the different panel-mounted components were oriented in various directions, some 90 degrees apart. Because the enclosure was a tight fit, I had to take a lot of extra care to make sure nothing was being stressed while tightening up all the nuts...heh...nuts.

Anyway, in my [small] experience, good design and careful assembly will get you everywhere in terms of board-mounting panel-mount components.

Watch your tolerances

[frank_p]

I've read somewhere in the army literature (perhaps old stuff) that components having user interface must be mounted on the chassis and the board on posts.  Now, a stompbox is not a warplane... 

In flexion, it's always the outer planes that whitsand the most traction and compression stress.  So basically where the copper traces are.

Ex:


If your nuts get loose you might have repetitive stress (fatigue) that might eventually develop into small cracks.

So you see where the red part is, is where there is more tension stress and at the bottom (the blue zone) there is more compression.  That zone is where the beam or plate is stuck in the wall (or bolted).  Basically where the degree of liberty is blocked and where the is more torque (Force * length of the lever).

The bad way of doing thing is like on the Boss square 9V plugs: not secured to the box and only secured by the PCB traces.  I've repaired some like this.  Cracked and lifted traces.

So if you want to write Milspecs on your box don't solder your jacks and pots on it.

A connection should not be linked to the structural part of the device if you want to work by the book.  Its function is for the circuit.  The more the circuit will be structurally or mechanically connected at many places and secured in holes of the box that may not be well positioned the more you will have regions of stress.

Plus soldering the tabs on the PCB will lead to more heat going to the inside of the switch and this is a cause of switch failure.  Now I don't think it's critical, just better done.

[R.G.]

I've posted my experiences. Take it for what it's worth.

The number of failures was a quite small percentage of the pedals, even after years in the field. However, that was the most common single place where the pedals as a whole failed, either by cracking solder or the switch simply going intermittent. It outnumbered all other failures put together. We chose to design it out instead of deciding it was small enough to ignore; kind of like the (probably apocryphal) Rolls Royce approach, where you find the weak places and then design those places to be stronger.
We voted for "more reliable" instead of "reliable enough". Probably a sufferer from MBA Disease would be horrified at that. 

How big was the percentage? Sorry, you'll have to do your own reliability engineering to come up with the numbers for your exact setup. It varies depending on the details of the box, board, other components, etc. Even if I told you, it would not be the same for your box/board/connections, etc. 

It might even be no problem to you at all.

[frank_p]

We voted for "more reliable" instead of "reliable enough". Probably a sufferer from MBA Disease would be horrified at that. 

When you can laugh at this, it mean you are not under their power.  These guys are dangerous.   

[Naz Nomad]

'Comedy Statement of the Year Award' goes to ...

"If your nuts get loose you might have repetitive stress."

[Processaurus]

If there is one thing not to PC mount, it would be the switch, as it is getting stomped on regularly with the kind of force that regular mass production consumer electronics (probably the origin of PC mount construction) would only see if they were dropped.  If the nut gets loose, like others have said, you're stomping on your PCB, torquing it against whatever is still holding it to the enclosure (the pots probably).  One conscientious way one could do it is have the PCB on standoffs on either side of the switch, so if the nut was gone, and you're mashing the switch against the PCB, it is still supported enough to withstand someone standing on it...

I like Line 6 and Tech 21's approach (and I believe visual sound's), using a sturdy actuator (like small bear sells) to press gently on a tact switch (or tiny DPDT, also small bear) on the PCB.  Loctite the actuator so it can't get loose, and it should be pretty bulletproof while being quick to manufacture.

[Naz Nomad]

If the nut gets loose, like others have said, you're stomping on your PCB, torquing it against whatever is still holding it to the enclosure ...

Loctite the actuator so it can't get loose, and it should be pretty bulletproof while being quick to manufacture.

Adding to this ... if you think your switch is insufficiently tightened (poor construction) and may work loose, why not just Loctite the nuts on the 3PDT?

[connie_c]

I do lots of repairs so here my 2 cents,

I repaired a big muff the other day, in the new die cast case they are using now.  It had a board mounted switch that had failed and it was a bitch to get out of the pcb. The switch had popped apart where the metal pinches the plastic and holds agains a raised edge (Ive seen this lots in amp footswitches).

It put me off the idea of board mounting and it also gave me the idea to put a little glue on the outside of the 3pdt so it wouldnt happen again. Ive done it to all the switches I have because i think it should definitley make them more reliable against this type of failure.

Also as a giging musician I know how easy it is to step on a pedal harder than you should. Energetic singer on small stage bumps you as you step on your boost. You can imagine.

I also come across plenty of broken solder on inputs to amps. I think board mounting input jacks on amps is a very bad idea and I always replace them with off board inputs if there is space in the amp. Some people plug in and out of pedals as often as their amp, not me because its all on a board but I think if you are building for other people it should be designed with maximum reliability in mind.

I think board mounted pots is a good idea though as it makes everything nice and neat and I would guess slightly increases reliability(i have no evidence for this) and it also allows the board to be supported without you needing to glue it in or whatever.

[Processaurus]

If the nut gets loose, like others have said, you're stomping on your PCB, torquing it against whatever is still holding it to the enclosure ...

Loctite the actuator so it can't get loose, and it should be pretty bulletproof while being quick to manufacture.

Adding to this ... if you think your switch is insufficiently tightened (poor construction) and may work loose, why not just Loctite the nuts on the 3PDT?

No, poor construction is planning on it never needing to be serviced, the actuator is the only thing that could make sense to Loc-tite, as it becomes a permanent part of the box, and is separate from the electronics.

[Naz Nomad]

Loc-titedoesn't make an unbreakable join, it just stops stuff coming loose until you need it to ... except maybe with skin. 

[dschwartz]

why you have to post this now? i was just designing a 100% PCB mounted pedal, since wiring each pedal takes more than 3 hours.

IMHO, PCB mounted 3pdt are less prone to fail due to overheating or loose wires (which have been the most common source of failure as far as know), since the pcb helps dissipating exxesive heat when soldering, and also, it can be soldered faster and with a more precise manner.

I think the best approach would be using a 3pdt pcb like the one smallbear sells (i make my own for that), avoiding the risk of bending the PCB if the 3pdt gets loose.

Anyway, i´d think that any less than smart musician would re-tighten the 3pdt if it gets loose.
A loose 3pdt will fail with or without PCB by copper or wire fatigue.

at the end, the best way to make switching reliable is using electronic switching. mechanic switching sucks on the reliability side.

[Naz Nomad]

... since the pcb helps dissipating exxesive heat when soldering ...

Would a copper wire soldered to an offboard switch not dissipate more heat than a PCB pad though?

[Mark Hammer]

My understanding is that in many instances the 3PDT is holding up the board, rather than imposing any stress on it.  That is, the switch serves as the standoff for the PCB.  The risk that might exist would be when the pots are also used as a standoff, and the chassis the switch and pots are attached to yields a bit.  So, if I had a folded-metal chassis (hmmm, where have I seen that before? ), and the board is secured to the stompswitch only, any give to the chassis when I press hard does not transfer to the board as stress; it simply moves up and down with the switch.  Now, if the pots were also secured to the chassis, or even the jacks, and stepping hard on the 3PDT resulted in stress being focussed on the point where the switch is secured to the board, THAT might be expected to invite risk of failure.

But all of that revolves essentially around managing the risk of uneven pressure or multiple sources of pressure.  If the chassis is a 1590B (which is about as rigid as chassis are likely to get), the odds of ANY sort of vertical pressure being transmitted to the board via the switch is slim to none.  If there is any risk to the board at all in those circumstances, I imagine it might come from:

• securing the pots to the chassis at an ever-so-slightly different distance from the PCB than the switch, placely stress on it, and risking some fracture down the line
• heat-related failure that comes from soldering all switch pins at once rather than doing one at a time and permitting it to cool off so the internal grease doesn't liquify

So, it CAN be the case that certain misuses, or uses with poor quality control, can place a PCB at risk when board-mounted 3PDTs are part of the equation, but when properly managed there is no inherent risk that comes from just using a board-mount stomp-switch.

[R.G.]

why you have to post this now? i was just designing a 100% PCB mounted pedal, since wiring each pedal takes more than 3 hours.

Just your buds looking out for you. 

IMHO, PCB mounted 3pdt are less prone to fail due to overheating or loose wires (which have been the most common source of failure as far as know), since the pcb helps dissipating exxesive heat when soldering, and also, it can be soldered faster and with a more precise manner.

Failure due to overheating is purely a manufacturing process

My understanding is that in many instances the 3PDT is holding up the board, rather than imposing any stress on it.  That is, the switch serves as the standoff for the PCB.  The risk that might exist would be when the pots are also used as a standoff, and the chassis the switch and pots are attached to yields a bit.  So, if I had a folded-metal chassis (hmmm, where have I seen that before? ), and the board is secured to the stompswitch only, any give to the chassis when I press hard does not transfer to the board as stress; it simply moves up and down with the switch.  Now, if the pots were also secured to the chassis, or even the jacks, and stepping hard on the 3PDT resulted in stress being focussed on the point where the switch is secured to the board, THAT might be expected to invite risk of failure.

But all of that revolves essentially around managing the risk of uneven pressure or multiple sources of pressure.  If the chassis is a 1590B (which is about as rigid as chassis are likely to get), the odds of ANY sort of vertical pressure being transmitted to the board via the switch is slim to none.  If there is any risk to the board at all in those circumstances, I imagine it might come from:

• securing the pots to the chassis at an ever-so-slightly different distance from the PCB than the switch, placely stress on it, and risking some fracture down the line
• heat-related failure that comes from soldering all switch pins at once rather than doing one at a time and permitting it to cool off so the internal grease doesn't liquify

So, it CAN be the case that certain misuses, or uses with poor quality control, can place a PCB at risk when board-mounted 3PDTs are part of the equation, but when properly managed there is no inherent risk that comes from just using a board-mount stomp-switch.

problem. It is entirely possible to make them solder properly and consistently. It may be hard to do this with manual soldering, since by definition every manual solder joint is a unique process.

I think the best approach would be using a 3pdt pcb like the one smallbear sells (i make my own for that), avoiding the risk of bending the PCB if the 3pdt gets loose.

It is a good way around some of the issues. The 3PDT board is so small and is mechanically disconnected from the rest of the mechanics so that it's much more robust about solder cracks. However, I really don't see the advantage of a micro-PCB for the switch if you then have to solder wires to the main PCB and to the baby PCB anyway if you're building by hand. There are other circumstances in volume manufacturing, but by hand, you have to solder the same number of wires into a hole one place or the other, and you have the expense of the baby board.

Anyway, i´d think that any less than smart musician would re-tighten the 3pdt if it gets loose.

You must have smarter musicians there than we have here. 

A loose 3pdt will fail with or without PCB by copper or wire fatigue.

Eventually, yes. The copper wire is much more forgiving because the stress is spread over a much larger length of wire, so the stress at any one place is lower. Much lower.

at the end, the best way to make switching reliable is using electronic switching. mechanic switching sucks on the reliability side.

Yep. The problem is then that non-technical musicians have been conditioned, Pavlov's-dog style, to salivate when they hear "true bypass... true bypass" whispered in their ear by boutique builders. 

My understanding is that in many instances the 3PDT is holding up the board, rather than imposing any stress on it.  That is, the switch serves as the standoff for the PCB.  The risk that might exist would be when the pots are also used as a standoff, and the chassis the switch and pots are attached to yields a bit.  So, if I had a folded-metal chassis (hmmm, where have I seen that before? ), and the board is secured to the stompswitch only, any give to the chassis when I press hard does not transfer to the board as stress; it simply moves up and down with the switch.  Now, if the pots were also secured to the chassis, or even the jacks, and stepping hard on the 3PDT resulted in stress being focussed on the point where the switch is secured to the board, THAT might be expected to invite risk of failure.

Correct! You sure you're not a mechanical engineer?

As long a the board is held in one plane, and the fixing in that plane is not in itself stressful (like, with pots which are not ...exactly... the same size as the switch standoffs, and the fixing substrate (i.e. the box) does not try to twist or flex the board, then the board is reinforced by the mounting, not abused by it.

But all of that revolves essentially around managing the risk of uneven pressure or multiple sources of pressure.  If the chassis is a 1590B (which is about as rigid as chassis are likely to get), the odds of ANY sort of vertical pressure being transmitted to the board via the switch is slim to none.  If there is any risk to the board at all in those circumstances, I imagine it might come from:

• securing the pots to the chassis at an ever-so-slightly different distance from the PCB than the switch, placely stress on it, and risking some fracture down the line
• heat-related failure that comes from soldering all switch pins at once rather than doing one at a time and permitting it to cool off so the internal grease doesn't liquify

Correct. I used all the background I had gleaned from IBM's mainframe mounting of PCBs in the Workhorse amps. Mount boards in one plane. Stiffen the PCBs if the unsupported length can allow mechanical flex bigger than the linear stress-strain range of solder alloy. Attach all devices subject to mechanical interaction by flexible connectors, which themselves flex less than the linear stress-strain range of their conductors in the flexed area. Take connectors off the areas right next to the mechanical fixing points. Do not use electrical connections for mechanical stiffening. And more. Here's a libation to Ralph Haus, who was willing to teach me over a period of years about some of this stuff without shooting me for both my ignorance and arrogance in his domain.

So, it CAN be the case that certain misuses, or uses with poor quality control, can place a PCB at risk when board-mounted 3PDTs are part of the equation, but when properly managed there is no inherent risk that comes from just using a board-mount stomp-switch.

Correct.

And quit posting while I'm composing one of my tomes; I keep having to update and I'm not that good a typist!