Leaving a space between components and PCB board?

[canidoit]

I have noticed recently that some people are soldering their components like resistors and caps with a space between the components and the PCB.

Most commercial electronics units I have seen just have the components up against the board.

Is it good practice to leave the space? Are people noticing a burning of the board through time, when they do not place a space?

Is it necessary and if so, what areas in a circuit should you always leave the space on, eg. power supply section?

 

[MagnetoSound]

You only need to do this if there is a reason to expect the components to be generating significant heat. I usually leave an air gap under zeners and dropper resistors of 1w or above, especially if they are going to be dissipating at close to their power rating. Lots of OEM designs ignore this issue entirely, which can lead to scorched boards, dry solder joints and in extreme cases, parts falling off the board completely.

Particularly in high gain/high impedance circuits, standing caps and resistors up in the air can make them more prone to vibration, which is a disadvantage if there is a likelihood of microphony in the circuit, so keep low power components close to the board and use silicone if they need physical support.

 

[sahib]

The components are only raised off the board if there is a requriement for heat dissipation. Otherwise they should rest against the pcb at all times as a good mechanical practice.

At audio frequencies long lead generally does not pose much electrical problem but at RF it is a single turn inductor. It only has to meet the correct frequency to inject trouble into the circuit.

 

[canidoit]

You only need to do this if there is a reason to expect the components to be generating significant heat. I usually leave an air gap under zeners and dropper resistors of 1w or above, especially if they are going to be dissipating at close to their power rating. Lots of OEM designs ignore this issue entirely, which can lead to scorched boards, dry solder joints and in extreme cases, parts falling off the board completely.

Particularly in high gain/high impedance circuits, standing caps and resistors up in the air can make them more prone to vibration, which is a disadvantage if there is a likelihood of microphony in the circuit, so keep low power components close to the board and use silicone if they need physical support.

They should make PCB boards high temperature resistant. I don't know why they don't?

Is the technology still behind to make PCB boards high temperature resistant or just too expensive to get boards made like that?

 

[wtmnmf]

Also done to provide enough isolation to avoid arcing during hi-pot testing. I used to manufacture a medical device that regularly failed hi-pot testing in production until I took over.  I put the unit in a darkened room and located the arcing which was between the underside of a resistor and a ground trace running underneath.  The potential would punch a hole through the solder-mask and the coating on the resistor.  I specified the resistors be raised, and a piece of capton tape placed below, to solve the issue.

Other reasons for lifting the components have to do with getting proper flow during wave soldering, keeping solder away from the component body, providing an adequate gap between the PCB and the component so that flux/flux remover do not get trapped between during board wash, leaving adequate lead length for thermal isolation during soldering.  Sometimes I lift components just so that I'll be able to see the traces underneath.  Also, it makes changing a component easier, as you can clip the leads before de-soldering,  and you may have enough lead length left to reuse the component.

 

[bruce0]

I know it is not common practice, but I put the low wattage fireproof resistors (used as fuses in some circuits) high up for two reasons.  Best reason is that if one goes, I can cut it out easily and leave enough lead length to grab it when de-soldering.  But the real reason is if I fry one during testing and debugging, it won't leave a burned mark on my nice clean circuit board  :-\.


Most common reason that makes sense for heat dissipation via airflow around component - but sometimes (and always with surface mount), this can be done well with a solder flow area as a heat sink

 

[Andy Peters]

They should make PCB boards high temperature resistant. I don't know why they don't?

They do; that's what the "94V-01" mark you see on all modern PCBs indicates. They're able to withstand high temperatures, and they're self-extinguishing, meaning if that a part catches on fire, the board won't continue to burn when the part stops burning.

Proper thermal design indicates that parts shouldn't get so hot that performance degrades (or worse).
You need to allow space around hot components to enable whatever kind of convection cooling is possible inside the chassis.

-a


-a

 

[gemini86]

I've heard that gloating the high value resistors (1g) in a mic amp circuit reduces hiss, but I've never a-b'ed it to know for sure.