Mechanical Design Question

[macduff]

Hi Folks,
I have a mechanical design question. What are the standard tolerances of components of a final board assembly?

Example:
If I have board with a known thickness I know it’s tolerance from the fabrication drawing. Then, I have a component and know its tolerance from the specification drawing. What I’m looking for is the final assembly tolerance when the assembler solders it to the board. Is there a gap between the board when the assembler solders it due to heat, which cause it to lift off the board? Or does it pretty much stay flush against the board?

I’m doing a tolerance stack-up, and need to know if I need to have tolerance added to my calculation. Shall I add an extra .005 in for good measure?

Thanks,

Colin


Macduff
Colin Fitzpatrick
Mechanical Design Engineer
Solidworks 2007 SP 4.0
Dell 390 XP Pro SP 2
nVida Quadro FX 3450/4000

 

[Skogsgurra]

It depends a lot on what components are involved. Some resistor, capacitor and transistor packages are almost flush with board surface (if SMD). Chips usually have little legs that raise them above board surface and through-hole pins have shoulders that keep components off the board surface. And, then again, there are countless odd shapes that you really have to look up in their data sheets.

All components have detailed data sheets. You have to look there.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[MikeHalloran]

When components had wire leads and boards had holes, it used to be standard practice to space components away from the board, to allow for flow of cooling air.

SMD parts do move around a little in reflow, and occasionally go edge-up instead of face-up. Whether that's a problem in the assembly is a design decision, i.e. it would be unacceptable in a pcmcia card, but might be okay in a generic box.

I'd add the .005" just for fun.



Mike Halloran
Pembroke Pines, FL, USA

 

[itsmoked]

There is no question the parts lift up on the solder though its not very much. 5mill is probably about right. You may need to actually test it as it will depend on a lot of variables.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[TurboXS]

The mechanical engineers we worked with used to hate us electronics engineers as we are generally very slack with our dimensions and tolerances, compared to mechanical engineers.

There are a few reasons for this;

1. Often we use multiple source parts or even generic parts so there are changes in dimensions.

2. Board assembly may result in parts "floating" in the solder bath which means that they don't sit flat on the board.

3. We like some clearances so that air can flow around the board.

4. Lead trimming can be either machine or by hand. So can be uncontrolled.

5. PCBs can vary due to whether they are routed or grooved. They can also have a warp. Hole sizes in PCBs can also vary.

As a result if you ask us what size enclosure we need we would add a few millimeters or more to accommodate the items above.

An example of this is the 19" rack system. The cage nuts allow at least a millimeter movement.

Just to annoy the mechanical engineers more, we work in both metric and imperial at the same time.

Of course there are exceptions to this such as camera, mobile phones etc. And I am sure others on this forum are much more precise/accurate than us.

 

[macduff]

Hello,
Sorry I didn't respond, our email was down for a couple of days.

I think I'm going with MikeHalloran comment and add .005 in my stack-up. I really appreciate everyone for taking the time to respond to my post. I really liked TurboXS's comment about Electrical and Mechanical Engineers sometimes not playing well together. It made me smile because it is so true. Bottom line is we come to a happy medium and get the product out the door.

Thanks again, and have a good week.

Colin


Macduff
Colin Fitzpatrick
Mechanical Design Engineer
Solidworks 2007 SP 4.0
Dell 390 XP Pro SP 2
nVida Quadro FX 3450/4000