Component Considerations

Component issues in your design can take on one of two common characteristics: landing patterns and thermal issues. Let’s look at both.

PCB manufacturing shops will drill, plate and stencil your board to match the landing footprint you used in your design. There is, however, no guarantee that the footprint you used is the exact match for the part you ultimately order. While this seems quite obvious, engineers of all levels of expertise still run into this problem.

An assembly house will tell you to be extremely diligent about double-checking the accuracy of the footprint symbol you use on your board. Duane Benson, Marketing Manager at Screaming Circuits, for example, says “Usually when I talk about parts library issues or footprint issues, I’m referring to tiny QFNs or new exotic chip scale parts or things like that. Well, even bigger, older parts can have library issues.

This thru-hole switch is a good example. It looks like the footprint is on a .1 inch pitch and the switch pins are at .09 or a metric pitch. It’s interesting that the tabs on the outside of the part are in the right spot, even though the pins aren’t.”

Benson goes on to say, “we’ve found a number of switches and relays with non-standard footprints. If you don’t find the exact part number in your component library and substitute something close, make sure to double check the part you selected for fit before sending the boards out for fab or assembly.”

Speaking of component selection, one should make sure not to overlook some basic pitfalls. With any board and a myriad of components and footprints to work with, it can be easy to make a library or BOM edit resulting in lost time and/or cost. Parts library and BOM verification can greatly reduce the potential for error.

The 3D Envelope

PMC cards are mezzanine cards. That is, they attach to another board and must be designed to exist in the space between two boards. Physical and thermal issues within this space can be of particular concern.

Carefully map out the thickness tolerances available to you between boards. Give careful consideration to the thermal flows. If you know that there’s a heat-generating part underneath your mezzanine card’s ultimate location, plan for that. Don’t place your main heat generating components over/near the other heat source.