EC-24.7 Thermal Design, and Designing for Manufacture and Test

What this is and why it exists

Two questions decide whether a good design becomes a good product.

Will it stay cool enough to last? And can it be built and tested by people who have never seen it?

Both are answered on paper, long before production. The thermal half is a resistance calculation almost identical to an electrical one. The manufacturing half is a checklist that costs nothing at design time and a great deal afterwards.

The vocabulary

• Junction temperature — the temperature of the silicon inside a package.
• Thermal resistance — degrees of temperature rise per watt dissipated.
• Ambient — the temperature of the air surrounding the product.
• Thermal via — a via used to conduct heat from a pad into an inner plane.
• Fiducial — a mark on the board that a placement machine uses as a reference.
• Reflow — the oven process that melts solder paste and forms the joints.
• Tombstoning — a small part standing up on one end during reflow.
• Stencil aperture — the opening that decides how much paste is deposited.
• Test point — a pad provided for a factory test fixture to contact.

The mental model

Heat flows like current. Power dissipation multiplied by a chain of thermal resistances gives a temperature rise, exactly as current times resistance gives voltage. Junction to case, case to board, board to air. Add them up and add the ambient, and you have the junction temperature.

On most boards the copper is the cooling system. An array of thermal vias under a package moves heat into an internal plane. How much it helps depends on that plane's area and on what else dissipates into it. A ground plane already carrying heat from three other parts helps less than the data sheet's figure suggests.

The ambient is where optimism creeps in. A calculation at twenty-five degrees is meaningless if the board sits inside a sealed enclosure in a warm room. The ambient used must be the one inside the product, not the one in the laboratory.

On the manufacturing side, a placement machine needs fiducials, adequate spacing and a consistent orientation. Violating any of them turns an automated build into a manual one, at many times the cost per board.

Reflow defects come from the layout. Pad geometry and thermal balance decide whether a small part sits flat or stands up on one end. Stencil apertures decide how much paste arrives. These are layout decisions with direct manufacturing consequences, and a manufacturer will point at them by name.

Finally, a factory has to prove each board before it ships. That needs physical access to key nets, or a scan interface reaching every pin. Adding that access after the layout is finished is rarely possible, so it has to be planned with the placement.

What you should now be able to explain or do

• Estimate a junction temperature from dissipation and a resistance chain.
• Design a thermal via array and say what limits its effectiveness.
• Choose the correct ambient temperature for a thermal calculation.
• List the layout features an automated assembly line requires.
• Explain what causes tombstoning and how layout prevents it.
• Plan factory test access before the layout is complete.

Check yourself