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# Heat Sink Calculator Use Instructions

How to use the online heat sink calculator to determine heat sink performance for a thermal solution using a solid metal base compared to a vapor chamber base. Input variables include fin height, fin thickness, heat source power, and heat sink dimensions. Since fin packs (zipper fins) are often used with two-phase devices, they will be used – rather than extruded, machined, bonded, or skived heat sinks.

The first thing you should calculate is your thermal budget which is simply Tcase max (or Tjunction max if bare die) that will be supplied by the chip manufacturer MINUS Max Ambient temperature at which the device is designed to operate. Thermal budgets above 40 oC usually only require an aluminum or copper base whereas tighter budgets may need to add a two-phase device such as a vapor chamber.

Heat Source Power (Q) – This is the thermal design power (TDP) which is the maximum amount of heat, in watts, generated by the chip. It should be provided by the chip manufacturer or ASIC engineer if done in-house.

Heat Source Length, Width, and Location – Self-explanatory especially with the provided diagram. Remember that chip location will greatly affect base delta-t for solid metal while its effect on the vapor chamber delta-t is negligible. A general rule of thumb is that vapor chambers become attractive as the ratio of base area to heat source area surpasses 10:1.

Base Information – The length and width dimensions you enter for the base of the heat sink will also be used as the dimensions for the fin pack. It’s important to note that LENGTH (of base and fin pack) will go in the direction of the AIR FLOW. Note: if you are in the early stages of heat sink design and need an estimate of the required heat sink size, refer to this online calculator.

Fin Pack Information – While fin height can vary considerably by application, we typically see figures in the 10-35 mm range. Fin thickness for fin packs (zipper fins) range from 0.2 to 0.6 mm while fin spacing/gap should be at least 2X the fin thickness. Next, input the fin pack material and the max operating temperature at which the device is designed to operate. You can use the ‘Fin Pack Temperature Rise & Pressure Drop” chart as a starting point for selecting the appropriate fan. Conversely, if you know the fan to be used, alter the fin and air flow variables to achieve acceptable pressure drop numbers. Lastly. choose the type of TIM to be used from the drop-down menu.

The Results – The first set of delta-ts are common to both the solid metal and vapor chamber base. In this example, we have a total fin pack delta-t of 27.3 oC, and a total TIM and Base to fin delta-t of 2.5 oC.

To this total of we need to add the base delta-T.

If your thermal budget (Tcase max – Tambient max) was initially calculated at 40 oC, we see that an aluminum base, with a total heat sink delta-t of 51.4 oC will not be acceptable. While changing both the fin and solid base material to copper (not shown) will get you below the thermal budget, it comes at a large weight penalty. In this case, a vapor chamber solution is clearly the better option.