Technical Information

Guideline for small size heat source(bare-chip)

Introduction

CORE

There are several issues to consider when attaching a heat sink to a heat source with a small contact area or bare die.


Review point
  1. Attachment method and reliablity
  2. Thermal interface for small heat source
  3. Heat sink for small heat source

1. Attachment method and reliablity

Typical attachment method and its suitablity for small heat source is as follows.

Attachment methodSuitabilityMemo
ADHESIVE Picture
Thermally conductive epoxy Not suitable Due to the small contact area, the epoxy will not provide sufficient bond strength. Also, the relatively high impedance of epoxy will affect the thermal performance significnatly.
TATE Picture
Thermally conductive adhsive tape Not suitable Due to the small contact area, the tape will not provide sufficient bond strength. Also, the relatively high impedance of tape will affect the thermal performance significnatly.
CLIP Picture
Plastic clip Not suitable Solder balls or chip substrate may be damaged during heat sink assembly due to localized stress.
Also, the heat sink size is limited to the chip/clip size.
ZCLIP Picture
Z-Clip Good Provides stable attachment to heat source and transfers load to mounting anchors/PCB.
PIN Picture
Push-Pin Good Provides stable attachment to heat source and transfers load to PCB.
Also, allows for tight control over mounting force and load placed on chip and solder balls.
SS Picture
Shoulder Screws Excellent Provides stable attachment to heat source and transfers load to PCB, backing plate or chassis. Suitable for high mass heat sinks.
Also, allows for tight control over mounting force and load placed on chip and solder balls.
About stability
SP Picture SP-32-23-1-SRU Picture

When a heat sink is attached to a small heat source, the heat sink can tip or tilt. If this happens, there is a possibility of damaging the die. To prevent this, we can apply sponge pads to the heat sink. The sponge pads help to stabilize the heat sink during attachment. We have several pad options, please click here for details.


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2. Selecting a thermal interface material for small heat source

Thermal Interface Material(TIM)is applied between the heat source and heat sink. When the heat source is small, the performance of TIM becomes far more significant. The following example illustrates the importance of TIM performance.

Product Size Input Power Thermal resistance Temperature Rise
TPCM585 35x35mm 10W 0.01degC/W 0.10degC
10x10mm 10W 0.13degC/W 1.30degC
TPCM905C 35x35mm 10W 0.03degC/W 0.30degC
10x10mm 10W 0.32degC/W 3.20degC

* at 69Kpa(10psi)

As this shows, the use of a high performance TIM has a very significnat impact when the heat source is small.
Also, phase change material or gap filler material requires a certain attachment pressure/load in order to perform properly. The following examples display the impact of mounting pressure on phase change material performance.

Product Load Thermal impedance
TPCM585 35Kpa(5psi) 0.050degC(-in2)/W
69Kpa(10psi) 0.020degC(-in2)/W
TPCM905C 35Kpa(5psi) 0.088degC(-in2)/W
69Kpa(10psi) 0.048degC(-in2)/W

Note: (-in2)/W = Thermal resistance of 1 inch square size

The use of a Z-Clip, Push Pin or Shoulder Screw for attachment allows tight control over the attachment pressure/load, ensuring acceptable performance from the TIM.


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3. Selecting heat sink for small heat source

HEAT Picture

It is far more difficult to cool a smaller sized heat source. This is due the spreading resistnace of the heat sink's base material. The heat must first travel through the base in order to get to the outer fins.


HEAT-COPPER Picture

Alpha's copper embedded heat sink makes it possible to spread the heat from a small heat source to the entire base far more easily. The copper embedded heat sink is efficient and reliable.

(Standard copper embedded heat sink)


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