Technical Information

Guideline for small size heat source(bare-chip)

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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.

  • Possibility of heat sink tipping/tilting.
  • Possibility of the exposed die being damaged during heat sink installation.
  • Contact area can be too small to form a strong bond with epoxy or tape attachment.

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 epoxyNot suitableDue 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 tapeNot suitableDue 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 clipNot suitableSolder 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-ClipGoodProvides stable attachment to heat source and transfers load to mounting anchors/PCB.
QSZ-CLIP Picture
QSZ-ClipGoodProvides stable attachment to heat source and transfers load to mounting anchors/PCB.
QuickSet Picture
QuickSetGoodThe anchor pins are located inside of the heat sink footprint, they will act as guides when t he heat sink is installed. The tight clearance between the anchor pins and the clearance holes will greatly reduce the ability of the heat sink to tip or tilt during installation, preventing contact with the edges or corners of the die.
PIN Picture
Push-PinGoodProvides 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 ScrewsExcellentProvides 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 PictureSP-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.

ProductSizeInput PowerThermal resistanceTemperature Rise
TPCM512535x35mm10W0.02degC/W0.20degC
10x10mm0.20degC/W2.00degC
TPCM712535x35mm0.01degC/W0.10degC
10x10mm0.10degC/W1.00degC
TPCM58535x35mm0.03degC/W0.30degC
10x10mm0.40degC/W4.00degC
TPCM905C35x35mm0.03degC/W0.30degC
10x10mm0.32degC/W3.20degC

* 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.

ProductLoadThermal impedance
TPCM512569Kpa(10psi)0.20°C-cm2/W
345Kpa(50psi)0.10°C-cm2/W
TPCM712569Kpa(10psi)0.10°C-cm2/W
345Kpa(50psi)0.06°C-cm2/W
TPCM58569Kpa(10psi)0.40°C-cm2/W
345Kpa(50psi)0.15°C-cm2/W
TPCM905C69Kpa(10psi)0.31 °C-cm2/W
345Kpa(50psi)0.19 °C-cm2/W

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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