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Novel Heat Sink Assembly for Components on an Organic Circuit Board

IP.com Disclosure Number: IPCOM000247531D
Publication Date: 2016-Sep-14
Document File: 5 page(s) / 415K

Publishing Venue

The IP.com Prior Art Database

Abstract

Problem:

Heat sinks (HS) help remove energy dissipated by electronic components such as microprocessors and power regulator modules. It is desirable to mount a HS assembly securely on a circuit board to maintain high preload and resist shock and vibration. However mounting a heat assembly securely on an organic circuit board can produce unintended negative effects. Due to differential thermal expansion between a heat sink and a circuit board, cyclic strain can be produced on solder joints during a thermal cycle. Furthermore, to ensure reworkability of components provision should be made to demount a heat sink without excessively straining the circuit board and associated solder connections.

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Novel Heat Sink Assembly for Components on an Organic Circuit Board

Background:

In a computer server or other power electronics there are single or multiple printed circuit boards (PCB) that facilitate the functionality of the server. Various high powered surface mount components such as a QFN (Quad Flat No Lead) power regulators, mounted on the PCB, produce excessive heat. These QFN's require heat sink solutions to dissipate the heat and allow the components to operate within a safe operating temperature. Critical to dissipating heat from a component to the heat sink requires the use of thermal interface material (TIM) and high pressure mounting to insure good thermal contact. The modulus of the interface materials can vary in stiffness from rigid epoxy interfaces to flexible thermal pads.

Unfortunately the method of mounting a heat sink to a PCB can create unwanted thermally induced mechanical strain, within solder interconnects of a QFN. This strain occurs due to the differential thermal expansion and contraction of the heatsink and the PCB. For example the CTE (Coefficient of Thermal Expansion) of a PCB can be on the order of 20 PPM (parts per million) while an aluminum heat sink can be around 23 PPM. During ATC (Accelerated Thermal Cycle) reliability testing the server node cards are heated to 100 C and then cooled to 0 C. This cyclic heating and cooling creates cumulative strain within the joint and can lead to interconnect fatigue failure. This ATC reliability testing is conducted to predict component life due to power on/off cycles in the field. To exacerbate the problem, strain from initial board mounting can contribute to the overall life strain. In the case of the surface mount QFN a mounting technique using a hard rivet assembly creates a rigid coupling between the HS and the PCB and adds to an initial strain within a solder joint.

An assembly (apparatus) is presented that provides sufficient mechanical freedom to a heat sink to move in the plane of a circuit board while maintaining a constant normal load on the thermal interface material. A novel feature avoids excess strain producing riveting process by providing a split-post based snap-on assembly where the split-post component is surface mounted to the same organic board.

Detail:

Figure 1 shows a representative 4-module assembly on an organic printed circuit board

with a heat sink (HS) securely attached at both ends. During a thermal cycle, the strain

within a solder of a module is generated as a result of external and internal thermal expansion/contract process. It...