today's market offers many heatsink attachment and thermal interface options. your design decisions usually aim to optimize product performance at minimum cost. secondary effects, not always sufficiently considered, significantly impact production cost, delivery performance, and reliability. these effects stem from the impact of the heat sink attachment design on the product assembly and disassembly processes.

a product designed for easy assembly will have more consistent quality, reliability, and delivery performance. the ability to remove and replace the heat sink without damaging the device, the heat sink, or neighboring devices can also improve production parameters.

to optimize assembly and repair processes consider these questions:

• is the heat sink easily removable for repair?
• how many different parts and materials are needed to install the heat sink onto the board?
• will personnel be exposed to any chemicals during installation or cleanup?
• will the assembly process cause ergonomic problems for personnel?
• are specialized tools or equipment required?
• how sensitive is the assembly process to surface flatness or contamination?
• will waste material for disposal be generated by the process or during cleanup?
• will removal cause damage ?
• are attachment reliability and thermal performance sensitive to assembly techniques that may be inconsistent among assemblers?

mechanical attachment

mechanical attachment generally consists of screws or clips that affix the heat sink directly to the device or to the pcb. the issues to consider here are the interface between the heat sink and the device, the number and type of parts needed, and the stresses imparted to the pcb and the device. a thermal interface material is often needed to reduce contact resistance at the interface. this interface material may be applied during board assembly (usually the case with thermal grease) or it may be pre-applied by the heat sink vendor as a dry material.

assembly and repair operations will usually benefit from a dry pre-applied interface material. this reduces assembly time and eliminates cleanup and re-application of grease and exposure to chemicals in case of repairs. the pre-applied material will usually give more predictable and consistent thermal performance because the application thickness and coverage area are well controlled.

when designing the fastening system, consider the number and type of fasteners as well as the effect on the device and the pcb of the forces imparted by the hardware. screws and clips generally permit the heat sink to be installed and removed without damage. clips also generally accomplish the installation with the fewest parts. when designing for adequate contact pressure, consider the effect of the hardware on the devices and the pcb. high contact pressure designs can impart substantial camber to the pcb.

depending on the complexity of the design, a mechanically attached heat sink is often the easiest type to remove and replace for repair. in particular, designs that clip the heat sink directly to the device are simpler to replace without damage to the device, the board, or the heat sink.

adhesive attachment

adhesive attachment is accomplished with double-sided tapes or dispensed adhesives such as epoxies. the advantages can be significant in the assembly process, however there are some drawbacks to repair. adhesive tapes offer many design and assembly advantages. tapes can be pre-applied to the heat sink for simple "peel and stick" assembly with little waste generation or chemical exposure. the attachment and thermal interface are combined in a single material. also camber-inducing stress on the pcb is minimized.

one concern with adhesive tape attachment is that interface resistance and attachment reliability may be affected by surface flatness or contamination. special tooling may be required to ensure proper bonding time and pressure are attained at assembly, particularly with delicate heat sinks. if the heat sink must be removed for repair, damage may occur and personnel may need chemicals or heat to remove adhesive residues.

the "wetness" of dispensed adhesives offers some advantages and disadvantages as compared with tapes. employees will likely be exposed to the adhesive and cleanup chemicals when working with the dispensed material. thermal performance may vary with adhesive thickness and coverage, thus tight process control may be required to ensure consistent results. waste for disposal may be created from dispensing equipment cleanup. however, these materials tend to be more forgiving of surface flatness and roughness.

repair

the cost and anticipated quality level of the components requiring heat sinks and the production volume and expected yield of the complete assemblies should influence the weight you place on repairability. a custom high power asic (application specific integrated circuit) may prompt a repair-friendly design. such devices may be subject to running design changes requiring component replacements on the fly or on recalled or upgraded products.

if a costly device is incorrectly installed it can be cost effective to remove, rework, and re-install that component provided the repair process is not too burdensome. an additional point is that quality improvement processes usually require fma (failure mode analysis) of certain defective components. this analysis is sometimes difficult or impossible if the component is damaged during removal. certainly the more costly and specialized the device is, the more important these repairability concerns are.

conclusion

design of heat sink attachment and interface systems must address manufacturing and repairability concerns in addition to thermal performance and reliability. a successful design will meet the thermal performance and reliability requirements and will have attachment and interface details appropriate for the product and manufacturing resources used.

david r. riese
lucent technologies