Choosing the proper attachment material when attaching a component to a printed circuit board or other material involves a diligent review of the properties of the component being attached. Some variables to consider include:
> Material properties of the device
> Properties of the mating material
> Surface conditions, such as plating on both the device and mating material
> Subsequent assembly processes
> Thermal dissipation requirements
> Post environmental conditions
Solders and epoxies can be procured in various forms. Solder for instance can be in the form of preforms, solder paste and wire. Epoxies can be selected as a pre-form or in a dispensable form. Epoxies can have a range in thermal conductivity and in some cases have a higher thermal conductivity than some solders.
Material properties between the device being attached and the mating material require choosing a bonding material that can accommodate those coefficient of thermal expansion (CTE) differences. In the case of using solder, if there is a large difference in CTE between the mating items then a soft solder should be selected.
In terms of epoxy there are a wide range of epoxies that could be used to help with CTE differences. In terms of thermal dissipation requirements, typically solder has been the material of choice. However with recent advances in epoxies such as sintered epoxies, the thermal conductivity ratings for the epoxy are in some cases 2X that of standard solders.
Post processing assembly steps and total cost of manufacturing can also greatly affect the most appropriate material to use. Solders, depending on the component type allow the user to select either no clean flux-based options or solders with a flux that require post assembly cleaning. For instance, for bottom terminated components (BTC) a no clean solder paste should be used. However, if post processing is needed where the flux becomes an issue trying to clean this is more difficult and costly. If there are post steps such as post die attach or wire bonding that require the surfaces to be clean such as in the case of a hybrid design, then the fluxes will need to be either burnt off completely during reflow or post cleaned. When reviewing the plating on the device being attached in some cases such as raw dice, devices are offered with different plating options to allow the device to be either soldered or epoxied.
As we can see careful review of all the properties and overall manufacturing process are key to making the right component attach choice. With the goal of having a greater first pass success rate, these quick checks and decisions can help save both time and money.