Thermal paste
The classical material used is silicon or silicon free paste. It can get rather sticky and in the past the thermal transfer was not the best.
However the paste products has evolved lately and much of the paste is silk printed in the production process, which avoids most of the contamination of paste. Therefore it depends more on you production process if you choose paste or PCM materials. T- grease 880, T-grease 1500 (silicon pastes) T-grease 2500 (silicon free paste).
For isolated semiconductors mounted direct too heat sinks the thermal paste is the most cost effective. However thermal paste is not accepted in some areas of production or in products. The PCM materials are alternatives.

Phase Change Materials (PCM)
A Phase Change Material, (PCM) has a homogenous surface. Some materials are sticky on the surface and others are dry in room temperature. The first time the material is used the temperature has to rise above the phase change temperature above 50°C. Then the surface melts and fill in all microscopic gaps removing air and result in a very large thermal surface. That dramatically improves the thermal transfer.

T-pcm 580 series has the lowers thermal resistance. It is supplied in several thicknessesfrom 0.125mm to 0.25mm.
It is a film based PCM and demands a minimum torque to achieve its optimal function. The phase change temperature is 50°C.

T-pcm FSF52 consist of only PCM material and is 0.125mm thick.
The advantage is it has a phase change temperatur of 52°C and do not need a specific torque to be efficient. As long as the material is between two surfaces it will not flow out. It fits therefore larger modules or between heat sinks.

T-pcm 900 consists of only silicon based PCM material and is available in three thicknesses 0.125, 0.25, 0.5mm.
The advantage is a homogenous material and only the surface is affected by the phase change temperature. The phase change temperature is not distinct and the material need to "be burned in" at 70°C. It also needs a specific torque to be efficient.

T-mate 2900 is made for processor applications. One side, towards the processor has a PCM surface that will not stick to the processor. It permits the user to change processor without changing the PCM material. It is based on the T-mate 900.

The advantage of phase change materials compared to silicon paste:

• Better reliability, silicon grease can spread and the silicon free paste has a tendency to dry.
• They are not sticky in production and you have a better control of thermal transfer assembly.
• Can be shaped after the component.

Advantages of the paste

• Modern paste has has a better total thermal tranfer.
• Screen print methods minimize contamination and material surpluses.

Graphite (Thermal and Electric conductive)
An alternative for mainly high power modules is the T-gon 800 a graphite based material. When mounted the upper surface is ”crushed” providing a very good electrical and thermal joint.  The graphite is conducting very well vertically and can be used to distribute hot spots. T-gon 800 has an extreme operating temperature range of -240 to +300°C. This is a perfect match for new high temperature silicon carbide high power semiconductors that can be used in a more efficient manner. The only drawback is maybe the price and the needed torque that needs to be comparably high.

My specified pad design?
All T-pcm materials can be cut out as separate pads according to you specification. It can also be mounted on feeder film.