Tweet

Posted on 28 November 2020

Power Electronics Packaging Technology

 

Power electronics packaging plays an inportant role in the design of semiconductor devices and systems. The packaging technologies used in the production of power electronics components largely determine the electrical, thermal, and mechanical properties of the resulting device. Selection of proper packaging materials is paramount. This article provides several examples and comparisons of packaging technologies commonly used in today's power electronics market.

 

Ceramic Packaging

Disc and stud thyristors with ceramic caps represent excellent examples of hermetic air tight packages made of ceramic and metals. At the junction of the ceramic and the metal, the ceramic is metallized using screen printing technology. The connection is then made by brazing with hard solder. The connection cable is made of a copper braid and a crimp connection is made using an 'H-Bar'.

Glass-metal packages are also hermetically sealed. The glass is fused to the metal to create the air tight seal. Glass-metal housings are often used in hybrid circuits for aerospace and similar applications which require high levels of reliability.

Synthetic Packaging

Synthetic packages are used when the following conditions apply:

  • High temperature resistance
  • High mechanical strength
  • Non-inflammable or non-latching
  • High surface leakage resistance (CTI-value)
  • Easy processing (low shrinkage)
  • Low water absorption

A few examples of synthetic packages are shown in the table below:

Packaging material PBT PPE/PS PPS PC PA
Manufacturer Crastin Du pont Noryl GE Ryton Phillips makrolon bayer Ultramid BASF
Tensile Strength (MPa) 140 83 165 50 110
Long term Temperature Index (°C) 140   220/240 125 140
Softening Temperature (°C) 212 150 260 148 220
Flammmability UL 94 V0 V0 V0 V0 V0
Water absorption rate [%] 0.1   0.02 0.32 4.7
Surface Leakage resistance 275 250 225 2 600

 

Insulation Substrates

Another important aspect of packaging technology is the selection of material to use for the substrate. Many options are available and selection largely guided by the thermal requirements of the application.

Substrate Material Selection

A number of ceramics provide a high degree of insulation abilities and good thermal conductivity. The coefficient of thermal expansion is usually small and well aligned with slicon. The purity of the raw materials determine the thermal conductivity of the ceramic. The purer the raw material, the better the thermal conductivity of the ceramic. The production of ceramic discs is done via sintering of ceramic powder at high temperatures while adding small amounts of sintering aid (for example Yttrium oxide, Y2O3) .

Diamond is an exception among insulating materials since its thermal conductivity is approximately 5 times higher than copper. Diamonds are, however, extremely costly which hinders frequent use. For less stringent requirements, synthetic materials are used as insulators. Their thermal conductivity is however extremely low compard to ceramics. The table below shows a comparison of the most important properties of insulating materials as well as silicon.

Material Thermal conductivity Coefficient of Thermal Expansion General Remarks
Al2O3 (Aluminium Oxide) 24 - 27 6.8 (than DCB) DBC and AMB possible
BeO (Beryllium Oxide) 290 5.9 dust is very poisonous; DBC and AMB possible
AIN (Aluminium Nitride) 180 4.7 (than DCB) DBC only after oxidation of the surface
Si3N4 (Silicon Nitride) 70 3.4 High mechanical strength
Epoxide 3 elastic Synthetic material (normal thickness 120 µm)
Polyimide 0.385 elastic Synthetic material (normal thickness 25 µm)
Thermal Paste ~0.8 Paste Base mostly silicon
Silicon 148 4.1 Fragile
Air 0.026 soft compressible  

 

Insulating Substrate: DBC - (Direct Bonded Copper)

Copper and copper oxide form a thin liquid eutectic at temperatures >1063°C. In the production of DBC substrates, an Al2O3 ceramic is coverd with this liquid and upon solidification, deep and strong bonds are formed between the copper and the ceramic.

The layers are usually Cu - Al2O3 - Cu. Typical thicknesses of the layers are as follows:

The thermal coefficient of expansion of the DBC is similar to the thermal coefficients of expansion of Al2O3 and Si.

Other oxide ceramics (e.g. BeO) can be used to produce DBC substrates. Non-oxide ceramics such as AIN have to be oxidised on the surface before they can be used to create DBCs.

The combination of copper and ceramic can only be done on one side at a time. The entire process must be performed again to apply a second copper layer. A copper-copper combination can be done as well during the DBC process.

For high temperatures, DBC substrates are normally stable. The temperature resistance is relatively high but decreases as the metal layer thickness is increased.

Material Selection: Other Insulating Substrates

In power electronics, DBC is the most commonly used insulating substrate. There are, however, other types of ceramic substrates in use. The following table shows a summary of these substrates:

Ceramic substrate General Remarks
AMB (Active Metal Brazing, Active Soldering) High temperature combination between metals (copper, aluminium) and ceramic (Al2O3, AlN, Si3N4 ) using an active solder. Can be used to substitute DBC
Thin Layer Ceramic (Thin film ceramic) Base is usually Al2O3 . Thin metallization through vapourising of sputtering.
Thick Layer Ceramic (Thick Film Ceramic) Base used is mostly Al2O3. Metallization through glass-based screen printing pastes. Can be used for up to 0.3 mm copper thickness as well as a combination of thin and thick copper, insulation layers and various metallisation layers
LTCC (Low Temperature Cofired Ceramic) Affordable ceramic oftem multi-layered and with interlayer connections. Screen print conductor and resistance pastes. Low thermal conductivity

 

Insulated Metallic Substrate (IMS)

An Insulated metallic substrate (IMS) is essentially a doped polymer covered on one or both sides by a thin copper layer. Power electronic components are attached to such a substrate using solder. General applications of IMS include use in power conversion, motor drivers, solid state relays, welding machines, uninterruptible power supplies, among others.

As opposed to DBC substrates, IMS substrates offer flexibility and can be adapted for use in smaller quantities. IMS substrates have relatively high coefficients of thermal expansion and thus limited temperature stability and cycle life, while the coefficients of thermal expansion in DBCs are similar to silicon. Whereas IMS have low current carrying capacity, DBCs offer high current carrying capacity. Both substrates exhibit good heat dissipation.

 

For more information, please read:

The Challenge of Packaging Small Power Devices

Power Electronics Packaging Revolution

Sinter Technology for Power Modules

A new 3D power module packaging without bond wires

 

VN:F [1.9.17_1161]
Rating: 0.0/6 (0 votes cast)

This post was written by:

- who has written 197 posts on PowerGuru - Power Electronics Information Portal.


Contact the author

Leave a Response

You must be logged in to post a comment.