IMS (Insulated Metal Substrate)

Choice of the appropriate IMS subtrate

In order to chose the appropriate IMS substrate for the respective application, the properties listed in the following charts should be considered..

1. Dielectric

	Dielectric with wovenglass reinforcement	Dielectric without wovenglass reinforcement
Breakdown voltage	high	lower
Cost	low	high
Breakdown voltage	lower	higher
Thickness uniformity	better	worse

2. Effect of Filler on thermal Conductivity and Breakdown Voltage

Filler type	Thermal conductivity (W/mK)	Breakdown voltage based on 70% filler
Al₂O₃	25~40	+ +
MgO	25~40	+
SiO₂	9.6	+
Si₃N₄	50	-
BeO	270	-
SiC	25~100	- -
AlN	120~220	+
BN	100~250	+

++ excellent, + good, - bad, - - poor

Typical Characteristics of IMS Substrates

	Thickness	Thermal conductivity (W/mK)
Copper	18 µm - 410 µm ¹⁾	380
Aluminium	0.5 mm - 3 mm	200
Dielektrikum	75 µm - 150 µm	0.4 - 0.8 / up to 2.5 ²⁾
Fr4		0.4

¹⁾ Copper thicknesses, ²⁾ woven-glass reinforced dielectric

What exactly is thermal Conductivity W/mK?

Thermal conductivity may not be confused with thermal diffusivity, i.e. the speed with which a change in temperature diffuses within a medium. Thermal conductivity exclusively refers to the heat conduction and not to the heat transfer by convection or thermal radiation. The heat conductance coefficient in Watt per Kelvin (W/K) is an index depending on the dimensions of a module. The thermal conductivity of materials is measured pursuant ISO22007-2 by means of the HOT-disk-method.
Should you have any questions regarding the ISO standards, please do not hesitate to contact us

When selecting a material for an application, the thermal impedance has to be taken into consideration as well. The thermal resistance (Rth) defines the interior thermal resistance of a material against a possible heat flow. The lower this value, the better the heat can be dissipated through the material. Thermal impedance is comparably characteristic as Rth, yet strictly referring to the constant of the contact surface.
If the breakthrough voltage is no decisive factor in the choice of the material, it frequently makes sense to chose a thinner dielectric and a lower thermal conductivity. Thus, it is possible to reach nearly the same thermal impedance (see chart).

By rule of thumb: The lower the W/mK value of the basic material, the lower is its price.

Thermal Impedance for different Values of thermal Conductivity

Dielectric	W/mK = 1.6	W/mk = 2.2
75 µm	0.074	0.054
100 µm	0.074	0.054
125 µm	0.074	0.089
150 µm	0.148	0.107

For most LED applications, materials with lower W/mK values are perfectly adequate.

CTE-Differences within IMS Materials

CTE Values for the individual IMS Materials with varying W/mK Values

Material e.g. Ventec:	VT-4A1	VT42	VT4B3
W/mk:	1.0	2.0	3.0
Dielektrikum:	with glass	with glass	without glass
CTE values (ppm/°C)
x,y:	13	12	21
before Tg (z axis):	32	25	44
after Tg (z axis):	150	140	198
Total (z axis):	2.00 %	1.80 %	2.70 %

IMS materials are offered with Tg values ranging between 90 °C and 170 °C.

In case of different lighting applications, the breakdown voltage is not decisive for the functional safety due to the fact that the operating voltage is relatively low. In this context, it is much more important that the individual modules are exposed to severe thermal cycling due to the seasonal ambient temperatures and humidity variations.

The CTE differences between copper, dielectric, aluminium and ceramic components result in shear stresses. At worst, these stresses and strains may cause fatigue cracks at the solder joints or in ceramic components and the de-lamination of the laminate. The use of glass reinforced dielectrics reduces the CTE mismatch between the substrate and the ceramic components and consequently also the default rate.

Effects of thermal Cycling on Breakdown Voltage and Peel Strength