Metal backed PCBs, MCPCB

 

Traditionally it has been the microwave industry that has requested PCBs being mounted on thick metal casings, mostly for thermal considerations.

The recent years development in LED technology have enabled designers from Many industries, to use a range of High intensity Surface Mount LED’s where once they may have used conventional bulbs or lamps. While this technology continues to expand in its usage, there is a problem with the heat generated by the new components. Especially when grouped closely together, this can cause burning of standard FR4 and result in failure of the circuit.

 

There are different production methods used to produce these boards and the materials used are typical aluminium, brass and copper. Thicknesses often seen are between 0.5 mm to 6.35 mm. The most common materials seen are from Bergquist, ITEQ, Laird, Arlon and Denka.

 

The most common one is a highly thermally conductive Insulated Metal Substrate (IMS) consisting of an aluminium base, an epoxy-based insulation layer with a high inorganic content filler giving a high thermal conductivity, and a conductive foil, thereby realizing thermal resistance equivalent to, or less than that of an alumina ceramic substrate.

But using heavy copper circuit boards with traces and copper planes of up to (210um) can still be an alternative to metal backed PCBs.  Heavy copper boards are a product that can be produced at a reasonable cost, and it is a well-proven technology and design.

Thermal conductivity or heat transfer coefficients is given in W/(m*K) And typical values for some materials are as follow: 

Material

Thermal Conductivity  /(m*K)

Air

~ 0.025

Pure Aluminium

~ 237

Aluminium alloys

~ 120-180

Brass

~ 109

Copper

~ 390

FR4 Low TG
FR4 Mid TG
FR4 High TG

~ 0.48
~ 0.62
~ 0.35

Gold

~ 318

Helium

Infinite


Thermally Conductive Dielectrics
Various thermally-conductive dielectrics available in the market. Thermal con-ductivity values range from 1.0-4.0 W/m.K,
and typical thickness of these dielectrics are 70um to 150um.  

Thermal conductivity and the thickness of the dielectric might look comparable on a data sheet, but they do not perform in
the same way when it comes to transferring heat from the LED.It is fair to say the key to the performance of the MCPCB lies in its dielectric layer.
Even though thermally conductive dielectric has higher thermal performance compared to stan-dard dielectric material it is still the weakest link in
the conduction thermal path of the MCPCB.  

         Material                Type                   Thermal Conductivity W/(m*K)          
Arlon 92ML 2.0
Bergquist   2.2 - 3.0
Doosan   2.0
Denka Hit Plate 2.0 - 8.0
DuPont CoolLam (Polyimide based) 0.8
ITEQ IT859 GTA 2.0
KingWon KW-ALG 1.5 - 2.0
Laird HKA 3.0
Polytronics   2.7
Sekisui   2.0
Ventec VT4A1 1.6


W/(*K) =  Watts/(Meter*degree Kelvin)

See .pdf file for full article with illustrations..   

                                                                                                                             Josse Elmatica Aug 2012

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