Power Electronics Technology

THERMALmanagement
els the temperature increase
becomes a factor in determining
ampacity because the
resistivity of the conductor
changes with temperature.
The relationship is linear, i.e.,
resistivity increases proportional to the change in temperature
at a rate determined by the temperature coefficient of
the conductor:
R T = R T0 × [(1 + (T - T 0 )] (1)
Where:
T = Temperature at which resistivity is measured
T 0 = Reference temperature (ambient)
= Linear temperature coefficient (copper = 0.004)
R T = Resistivity at measurement temperature
R T0 = Resistivity at reference temperature
For a copper conductor, every 25°C increase in temperature
means a drop of about 5% in maximum ampacity
due to an increase in conductor resistivity, RT . Since
this presents the probability of further power dissipation
and temperature rise, MiB design practice must consider
effective methods not only to control and reduce conductor
resistivity, but also to provide low thermal resistance
pathways for heat dissipation.
In a printed circuit board, ampacity depends on a number
of different factors:
• Conductive + convective capability provided by spreading
layers, ground layers, stack-up
• Ratio of track width to thickness
• Ambient temperature
• Adjacent high current tracks
• AC or DC current
• Presence and frequency of partial crosssection
shrinkage
• Presence, number, and conductive
cross-section of plated through holes in
series with the conductor.
Therefore effective design needs to
consider more variables than are normally
addressed by the IPC-2152 current vs.
temperature charts.
MIB
“Metal in the Board” or “MiB” includes
a number of approaches where MiB
building blocks are combined to provide
effective high-current solutions.
The “DWPCB” (discrete wire PCB) type
Fig. 1. Gullwing heat sink on TO-252
power device.
External Only
Low heat spreading
Low heat spreading
Wire Bonder
Bonded Elements
Etched Conductor
Pattern
Substrate Cross-Section
Fig. 2. “Discrete Wire” process- bonding high current elements to inner layer.
is one of the most versatile. One of the commercially
available versions of DWPCB is HSMtec, developed by
the Austrian PCB manufacturer Häusermann GmbH.
HSMtec uses 0.5 mm diameter copper wire and rectangular
sectioned 0.5 mm thick copper strips (“profiles”)
to provide discrete low resistance electrical and thermal
pathways in the board as shown in Fig. 2.
There are a number of advantages to this solution compared
to conventional thick copper or metal core boards:
• Conventional PCB processes ensure consistently high
reliability
• Enhanced thermal and current pathways only where
needed
• Cost of MiB limited to those nets needing MiB
• Wiring densities up to and including HDI enable logic
and power integration
• FR-4 materials reduce CTE mismatch common to aluminum
based substrates
• Board may be folded during assembly, providing photometric
solutions for LED luminaires and eliminating
daughter boards/connectors
The profiles and wires that make up the MiB compo-
Internal + External
Heat spreading
Heat spreading
Fig. 3. Effect on heat spreading of thermal dissipation plane (source: Häusermann GmbH).
8 Power Electronics Technology | June 2013 www.powerelectronics.com