New circuit board technologies meet the challenge of digital power
New Families of Power Packages Drive Board Design
In their just-published report: "Metal in the Board - Opportunities for Printed Circuit Boards Providing Enhanced Thermal and Power Management," BPA Consulting have identified new families of power semiconductors which require new approaches in board design for thermal and power management. A number of board level solutions are emerging in response. The report has identified 12 major types segregated by:
- thermal management technology- heat pipes, inlays, dissipation planes, etc
- current management- copper planes, embedded bus bars, discrete wiring or strips
- board layup- number of layers, use of internal/external dissipator planes
These types offer a wide range of thermal and power management capabilities, and have been characterised in terms of the power densities they are capable of managing with less than 10°C temperature rise through the board thermal path. Solutions typically range from 0.25W/cm2 up through 35W/cm2- through the board.
This range of capabilities is proving to be critical as power conversion, management, and control becomes digital and advances in power semiconductor technology combined with the cost advantages of automated assembly drive development of new and smaller surface mount packaging.
High Power in Small Packages
One of these is the "isometric" family of packages, developed by International Rectifier and marketed as the "DirectFET®." A similar package is offered under license to IR by Infineon - the "CanPAK™." These devices are "isometric" because they provide a balanced thermal pathway both into the board and, if necessary, out through the top of the package as shown in Fig. 1.
In addition to thermal challenges, the power applications typically serviced by MOSFETs or IGBTs in isometric packaging involve currents from 50 to several hundred amperes. This is well in excess of the 10 - 15A typically considered the top end for conventional printed circuit boards, and managing these current levels involves a different and unique set of board design criteria.
Ampacity - A Critical Parameter in Power Packaging
Current flow through a conductor causes resistive power losses (I2R) in the form of heat, and at high current levels 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:
(1) RT = RT0 × [(1 + α (T - T0)]
T = temperature at which resistivity is measured
T0 = reference temperature (ambient)
α = linear temperature coefficient (copper = 0.004)
RT = resistivity at measurement temperature
RT0 = 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, stackup
- Ratio of track width to thickness
- Ambient temperature
- Adjacent high current tracks
- AC or DC current
- Presence and frequency of partial cross-section shrinkage
- Presence, number, and conductive cross-section of plated through holes in series with the conductor
Therefore the design problem needs to consider more variables than are normally addressed by the IPC 2152 current vs. temperature charts.