Herzlich Willkommen beim beam-Verlag in Marburg, dem Fachverlag für anspruchsvolle Elektronik-Literatur.


Wir freuen uns, Sie auf unserem ePaper-Kiosk begrüßen zu können.

Aufrufe
vor 6 Jahren

4-2017

  • Text
  • Ems
  • Mikrotechnik
  • Nanotechnik
  • Qualitaetssicherung
  • Loeten
  • Loettechnik
  • Bestuecken
  • Leiterplatten
  • Displayfertigung
  • Halbleiterfertigung
  • Elektronik
  • Bauteile
  • Productronica
  • Baugruppen
  • Produktion
Fachzeitschrift für Elektronik-Produktion - Fertigungstechnik, Materialien und Qualitätsmanagement

Rund um die Leiterplatte

Rund um die Leiterplatte High efficient heat dissipation on printed circuit boards Figure 1: Heat flux in a PCB Abstract This paper describes various techniques for dissipating heat from heat generating electrical components on printed circuit boards (PCBs). Small copper coins that are matching the shape of the electrical components are located underneath the component and are integrated into the PCB construction. The heat from the component will be dissipated by the copper coin to a heat sink or cold plate. The thermal conductivity of such kind of copper coin is about 10 times higher than usually achieved with so called thermal via arrays. Several different methods of integrating copper coins into the construction of PCBs have been developed and will be discussed. New developments such as the “Chip-on-Coin” technique are providing solutions for highly miniaturised electronic circuits and micropackaging. The integration of copper coins into PCBs is suitable for all common substrates including RF and microwave substrates as well as for conventional PCB substrates. (Key words: PCB, heat dissipation, thermal via, copper coin, press-fit, bare die attach.) Introduction Controlling the heat loss of electronic and microelectronic systems is a more and more challenging Markus Wille Schoeller Electronics Systems GmbH www.schoeller-electronics.com task as miniaturisation is increasing, and the growth in functionality is driving the components to their limits, which means that they are generating more heat loss. Printed circuit boards are the carrier of the components and are therefore also highly involved in the matter of controlling the heat. The PCB by its nature is not a good thermal conductor. It is made of substrate materials that are insulating electrical interconnections between components. The thermal conductivity of a typical substrate material is about λ ~ 0.2 W/ mK. However, copper, the material of the conductive traces of a PCB, has a high thermal conductivity of λ ~ 390 W/mK. Depending on the copper distribution the heat flux in a printed circuit board is normally better in the x-y plane compared to the heat flux in the z-axis (Figure 1). A power or ground plane has a bigger influence on the heat flux. The heat flux and direction is mainly dominated by the thermal conductivity of the materials and the ∆T in a given area. The conductive traces of a PCB in practise cannot be used as a good and efficient thermal conductor. Their cross sectional area is simply much too low. Many microelectronic components are designed with a predetermined thermal pathway inside their packages (Figure 2). Figure 2: Thermal pathway inside component packages Figure 3: Thermal vias in a PCB The thermal loss of a plastic ball grid array (P-BGA) for example is dissipated via the base of the package whereas in a flip chip ball grid array (FC-BGA) the thermal loss is guided to the top surface of the package. The principle that the thermal loss of a component is transferred to the base of the package provides the ability to integrate a path for heat dissipation into the physical construction of a printed circuit board. A very common approach is to place an arrangement of vias as so called thermal vias in the PCB underneath the component (Figure 3). The base of the component is connected to the thermal vias on the top side of the PCB. The heat flux is transferred through these vias down to the bottom side of the PCB and then coupled into the heat sink or a cooling plate. For heat spreading the thermal vias are sometimes connected to power or ground planes of the PCB. This principle is widely used at nearly any extra charges because PCBs consists of lots of lots of vias anyway. The question is how efficient are thermal vias? They may work fine for many applications but the effective thermal conductivity of thermal vias is low due to the small amount of conductive materials that are involved. The heat flux flows mainly only through the very small cross sectional area of the copper plating at the hole wall of the vias. The centre of the vias remains usually open and unfilled, and the surrounding material is the substrate material of the PCB which is a good insulator. Local Heat Dissipation By Copper Coins To create a much more efficient path for heat dissipation the idea is to replace the arrangement of thermal vias by some piece of solid 56 4/2017

Rund um die Leiterplatte Figure 4: PCB with press-fitted copper coin metal to increase substantially the amount of conductive material of thermal via arrays. The goal was to find methods and techniques that are compatible to the constructions and the manufacturing processes of printed circuit boards and that are suitable for any assembly processes. The material of choice is copper because of its high thermal conductivity and its excellent compatibility to PCB production processes. Several methods have been developed: • press-fitted copper coins, • adhesive bonded copper coins, • embedded copper coins. All these methods are using solid pieces of copper that are integrated into the mechanical construction of the printed circuit board during its origin production process. Press-fitted copper coins The insertion of copper coins in printed circuit boards by means of the press-fit method is a very cost effective technique that is practised e. g. on PCBs for engine controls in the automotive industry or for power amplifier in base stations of wireless networks. Copper coins are pressed in an intermediate production step into appropriate openings of printed circuit boards. The openings can be plated or non-plated. After the coin insertion the normal production process flow continues. Figure 4 shows a segment of a printed circuit board with press-fitted copper coins. The copper coin is designed with a number of specific ribs along the Figure 5: PCB with adhesive bonded copper coin outer peripheral surface helping to control how strong the coin is fastened in the cut-out of the printed circuit board. The ribs are also maintaining the electrical connection between copper coin and PCB, e. g. the grounding. Adhesive bonded copper coins Another method is to attach the copper coins onto the PCB when the normal fabrication process has been finished. The copper coins are bonded to the PCB in defined locations by using thermally and electrically conductive film adhesives (Figure 5). The copper coin in Figure 5 has a flange the spreads the heat and enables a better thermal connection to a heat sink or cold plate by enlargement of the effective surface area. It also carries the adhesive preform (grey colour). The bond strength of the bonded copper coins depends on the adhesive used, the type of surfaces, and also on the size and geometry of the bonded area. Depending on the selected adhesive the coin can be thermally and electrically connected to the PCB or insulated or only thermally or only electrically connected. Embedded copper coins If the copper coin is integrated into the construction of the printed circuit board at the same time and with the same process when all other layers of the PCB are laminated together then this method is called embedded copper coin. Window cuts are prepared into the cores and prepregs of a PCB stack and when the stack-up construction is assembled prior lamination the copper coins are placed into the Figure 6: PCB with embedded copper coin window cuts. The embedded copper coins can be electrically connected to the PCB by plated through holes and galvanic copper deposition on the surface layers (Figure 6). The embedded copper coin is fully integrated in the layer construction and lies flush in plane on both sides of the printed circuit board. The press-fitting of copper coins is the most cost efficient technique of the three described methods in this paper. However, the press-fitting technique is limited to a maximal size of approx. 40 mm x 40 mm to avoid overstressing the PCB with a too high mechanical load during the press-fitting process. Therefore, the two other methods can be seen as back-up solutions for the case that press-fitted copper coins are not the best suitable technique for a specific application. Further Designs And Developments Copper coins with cavities Some high power transistors are housed in packages with metal flanges for heat spreading and dissipation. They are normally soldered or bolt down on heat sinks or cold plates. The copper coin technology provides a solution to assemble such devices directly onto a printed circuit board (Figure 7). The copper coin of Figure 7 includes a cavity in which the flange of the power transistor will be placed for assembly. The design of the cavity matches perfectly to the shape and 4/2017 57

hf-praxis

PC & Industrie

© beam-Verlag Dipl.-Ing. Reinhard Birchel