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How do you create Stack-up for Aluminum PCBs?
Posted on 29/03/2019

Introduction

Metal Clad Printed Circuit Boards or MCPCBs offer high thermal conductivity to remove heat from components on the PCB that are likely to run at high temperatures. The most common type of MCPCB has an aluminum base, although copper is used instead if superior heat conduction is necessary. Aluminum is more popular as it is less expensive than copper is. Irrespective of the metal base used for the PCB, the stackup design requires careful consideration for the MCPCB to succeed in its ultimate aim of heat removal.

The complication with the stackup arises from various options the designer has of placing the metal base within the stack. Compare this with the regular multilayer board, where the stackup consists of alternate layers of copper foil and prepreg/dielectric, and is a balanced structure. In contrast, an MCPCB may be of a single-layer type, a two-layer one SMD-layer type, or a double-layer type.

Single-Layer MCPCB

This is the most common type of MCPCB, and the LED industry uses them widely. The metal or aluminum base has a copper foil attached on one of its sides, with a layer of prepreg or dielectric separating the two. During manufacturing, the copper foil is etched to represent the circuit required to connect the SMD components such as resistors and LEDs. The heat generated by the SMD components when operating passes through the prepreg to the aluminum base. Therefore, the prepreg has to be electrically neutral but thermally conductive.

Two-Layer, One SMD Layer MCPCB

If the electrical circuit involved is more complicated and the single-layer MCPCB is incapable of handling the density, a two-layer board is necessary. Eventually, this translates to two copper foils forming the two layers, bonded together by a prepreg or substrate between them. The design now proceeds in the same way as that for a regular two-layer PCB, including vias connecting the two layers, but with a single major difference.

One of the two copper foils will finally be blind-sided by the aluminum base, electrically insulated from it with a thin layer of prepreg. As this copper layer will have to be entirely attached on to the aluminum base, it cannot have any components soldered on to it. The designer has to be careful not to position any SMD components on this copper layer, but use this copper layer only for routing purposes. Therefore, the finished board will have SMD only on one of its sides just as the single-layer board does, but unlike for the single-layer board, the tracks will appear on two layers, increasing the density of the entire board.

Double-Layer MCPCB

It is also possible to manufacture an MCPCB in a balanced manner, in which the aluminum base has copper foils on both its sides, rather than on only one side, separated from it by prepreg. Vias connecting the top copper layer to the bottom will necessarily pass through the aluminum base, and the designer has to be careful to avoid a short. The manufacturer has to pre-drill the aluminum base with a hole diameter larger than for the via, and seal it with an insulating material before the copper foils are bonded on. Once the copper foils are in place, the entire structure is through-drilled for creating the vias.

Conclusion

The use of aluminum in PCB’s has many benefits in terms of its use and application. For more information on the stack-up abilities of aluminum PCB’s, please contact our team at sales@pcbglobal.com