10 – Design guidelines for aluminium core circuits

Aluminium-core circuits have the task of dissipating the heat produced by power components. We therefore recommend housing the pure power part in this section of the circuit. The control part should be accommodated on a separate standard PCB.

The most important criteria for this is the minimum distance for drill holes required for this type of PCB.

Fig. 1 (on the left) – Drilling distances and diameters

In case of double-sided aluminium-core circuits, the aluminium core must be insulated against throughplating. This is done using an excess of resin when press-moulding the aluminium core with prepregs and copper or by plugging. To this end, the aluminium core must be pre-drilled.

The smallest drilling diameter in the aluminium core is 1,0 mm, the smallest final diameter in the PCB is 0,3 mm. So that the drills are not damaged in these close-set drill holes, a minimum spacing of 1,2 mm is required. This is generally the knock-out criteria for a densely-packed control unit.

10.1 – Variants of aluminium core circuits

10.1A – Single-sided PCBs on aluminium carriers

A copper foil is laminated onto an aluminium carrier using a prepreg.
Normally, this variant only has drill holes for fastenings.

Fig. 2 (on the right): Single-sided structure

10.1B – Multilayer PCBs with aluminium core

Copper foils are laminated onto both sides of an aluminium core using prepreg. The PCB can be through-plated. This means that it is also possible to produce multilayers with a 0,5 mm aluminium core.

Fig. 3: 2-sided structure with aluminimum core

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Fig. 4: 4-layer structure with aluminium core

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10.1C – PCBs on metal heat-conducting sheets

Completed PCBs are press-moulded to an aluminium carrier using a prepreg. Benefit: Multilayers can also be used (only single-sided SMD). Partial aluminium carriers are possible. Disadvantage: Poor heat dissipation, as the heat has to be dissipated through the entire PCB.

Fig. 5: Structure of the metal heat-conducting plate

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Fig. 6: Partial aluminium carrier

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In this variant, the aluminium carrier can also be partially applied. What is important here is that the carrier extends over 2 ends of the PCB, otherwise accurate positioning is not possible.

10.1D – Aluminium-flex

One further possibility is a rigid-flex PCB structure, where the aluminium carrier works as the rigid area of the PCB. This means that it is also possible, for example, to connect a control unit as a plug connector over the exposed flex area.

Fig. 7: Aluminium-flex structure

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Technical data

Parameters
• Thickness of the aluminium core: 0,5–2,0 mm
• Copper thickness: 18–105 μm
• minimum drill hole diameter PCB (PTH): ≧ 0,3 mm
• minimum drill hole diameter PCB (NPTH): ≧ 1,0 mm
• minimum drill hole diameter for aluminium: ≧ 1,0mm
• Drill hole/drill hole spacing: > 1,2 mm
• minimum router diameter: ≧ 1,6 mm
• Surfaces: HAL, HAL unleaded, OSP, chem. Sn (not recommended!)
• Solder resist varnish: green, white

Machining
Produced like standard PCBs in multiples or individually, as well as being milled and scored.

Cost factors
• Aluminium thickness (raw material price)
• Copper thickness (raw material price)
• Number of drill holes (tool wear)