Metal-core PCB design, including DFM, follows many of the same basic design rules as typical PCBs on FR4. If you’re designing a new product in any of the above areas, a metal-core board might be the solution you need to keep temperatures in check. In this article, I’ll briefly go over the structure of a metal-core PCB and some important design points to consider before planning to work with metal-core PCB designs. These boards carry particular fabrication requirements, but the right design firm can help you navigate these requirements and ensure your board will be manufacturable at scale.

Applications of Metal-Core PCB Design

Metal core PCBs find their home in nearly any application where high amounts of heat are generated as the device operates. These boards aren’t a great substitute for ceramics as they are a lower cost option, and they provide higher thermal conductivity to remove heat from important components. They tend to be a starting point when looking for a board for a rugged system with high heat dissipation. Some applications of metal-core PCBs include:

• LED lighting units. Boards with high power LEDs are normally fabricated on metal core PCBs. These boards can provide a rugged base for high power LEDs (SMD and through-hole) while dissipating high heat into the metal core.


• Power conversion and management. Hybrid vehicles, industrial equipment, base station telecom equipment, and municipal power distribution systems all operate at high power. Metal-core PCBs are common in these areas.


• Photovoltaics. Solar power equipment needs to be especially rugged and operate at high temperatures, as well as high DC voltage/current. Similar board designs could be implemented in geothermal facilities.


• Defense (e.g., submersibles, aircraft). Metal-core PCBs provide fast heat dissipation away from electronics that might sit near a high heat source, such as an engine unit or exhaust system.

There are many other areas where high reliability and structural rigidity are critical, making metal-core boards an excellent option. Once we start looking at the stackup and layout requirements for these boards, it becomes less obvious how they should actually be designed. Can you do a multi-layer metal-core board? Can it be double-sided? How are vias handled during manufacturing? These are all important questions relating to DFM for metal-core PCBs. After working through a recent project for an LED board, here’s what we found and how we would approach future metal-core PCB design projects.

DFM for Metal-Core PCBs

Just like any other PCB, you need to follow some basic DFM guidelines for your particular board if you want to ensure a successful fabrication run. Metal-core boards follow a different process than typical PCB stackups involving glass-weave laminates, thus they tend to carry different DFM rules. The image below shows the typical stackup for a double-sided metal-core PCB.

 

 

Note that this stackup can technically be adapted as a multi-layer board, where multiple dielectrics on each side of the metal core. Alternatively, you can have the board be single-sided, where the metal core is exposed through the backside. Here are some of the important manufacturing points to remember when working through your metal-core PCB design:

Grounding the Metal Core

The metal backing on a PCB can act like a large ground plane or a large heat sink. If the board needs to have high speed/high frequency circuit blocks, then using the backside metal plate as a large ground plane provides some shielding. It can also provide some plane capacitance if a power plane is used in the board.

In addition, the metal core can be used as a large heat sink, especially if left exposed. latter aspect is very useful when the board needs to be mounted near a high heat source. In this case, it is probably best not to ground the back side when you are connecting the top side to a standard power source; this will prevent ground loops. This will also provide direct heat dissipation into a very large heat sink, which helps the surface temperature low.

Holes in Single-Sided Boards

Holes can be placed in a metal-core PCB, either as mounting holes or as standard through-hole vias on a double-sided board. If holes are simply used for mounting through-hole components on a single-sided board, these holes should not be plated to prevent short circuits. This is done by backdrilling the mounting hole and filling the hole with a non-conductive epoxy or gel. This then plugs the hole so that mounting can be applied in an upper layer.

Holes in Double-Sided Boards

In a double-sided metal-core PCB, both sides might carry some components and plated vias will need to be placed between signal layers. This creates some difficulties in fabrication as a pre-drill/insulating filler/re-drill/plating process is used to form plated through holes. This process takes extra time and incurs extra costs, but it is designed to prevent the vias from shorting. In the PCB layout, it’s best to apply an antipad to denote the region around the via hole that requires filling with insulating filler material. Be sure to size the via antipad so that the IPC-2221 standards are followed.

Other aspects of DFM in metal-core PCB design do not differ so much from standard PCBs, although CAD tools are not designed to work with these boards. There are some minor rules on the front-end that need to be followed for power electronics, particularly IPC-2221 (creepage and clearance rules), as well as other standards for defense and aerospace. The right design firm can help you navigate these standards to create a manufacturable and reliable metal-core PCB design.

 

If you’re ready to start your next metal-core PCB design, you need to work with a firm like NWES. We help electronics companies design modern PCBs and create cutting-edge technology for automotive, aerospace, and defense systems. We've also partnered directly with EDA companies and advanced PCB manufacturers, and we'll make sure your next layout is fully manufacturable at scale. Contact NWES for a consultation.

 

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