As the power demands of devices increase to keep pace with performance, the thermal capabilities of standard PCB materials cannot provide the requisite characteristics for reliable operations. Traditional active and passive cooling approaches can impede project goals and lead to sub-optimal solutions, especially constrained designs due to enclosure size, package weight, or power efficiency. Increasingly, aluminum sheets built into the stackup fabrication are helping designers and manufacturers balance performance and the myriad of competing DFM tradeoffs. Working with aluminum core PCBs is exceedingly similar to standard fabrication practices, but designers will want to be aware of the restrictions and challenges of the material.
Conventional Assembly vs. Solderless Aluminum Assembly Steps
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Aluminum Core PCB Thermal Characteristics
The addition of aluminum in the PCB core primarily functions as thermal relief: aluminum’s excellent conduction capabilities make it well-suited for high-power applications where traditional substrate core material cannot provide the requisite heat transfer. Working with aluminum has numerous manufacturing benefits, even if the overall process diverges somewhat: its natural abundance makes it readily affordable, its recyclability lessens the environmental impact, and thermal isotropic qualities offer less directional wear over the device’s service life. Single- and double-sided assemblies use the aluminum core analogous to traditional stack-ups, but multilayer boards will use the aluminum as a base – this configuration will preclude through-hole components and double-sided assemblies while being compatible with HDI designs with tight pitches and high component fanouts. In the latter stackup design, a completed PCB attaches to the aluminum heat-conducting sheets, so the impact on manufacturing complexity is minimal.
Adding aluminum to a PCB is akin to inserting a plane-wide heatsink into the board, and many of aluminum’s material properties dovetail nicely with copper and the common substrate materials they replace. While the z-axis expansion of substrate is approximately four times that of in-plane substrate CTE and copper, aluminum’s similar CTE value in all dimensions ensures minimum stress-strain development around the via pad during thermal excursions (baking, soldering, etc.) While aluminum has a density 50% greater than common substrate materials, it’s a minor concern except for applications where weight optimization is paramount (like aerospace).
Manufacturing With Aluminum and Solderless Assembly
An additional wrinkle of working with aluminum substrates is their ability to eschew solder in their assemblies; accomplishing this process requires extra steps to standard substrate materials:
Steps for Solderless Aluminum Manufacturing
- Milling of the base for feature/component placement.
- Lamination of aluminum and placement of the components with an insulator coating.
- Hole drilling combined with insulative material fill.
- Via drilling to access component terminations and interconnect routing.
- Electroless plating and seal of the circuit pattern on outer layers.
- Pattern plate or print and etch.
- Coat exposed openings with mask (save for test contacts).
While the approach differs, the result is a board that is easier to produce while carrying a significant upside:
Strengths of Aluminum Core PCBs
- Cost – Eliminating solder processes eliminates the up-front cost of materials used for solderability or to support material ruggedness due to the high temperatures required for soldering. Post-soldering surface finishes are also unnecessary. The board’s reliability increases as it does not experience the material aging inherent to wave or reflow soldering.
- Electrical parameters – Parasitic capacitance decreases since the construction almost exclusively utilizes via-in-pad termination, which shrinks the landing pad size; just like a non-metal core PCB, the routing area improves with the most direct connections. The metal sheet layer also acts as a “free” power or ground layer that reduces impedance to the most accessed networks for great power response and signal integrity.
- Mechanical performance – The tight fit of a metal core PCB assembly eliminates the shock and vibration by constraining component package movement. Moreover, while microvias have some reliability concerns, they generally prove superior to solder joints in service life.
- Security safeguards – Placing components in recessed compartments makes simple viewing and reverse engineering of the product more complex. The arrangement also makes component extraction more difficult and prevents second-hand components from reentering the market as counterfeits of varying and unknown quality.
- Reliability – Since material aging due to soldering and joint failure are among the two most common sources of board defects/rejection, eliminating the step cuts down device infant mortality and general failure rates by outright eliminating a failure mode. Material complications of soldering, like tin whisker formation and filament growth that spontaneously short board connections, also become avoidable.
- Environmental compliance – REACH, RoHS, and similar legislation like the Toxic Substance Substances Control Act (TSCA) are the basis for material manufacturing regulations that aim to prevent dangerous substances’ environmental and biological accumulation. Aluminum requires minimal processing pre- and post-service life while being an easily recoverable material overall.
Your Contract Manufacturer Takes the Heat Off High-Performance Design
An aluminum core PCB is an excellent solution for the thermal regulation of today’s high-end performance boards. While aluminum processing requires additional steps compared to traditional substrate, the ability to add significant passive cooling for a minimal cost and weight increase is an attractive option for many designs. Working with an established contract manufacturer when dealing with a more exotic board material can ease DFM concerns and ensure production proceeds without the need for constant revisions. Here at VSE, we’re a team of engineers committed to building electronics for our customers; together with our manufacturing partners, we’ve been realizing life-saving and life-changing products for over forty years.
