IEEE Power Electronics Magazine - December 2021 - 17

Magna-Power has nearly fully in-sourced manufacturing
operations under one roof, the strategies presented can
also be implemented through close vendor relationships
and understanding of the constraints from various production
processes and machinery.
Printed Circuit Board Design
PCB fabrication is one of the few manufacturing components
that the company outsources. It is also the most complicated
component to specify, revise, and manage inventory.
Correctly communicating design requirements to the
PCB manufacturer, along with observing their individual
fabrication constraints, allows designs to be fungible across
manufacturers; such a process has been refined over many
years through experimenting with many different vendors.
Supporting multiple product lines requires maintaining a
pool of manufacturers with different specialties. Some excel
at fine pitch and multiple layers, others specialize in highounce
copper and/or high-temperature tolerant fiberglass,
and some for quick-turn around prototypes that operate
24 hours/day, 7 days a week. Multiple manufacturers from
different regions have added much resilience to our supply
chain, which is advantageous when a country shuts down
for holiday, and recently due to COVID-19.
The next two sections explore manufacturing considerations
for high-voltage and high-current layout design and
fabrication. Each consideration is presented followed by
images of real-world consequences when ignored.
High-Current Printed Circuit Boards
The current a PCB can handle is governed by the thickness
of copper, specified in ounces, and the number of conductors,
specified in layers. Working with thick copper (high
ounce) clad FR4 introduces challenges both on the board
fabrication side as well as the circuit assembly, mostly
rooted in chemical and machinery limitations. Thick copper
(which for Magna-Power is 4 oz or more) is more difficult to
acid etch, and the PCB fabricator will have wider tolerances
and alternative design guidance. For example, one of our
board manufacturers recommends 5 mils minimum trace
width for 1 oz copper. This recommendation jumps to
15 mils for 4 oz copper. Wider trace limits the selection of
digital ICs that can be installed on a board. Mobile computing
has been driving chips smaller, shrinking the pitch on IC
packages, whose leads are often too tightly spaced for trace
routing on power boards. The workaround for this problem
is to consolidate logic and control circuitry to a light-ounce
PCB that mates with high-ounce PCBs, as shown in Figure 1.
Copper is an excellent electrical and thermal conductor.
The latter is undesirable for manufacturing as it sinks heat
away from joints, lowering the temperature below what is
necessary to melt solder completely. Thermal reliefs, as
shown in Figure 2, are recommended for any copper pour
to pad connection so that production can solder more easily.
Single high-current PCB pads (terminals, bus tabs, etc.)
should be split into multiple parallel connections. Doing
FIG 1 Mixing PCBs with different copper ounces.
so produces the same current output while increasing the
thermal conductance of each joint, which eases soldering,
as shown in Figure 3. Since most through-hole assemblies
pass through a wave solder machine, where joints are soldered
all at once, the added connections have no impact on
labor and makes any solder iron rework more efficient.
Most Magna-Power PCBs retain a thickness of 62 mils,
which is the common layer stack-up in industry. For highcurrent
PCBs, each copper layer is 5.6 mils, so when multiple
layers are needed, the core thickness is increased to
keep the final assembly rigid. On a 4 oz, 4 layer PCB, MagnaPower
designs use a 59 mil dielectric core and a total board
stack-up of 92 mils. The core thickness will keep the board
flat on the pick and place machine, leading to more accurate
component placement, and prevents warping caused by
FIG 2 Copper bus mounting using pads with thermal reliefs.
FIG 3 Multi-path conduction for increased thermal conductance.
December 2021 z IEEE POWER ELECTRONICS MAGAZINE 17
IEEE Power Electronics Magazine - December 2021

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