Table of Contents
1. What Makes a Motor Control
PCB Different?
2. Why Motor Control PCB
Assembly Is More Demanding
3. Preparing a Motor
Control PCB for Reliable Manufacturing
4. Motor Control PCB
Assembly Process
5. Inspection and
Testing of Motor Control PCB Assemblies
6. Matching the
Assembly Strategy to the Application
7. Conclusion
8. FAQs
A stuttering robotic arm during
movement. An electric bike control system that does not work due to overheating
while climbing an incline. A conveyor motor that causes an overcurrent fault,
even when there’s nothing wrong with it. All of these situations can be related
to problems in the motor control PCB assembly, rather than some flaw in the
motor itself.
Motors use electric energy to
perform mechanical work within almost any industry sector – from automated
production facilities to electric vehicles – and modern motor-control systems
rely on electronics that must handle repeated current switching. Motor control
PCB assembly can face electrical and thermal stresses that a typical low-power
logic board may not, which creates distinct manufacturing challenges.
Industry analysis of the electric
motor and drives market has repeatedly indicated the growth prospects that lie
ahead in the coming years. This is due to the automation trends,
energy-efficiency policies, and electric mobility that will increase demand for
reliable motor-control electronics. All these require a control board that has
the ability to perform its function under their special electrical and
environmental conditions.
This article will guide you on how
a motor control PCB differs from other electronic components, reasons why
making one is difficult, and the processes used to assemble and test it.
What Makes a Motor Control PCB
Different?
A motor control PCB is not just an
intermediary between two chips; depending on the application, it must detect
motor position or current, do some processing, and drive a power stage that
switches current. Some switching and control events occur on microsecond
timescales.
Control, Power, Sensing, and Protection on One PCB
Most motor controller circuit
boards include four regions on a single circuit board: the control logic, which
may be realized by a microcontroller or a DSP; the switching region, which
includes a gate driver and power stage; a feedback region, which contains a
current or position sensor; and protection circuitry for overcurrent and
overvoltage events.
It is precisely the task of
maintaining appropriate electrical separation and clean signal paths between
these regions while placing them on a single motor control circuit board that
makes the design of motor control PCBs challenging.
A design that positions the noisy
switching region near the delicate current-sense trace may work under light
bench conditions but malfunction or become unstable once the motor starts
spinning under load.
Motor Control PCB vs. Motor Driver PCB
The two are sometimes used
interchangeably, but for practical purposes, a motor driver PCB usually refers
primarily to the power-switching stage, while a motor control PCB may also
include control logic, sensing, feedback, communication, and protection. A
closed-loop motor control system uses feedback to adapt its behavior to
changing operating conditions.
Some applications do require only a
basic motor driver PCB. Examples include a fan that works at a fixed speed and
does not necessarily require any kind of feedback. There are, however, some
applications that require the entire feedback system.
Why Motor Control PCB
Assembly Is More Demanding
General electronics assembly rarely
has to handle the combination of stresses that a motor controller board faces
every day.
High Current and Power Components
Some high-current motor control PCB
runs tens or even hundreds of amps through traces and connectors depending on
the application, which have to remain cool when carrying their rated load. The
calculation of trace width, copper weight, and vias becomes necessary rather
than an option because any underdimensioned copper starts acting as a heat
generator the moment the current is drawn.
A board identical to a less
powerful one in the schematic may act totally differently upon carrying real
current because of the necessity of current-carrying capability verification.
Thermal Management and Solder Joint Reliability
Power components such as MOSFETs
and drivers produce heat which needs to be removed from the board very quickly.
Heat is removed by the means of thermal vias, copper pours, and in some cases a
metal substrate, while solder joints of power devices should be strong enough
to withstand thermal cycling without cracking.
A motor control PCBA which does not
go through this process will silently deteriorate, thermal cycle after thermal
cycle, until one joint cracks. At that point, it would be difficult to
distinguish between a random field return and a design defect.
Keeping Power Switching Away from Sensitive Signals
The fast switching edges in the
power section emit electrical noise which may affect the current measurement
through the current sensor or the encoder input signal. The correct placement
ensures that the switching paths and the analog sensing paths are kept separate
and the gate drive paths are as short as possible.
This would not be apparent when
doing a bench test under a light load, but will only become clear when the
motor actually starts rotating with a torque load.
Preparing
a Motor Control PCB for Reliable Manufacturing
Before a single component reaches a
pick-and-place machine, a set of design and review decisions determines how
smoothly motor control PCB manufacturing will go.
PCB and Stack-Up Considerations
Some stack-up considerations for motor
controller PCB manufacturing may include heavy copper layer use on the power
plane, and separate planes for gate drive return planes. The number of layers
used and copper thickness should be based on electrical, thermal, mechanical,
and isolation requirements that the board will draw in service, not in bench
test conditions.
Component and BOM Review
For a review of the bill of
materials of a motor control board assembly, any component that has a
marginally low current and/or voltage capability relative to the application
should be identified. Gate drives, MOSFETS, and current sensing resistors must
all have sufficient margin to handle transient spikes.
Design for Assembly and Test
Testing points on gate signals,
current sensing, and important voltage buses save an incredible amount of time
at bring-up and manufacturing tests. A design without these points necessitates
probing the fine pitch contacts directly, which makes testing each unit coming
off the assembly line extremely slow.
Motor Control PCB
Assembly Process
The standard assembly processes for
PCBs remain relevant, although each stage has additional importance considering
the fact that the board being assembled is a motor controller, and ignoring
this fact is how a general process loses its importance in this particular
case.
Solder Paste, SMT Placement, and Reflow
Solder paste volume is particularly
important for a motor control PCB assembly when power MOSFETs use exposed
thermal pads, because these pads require adequate solder coverage for effective
heat conduction. If this is not properly controlled during stencil design and
reflow, excessive voiding can occur, reducing heat transfer and potentially
creating localized hot spots beneath the device.
Through-Hole and Mixed-Technology Assembly
Some high-current connectors, large
capacitors, and inductors may use through-hole mounting and not surface
mounting because these components have to withstand mechanical stresses which
the surface-mount pads alone cannot bear. For mixed-technology motor controller
PCB assembly, there is a reflow and wave or selective soldering process
sequence which will protect heat-sensitive components mounted on the PCB.
Cleaning, Protection, and Traceability
Flux residue in proximity to
high-voltage traces acts as a leakage path when humidity comes into the
picture, making cleaning more important in this case compared to that for a
low-voltage logic board. Conformal coating is applied in many motor control
board assembly projects for moisture and dust protection.
A batch code and component lot code
are just as important, as tracing the field failure needs to happen right down
to the production batch and component lot.
Inspection and Testing of
Motor Control PCB Assemblies
Motor
control PCBA testing must include much more than simple continuity because a
board that tests fine with respect to basic electrical properties may very well
fail when put into actual use.
SPI, AOI, and X-Ray Inspection
Solder paste inspection helps to
detect paste quantity issues prior to reflow, and automated optical inspection
detects misaligned or missing components commonly after reflow. X-ray
inspection plays a significant role when it comes to power components featuring
hidden thermal pads, since visual inspection cannot verify that the solder
joint is sound.
Electrical Testing and Programming
In-circuit testing can detect
opens, shorts, incorrect component values, and other assembly faults in gate
drive and current sensing circuitry before firmware is loaded. Motor controller
PCB testing at this point can identify wiring and component issues before
attempting to spin the motor.
Moreover, motor controller PCB
testing is not limited to detecting continuity issues. During powered or
functional testing, technicians can verify that protection thresholds trip when
they should, because proper protection behavior is essential to the circuit.
Functional and Load Testing
PCBA functional
testing should be performed using the board with a real motor or a
realistic load in the form of an electronic device that behaves like a motor, rather
than relying on a resistive dummy load. PCB assembly for motor controllers
should include checks for aspects such as current limitation, speed ramping,
and behavior when stalled.
These are the conditions that
differentiate between a good design and one that works on paper alone, and
precisely the kind of conditions that would be missed by a generic functional
test.
Matching the Assembly
Strategy to the Application
Motor controllers for small fans
have completely different specifications than controllers for traction motors
in electric vehicles; accordingly, the correct choice of assembly strategy will
depend on these specifications.
When choosing a motor control PCB
manufacturer, the OEM should make sure the company has experience with
high-current PCB design and can support testability of the protection
circuitry, not only SMT capability. The manufacturers who publish their process
for power-stage PCBs usually notice the issues that generic PCB shops would
never see.
PCBasic supports motor control PCB assembly projects with DFM review, thermal design
considerations, and functional testing based on specific project requirements.
Conclusion
A motor control PCB assembly
succeeds or fails long before the product reaches customers, based on the
decisions that have been made about the copper weight, the thermal design, and
testing months before.
Making the right decisions from the
outset would save a lot of money compared to fixing problems later. If you are
looking for a manufacturing partner who understands that motor control PCB
assembly is a separate discipline, not just a standard PCBA assembly, PCBasic
would love to talk to you.
FAQs
Q1:
What is a motor control PCB assembly?
It is an assembly of a printed
circuit board that is designed to drive and control the operation of an
electric motor on a single board.
Q2: Why does a motor control PCB assembly need
more thermal design than a standard PCBA?
Components of power-on motor
controller boards will generate heat when loaded; therefore, thermal vias,
copper pours, and even some metal core substrates are needed to maintain the
temperature of the components and solder joints at the rated limit.
Q3: What is the difference between a motor driver
PCB and a motor control PCB?
A motor driver board will typically
include only the switching stage, whereas a motor controller board will include
additional circuitry required for closed-loop control.
Q4: How is testing different for a motor control
PCBA compared to a typical board?
Testing must go further than simply
verifying continuity; it must be able to verify real-life performance
characteristics such as current limiting, speed control, and protective
response while the motor is loaded.
Q5: What should an OEM look for in a motor
control PCB manufacturer?
Look for experience in high-current
design, heat dissipation, and functional testing under load because these have
a direct effect on the way that the board will operate in its environment.
