Surface Mount Technology (SMT) is a common manufacturing method for assembling electronic circuit boards. This automated technique has revolutionized the assembly of printed circuit boards (PCBs) in the electronics manufacturing industry. Unlike through-hole methods, SMT components mount directly onto the board, enabling faster production, smaller designs, and more automation.
This post provides a step-by-step overview of the general SMT process, from stenciling to inspection, shares common challenges, and reviews industry standards.
There are many steps to the SMT process, and each must be done in sequential order. Equipment and production procedures may vary per company and application.
The stenciling process begins by applying solder paste to the bare PCB. This paste is a mixture of solder particles and flux and is applied only to the pads where components will be terminated. This is done using a stainless-steel stencil that is aligned over the board. A squeegee spreads the paste, filling the cutouts. When the stencil is lifted, precise deposits of solder paste remain on the board. This step is critical. Poor stencil printing causes up to 60% of SMT assembly defects, including excess and/or insufficient paste, resulting in weak joints and bridging.
Next, the board moves to the pick-and-place machine, the heart of the SMT assembly process. This high-speed automated machine uses vacuum nozzles to pick up surface-mount components from reels or trays. They are then accurately placed onto the solder-pasted pads from the previous step. These components can range in size from 0201 chip components to 100-pin quad flat pack (QFP).
Pick-and-place machines can handle thousands of components per hour with precision down to the micron level. They rely on electrical and visual systems to align parts that ensure correct orientation and polarity for components like capacitors and diodes.
Once components are in place, the board enters the reflow oven through a conveyor system. The multi-zone oven gradually heats the board to melt the solder paste and form strong electrical and mechanical connections.
The reflow process follows a controlled temperature profile of four steps:
A well-tuned reflow profile ensures reliable soldering without damaging sensitive components. Parameters like conveyor speed and zone temperature can be altered to match these specific board profile needs.
After reflow soldering, assemblies require cleaning to remove flux residues—contaminants like dust, hair, and moisture are considered foreign object debris (FOD). FOD cannot be present in assemblies as it can cause corrosion, short circuits, or violate minimum electrical clearance. Products that demand high reliability (like in military or aerospace applications) have a lower acceptance criterion for levels of residues.
Cleaning can be done using aqueous (water-based) or solvent-based systems, often in specialized machines that spray, scrub, and rinse the boards. Clean boards are less prone to electrical failures and long-term reliability issues, and ensure good adherence of staking and coating materials.
The final step is inspection, which evaluates and judges all components on placement and solderability. This can involve several methods:
Some manufacturers also perform functional testing to verify that the board works as intended before it moves to final assembly. This testing includes bed of nails (fixture-based) and flying probe (in-line) testing methods.
Although Surface Mount Technology delivers fast and accurate assembly, it still comes with its share of hurdles. Engineers and manufacturers must navigate a range of limitations and considerations to ensure high-quality, reliable assemblies.
To maintain consistency and quality across the industry, manufacturers rely on IPC standards. Global guidelines were developed by the Association Connecting Electronics Industries. Here are the standards that pertain to SMT and PCB assembly:
Many companies require technicians and inspectors to be IPC-certified. This ensures they understand and can apply these standards correctly. Training programs are available globally and often include hands-on labs, exams, and continuing education. At ACT, we have IPC-certified instructors to ensure our own SMT process and assembly meet the necessary requirements of these standards.
Surface Mount Technology (SMT) is a foundation of modern electronics. It enables compact, dense, high-performance devices, and it demands careful design, process control, and industry standards.
Whether you’re a hobbyist, engineer, or manufacturer, understanding the SMT workflow and its challenges can elevate your work and ensure long-term reliability.
For decades, we’ve been designing and building military-grade power supplies for use in some of the most demanding environments. SMT is a crucial part of our manufacturing process and helps to ensure high reliability. Our vertically integrated manufacturing facility features SMT and through-hole board assembly, which allows us to have flexible circuit expertise.
At ACT, all products are designed and built with a quality mindset. All workmanship complies with MIL-HDBK-454 and J-STD-001 Class 3 standards—at a minimum—and our quality system is certified to AS9100D and ISO 9001:2015. Contact us for power solutions you can depend on.
