Here are important techniques to consider to control the quality of PCB SMT assembly.
Surface mount technology (SMT) assembly is one of the broadly used component assembly techniques in the PCB manufacturing industry as it provides high reliability at a reduced cost. SMT PCB assembly is the most sought-after component mounting method in all the recent electronic applications like telecommunications, the aerospace sector, and automotive, to name a few.
The benefits of using SMT assembly are reduced manufacturing cost, efficient space utilization, and high repeatability due to automation of the process. It demands closer monitoring of the assembly line and focuses on details to achieve quality output.
Many factors affect the PCB quality like substrate material, design strategies, and equipment efficiency. But the SMT assembly process itself has various steps that can introduce defects and reduce the quality of the PCB assembly.
We shall briefly understand the SMT assembly process that consists of the following steps:
Material preparation and check: PCB and components are checked for any visual faults. Stencils are prepared to provide a fixed position for solder paste printing.
Solder paste printing: Solder paste is a mix of tin and flux. It is applied on the PCB with a stencil to connect the surface mount components on the solder pads of the PCB.
Surface mount component placement: Using a pick and place machine, all the surface mount components are placed on the bare PCB.
Reflow soldering: In this stage, the components are soldered onto the pads as they pass through the soldering oven, reflow zone, and cooling zone.
Clean and inspection: After soldering, the PCB is cleaned and inspected for any defects like bridging, solder beading, etc.
In the SMT assembly sequence, defects are usually introduced in the three main stages which are solder paste printing, component placement, and the soldering station. These possible faults can affect the quality of the SMT assembly process. If process control methods are not followed during these three stages, then the quality of the SMT assembly will be compromised.
Some of the important techniques to control the quality of PCB SMT assembly are listed below.
Best practices during Solder Paste Printing:
The quality of the solder paste depends on its storage temperature and humidity. It has to be refrigerated at 5°C and has to be opened at room temperature just before the usage. The ideal application temperature is approximately 20°C with humidity in the range of 30-50%. If the moisture content is high, then soldering balls may be created due to quick vaporization and can cause solder ball defects.
Soldering quality depends on the speed and pressure of the scraping blade during solder paste printing. The printing parameters should be set to the optimal value for ensuring the process quality.
Printing should be done starting from the core to the board edges and not vice versa.
If any turned-down edges are found on the PCB surface, then verify that no blockage occurs in the stencil holes.
If any deviation occurs due to the printing effect, then the stencil location has to be modified accordingly.
Best practices during component mounting:
Component size and position have to be accurate and should meet the chip mounter tolerance else component misplacement or bridging defects will occur.
As the SMT assembly uses automation like pick and place methods for component placement, the software used should be highly accurate to avoid possible errors.
Ensure that the component feeders and surface mount devices are well connected to avoid any delay during placement.
Chip mounter device has to be checked and serviced in between to confirm the smooth running of the process.
Any recurring faults have to be identified and analyzed to regulate the process.
Best practices during reflow soldering:
Soldering defects like bridging, tomb-stoning, solder beading, etc. can significantly reduce the performance of the PCB and can cause permanent damage to the board also. Using appropriate solder temperatures is one of the ways to avoid these defects.
Any vibration during soldering should be avoided and soldering direction should be as per the guidelines.
Multiple assembly and fabrication issues are discussed below along with the solutions to improve the quality of the overall PCB assembly process.
A solder bridge or electrical bridge is a defect caused due to a solder joint on the adjacent pins of a component or two traces. This causes an electrical short and can damage the PCB. Excessive solder pastes or overheating of solder paste can be the reason for such solder bridge issues.
A solder ball may be seen near the traces and can cause unnecessary signal conduction between adjacent traces. This occurs mainly due to water vapor and hence the printing process should be followed by appropriate baking of the PCB to eliminate water vapors if any.
Tombstone defect is the partial or complete lifting of the passive surface mount components from the PCB pad. Using recommended pad size and footprint construction during the design stage can avoid this assembly error.
Appropriate usage of solder paste and flux can reduce many assembly issues related to excessive or insufficient solder joints.
SMT assembly process provides the advantage of lesser manufacturing cost and compact design capability. Consumer devices like tablets and mobile phones make the best use of SMT assembly technology. But the SMT assembly issues like solder bridge, excessive fillets, etc can degrade the PCB performance. Lightweight components and thermally sensitive components are not suitable in the SMT assembly method. Hence, adhering to strict quality control during the process is essential to improve the SMT assembly yield.
There are different testing services like automated optical inspection, x-ray testing, Flying-probe test which can detect any defects in the SMT assembly. Contract manufacturers usually provide these facilities along with the PCB fabrication and assembly services. An experienced CM will be aware of these techniques to improve the PCB SMT assembly output.
About the Author
Suresh Patel has worked as a Sales Engineer and other management roles at Mer-Mar Electronics. He brings 25 years of experience in printed-circuit-board sales and technical client service and managing business. You can connect with him on Twitter.