Essential approaches for SMT assembly process control

No matter how SMT (surface mount technology) develops today, it’s still core to essentially improving the quality and performance of assembled PCBs (printed circuit boards), which directly affects the reliability of electronic products. As far as an SMT assembly manufacturer is concerned, assembled product quality not only indicates the manufacturing standard of the factory, but also the capacity and potential of the electronics manufacturer.

To acquire top-quality assembled products, a rational and effective manufacturing quality process control plan has to be implemented together with the consideration of practical manufacturing. Therefore, process control for SMT assembly is playing a fundamental role in optimising the SMT assembly process. Effective process control is helpful in finding out any issue that may stop assembly manufacturing from smoothly running and minimising the failure rate of products, so that economic loss can be finally avoided due to disqualifications.

Although SMT assembly features a complicated PCBA process, its process control mainly occurs as part of the leading steps of the whole process; that is, printing, mounting and reflow soldering. So this article will talk about some approaches concerning those steps for SMT assembly process control. All the approaches are based on practical manufacturing experiences of the PCBCart factory.

Solder paste printing

IQC on PCB

IQC (incoming quality control) has to be carried out on each batch of PCBs even though they are fabricated under one roof.

The aspects to be inspected concerning PCBs prior to SMT printing include:

• checking whether deformation occurs in circuit boards;
• checking whether oxidation occurs in circuit board pads;
• checking whether scratches, disconnection or copper exposure occur on board surfaces;
• checking that the PCB surface is flat, smooth and uniform.
   

In the process of solder paste printing, the following aspects have to be taken into consideration to suitably deal with PCBs:

• Gloves have to be worn when catching circuit boards.
• Visual inspection should be implemented, with the distance between eyes and board being in the range of 30–45 cm and angle being in the range of 30–45°.
• Boards should be picked and placed with great care to avoid collision and falling, and they should never be stacked up or vertically placed to stop disconnection.
• Fiducial marks on board should be inspected to make sure complete matching occurs with location holes on stencil.

Application and administration of solder paste

In the process of solder paste application, the following regulations should be conformed to:

• The workshop environment should be controlled, with its temperature being approximately 25°C and relative humidity being in the range of 35–75%.
• Solder paste temporarily not in use should be far away from the manufacturing line in order to avoid using it mistakenly.
• If newly opened solder paste is used with ‘old’ solder paste, they should be mixed at a ratio of 3:1.
   

The storage of solder paste is similarly important and the following aspects should be noted:

• Validity of solder paste should be strictly monitored and expired solder paste mustn’t be used.
• Solder paste should be kept in fridge when it is not used.

Process control measures during printing

To guarantee the quality of solder paste printing, the following process control measures should be observed:

• The printing part should be complete. If it is not, parameters should be modified on the circuit board, stencil and printing blade.
• Bridging mustn’t be seen on printing.
• Printing thickness should be uniform. If it is not, the strength of the scraping blade should be modified.
• The pad has to be inspected to ensure whether a turned-down edge is available or not. If it is, stencil holes should be checked to ensure they are not blocked.
• The printing effect should be inspected to see whether deviation occurs or not. If there is deviation, the stencil location should be modified.
   

In addition, the stencil should be cleaned to stop flux from being dried on the stencil and blocking the stencil holes. When it comes to electronic products that have to strongly vibrate in practical application, solder paste thickness should be modified to guarantee solderability and reliability of products.

Chip mounting

As a key device in the SMT assembly manufacturing process, a chip mounter is capable of sticking SMDs (surface mount devices) to corresponding pads on PCBs. Therefore, high accuracy is required in this step, which is especially true for material supply, programming, testing and assembly.

Process control measures during chip mounting

• Measure #1: All SMDs have to be completely correct, which is compatible with design files.
• Measure #2: A control instruction signal and its editing procedure have to be implemented with high accuracy. Additionally, corresponding data has to conform to the chip mounter program manual.
• Measure #3: The assembly between SMDs and component supplier should be as accurate as possible to stop errors from repeatedly occurring.
• Measure #4: Debugging should be accurately implemented on the chip mounter before assembly manufacturing and defects have to be suitably dealt with during SMT assembly procedure.

Process control measures after chip mounting

Chip mounters feature a complicated structure, containing a transmission mechanism, servo system, recognition system, sensors, etc. Defects tend to be caused during SMT assembly process; their solutions are provided below.

• Solution #1: The working order and logical relationship between transmission parts of the chip mounter should be analysed.
• Solution #2: The location, link and extent of the defect in the process of SMT assembly running should be known and different defects can be classified and recognised through bizarre sounds.
• Solution #3: The operating process should be clarified prior to defects.
• Solution #4: Defect occurrence location should be clarified.
   

As high-precision electronic manufacturing equipment, a chip mounter has to load a huge amount of SMT assembly mounting. A maintenance schedule has to be made in order to keep the equipment in a good state in order to better run.

Reflow soldering

Reflow soldering aims to attach SMDs to a PCB board through heat convection. As the temperature rises, solder paste connecting components and pads begins to be melted. As the temperature cools down, the solder paste will be cured with components permanently stuck to the board.

Process control requirement for reflow soldering in SMT assembly include:

• Appropriate temperature curve for reflow soldering should be set and real-time test needs to be made.
• Soldering should be implemented in accordance with the soldering direction regulated in PCB design files.
• Vibration should be avoided in the process of soldering.
   

To testify the performance of reflow soldering, the following aspects can be used as a reference:

• The soldering part of the component should be complete.
• Soldering joints should feature a smooth surface.
• Solder joints should be in a semilunar shape.
• The surface of the circuit board should be free of soldering balls and residues.
• Bridging and pseudo soldering should not be available.
   

Image credit: ©stock.adobe.com/au/servickuz

Originally published here.