No matter how SMT (Surface Mount Technology) develops today, it’s still a core to essentially improve the quality and performance of assembled PCBs (Printed Circuit Boards), which directly affects the reliability of electronic products. As far as SMT assembly manufacturer is concerned, assembled product quality not only stands for manufacturing standard of the factory, but also indicates capacity and potential of electronics manufacturer. To acquire top quality of assembled products and promote manufacturing process towards reasoning, regulation and standard, 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 a start playing a fundamental role in optimizing SMT assembly process. Effective process control is helpful in finding out any issue that may stop assembly manufacturing from smoothly running and minimizing 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 to 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 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:
a. To make sure whether deformation does occur to circuit boards;
b. To make sure whether oxidation does occur to circuit board pads;
c. To make sure whether scratches, disconnection or copper exposure occur to board surface;
d. To make sure 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:
a. Gloves have to be worn when catching circuit boards;
b. Visual inspection should be implemented with the distance between eyes and board being in the range from 30cm to 45cm and angle being in the range from 30° to 45°;
c. Boards should be picked and placed with great care to avoid collision, falling and they can never be stacked up or vertically placed to stop disconnection;
d. Fiducial marks on board should be inspected to make sure whether complete matching does occur 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:
a. The environment of workshop should be controlled with its temperature being approximately 25℃ and relative humidity being in the range from 35% to 75%;
b. Temporarily-not-used solder paste should be far away from manufacturing line in order not to use it mistakenly;
c. If newly-opened solder paste should be used with “old” solder paste, they should be mixed at a ratio of 3:1.
It’s evenly significant to keep solder paste and the following aspects should be noticed:
a. Validity of solder paste should be strictly monitored and overdue solder paste mustn’t be used;
b. 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 done:
a. Printing part should be complete. If it is not, parameters should be modified on circuit board, stencil and printing blade;
b. Bridging mustn’t be seen on printing;
c. Printing thickness should be uniform. If it is not, strength of scraping blade should be modified in time;
d. Pad has to be inspected to ensure whether turned-down edge is available or not. If it does, stencil holes should be checked to ensure they are not blocked;
e. Printing effect should be inspected to see whether deviation occurs or not. If there is deviation, stencil location should be timely modified.
In addition, stencil should be cleaned to stop flux from being dried on stencil and blocking 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, chip mounter is capable of sticking SMDs (Surface Mount Devices) to corresponding pads on PCB. 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: Program plays a role as a control instruction signal and its editing procedure has to be implemented with high accuracy. Additionally, corresponding data has to conform to 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 chip mounter before assembly manufacturing and defects have to be suitably dealt with in time during SMT assembly procedure.
•Process Control Measures after Chip Mounting
Chip mounters feature a complicated structure, containing transmission mechanism, servo system, recognition system, sensors etc. Defects tend to be caused during SMT assembly process, their solutions will be provided below.
Solution#1: Working order and logical relationship between transmission parts of chip mounter should be analyzed.
Solution#2: Location, link and extent of the defect in the process of SMT assembly running should be known and different defects can be classified and recognized 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, chip mounter has to load a huge amount of SMT assembly mounting. Maintenance schedule has to be made in order to keep the equipment an excellent state to be able to better run.
Reflow Soldering
Reflow soldering aims to attach SMDs to PCB board through heat convection. As temperature rises, solder paste connecting components and pads begins to be melted and as temperature cools down, the solder paste will be cured with components permanently stuck to board.
Process control requirement for reflow soldering in SMT assembly includes:
1. Appropriate temperature curve for reflow soldering should be set and real-time test needs to be termly made;
2. Soldering should be implemented in accordance with the soldering direction regulated in PCB design files;
3. 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:
1. To ensure soldering part of component is complete;
2. Soldering joints should feature smooth surface;
3. Solder joints should be in semilunar shape;
4. The surface of circuit board should be free of soldering balls and residues;
5. Bridging and pseudo soldering should not be available.
