The constant and increasing application of SMT (Surface Mount Technology) assembly manufacturing in electronics industry leads performance and reliability to be people’s core concern on electronics products. SMT assembly manufacturing quality not only stands for the level of manufacturing workshop, but guarantees the long-term development of electronics products. To substantially ensure the performance of products and to lead manufacturing process to be reasonable, regulational and standardized, a rational and effective process control system has to be established to be compatible with practical manufacturing requirement. SMT assembly manufacturing has to start with rigorous process control that plays a fundamental role in the whole manufacturing process because effective control is capable of exposing quality issues in time to minimize the throughput of rejected products so as to avoid economic loss resulting from disqualification. Therefore, it’s of huge significance to carry out process control measures in the process of SMT assembly.
SMT assembly process mainly consists of three steps: solder paste printing, components placement and reflow soldering. Process control measures have to be implemented in each step so that high reliability will be obtained.
Process Control Measures in Solder Paste Printing
• PCB Quality Control
Sampling inspection should be carried out on all batches of PCBs prior to solder paste printing. Inspection items include:
a. Whether deformation occurs on PCB;
b. Whether oxidation takes place on PCB pad;
c. Whether scratches, shorts and copper exposure takes place on PCB surface;
d. Whether printing is evenly smooth or not.
In the process of process control on PCB performance, sufficient attention should be paid from the beginning to the end. Firstly, gloves have to be worn when picking up PCB boards. Secondly, when visual inspection is applied, the distance between naked eyes and inspected boards should be in the range from 30cm to 45cm with the angle about 30° to 45°. PCB boards should be gently dealt with during inspection to avoid collision or falling and they mustn’t be stacked up or kept upright to stop circuits from being cut. Meanwhile, location holes on boards should be inspected to ensure that stencil openings are compatible with pads on PCB.
• Application and Storage of Solder Paste
In the process of SMT assembly, validity of solder paste has to be rigorously monitored to maintain its high reliability. Overdue solder paste mustn’t be applied and purchased solder paste should be kept in cold closet of refrigerator. Uncovered solder paste has to be used within one week. In the process of solder paste application, the workshop temperature should be controlled at approximately 25℃ and RH (Relative Humidity) should be controlled within 35% to 75%. Temporarily-not-used solder paste should be placed away from workshop to stop it from being mixed up with in-use solder paste. When “new” solder paste has to be mixed up with “old” solder paste, the mixing ratio should be 3:1.
• Some Control Measures in Solder Paste Printing
Successful solder paste printing should be compatible with the following requirement:
a. Printing should be complete;
b. No bridging takes place;
c. Printing thickness should be evenly smooth;
d. No turn-down edge occurs on pad;
e. No deviation occurs in printing.
If solder paste printing is found incomplete, PCB board, stencil and scraping blade should be adjusted to make it complete. If bridging takes place in solder paste printing, chips should be inspected with the finest pitch, normally CPU. If solder paste printing isn’t done evenly smooth, scraping pressure should be adjusted. If turn-down edge is found at pad, stencil opening should be inspected to ensure that no blocking is available. If deviation is found in solder paste printing, stencil position should be in time adjusted.
Process Control Measures in Chip Mounting
As the key equipment applied in SMT assembly manufacturing, chip mounter is capable of fast and accurately placing components onto corresponding pads through a series of actions including absorbing, moving, positioning and placing.
• Mounting Requirement
a. All SMDs (Surface Mount Devices) should be guaranteed to be sufficiently and correctly used;
b. Programming should be accurately edited so that corresponding parameters are compatible with programming requirement;
c. SMDs and feeders should be accurately combined to avoid the reoccurrence of errors;
d. Chip mounter should be accurately debugged prior to chip mounting and breakdowns should be in time dealt with during SMT assembly process.
• Solutions to Defects in Chip Mounting
Chip mounter features such a complex structure that consists of transmission mechanism, servo system, recognition system and sensors. Different defects tend to be met in chip mounting and measures to deal with defects will be discussed below:
a. Working sequence of chip mounter should be analyzed and the logics between transmission parts should be known;
b. In the process of equipment running, defects can be known in terms of their positions, link and extent and they can be achieved through strange sounds;
c. The operation process should be clarified before defects;
d. Defects should be clarified to determine whether they take place at some fixed positions such as mounting head or nozzle;
e. Defects should be clarified to determine whether they take place at components feeder or SMDs;
f. Defect redundancy should be studied to determine whether it occurs to particular amount or time.
As high-density electronic equipment, SMT chip mounter is responsible for SMT chip mounting and should be inspected every day to make it smooth for assembly manufacturing.
Process Control Measures in Reflow Soldering
Reflow soldering refers to the process in which gas reaches a high temperature through internal circulation flow to make SMCs and SMDs stuck to PCBs.
Reflow soldering should be compatible with the following requirement:
a. Reasonable reflow soldering temperature curve should be set and practical test should be carried out at regular intervals;
b. In the process of reflow soldering, reflow direction should be conformed to what is designed for PCB;
c. Vibration should be avoided on transfer belt in reflow soldering process.
As far as solder paste is concerned, higher metal oxide content always leads to a higher combination resistance among metal powder. Thereafter, insufficient wettability is caused between solder paste, pads and SMDs, reducing their solderability. It’s been summarized that the occurrence of solder balls is directly proportional to metal oxide. Therefore, the oxide should be rigorously controlled below 0.05% in solder paste so as to stop solder balls from being generated.
As reflow soldering comes to an end, soldering effect can be determined through the following aspects of inspection:
→ To see whether soldering part is complete on components;
→ To confirm whether solder joints feature smooth surface;
→ To see whether solder joints feature semi-lunar shape;
→ To determine whether residues are available on PCB surface;
→ To see whether bridging and cold soldering do take place through microscope.
Flexible temperature curve can be applied and modified at any time during the process of reflow soldering to cater to different changes in terms of environment and product performance.
