The overall process of SMT assembly mainly covers solder paste printing, placement, soldering and inspection among which solder paste printing, placement and soldering top the list.
Defects Caused in Solder Paste Printing Process
As the beginning of SMT assembly process, solder paste printing plays a direct role in determining the quality of both PCB and end product. If rigorous control isn’t applied during solder paste printing, soldering balls will possibly be caused in later procedure of SMT assembly. For example, unwetting will cause metallic particles to be oxidized or metallic particles will possibly suffer from irregular shapes due to insufficient solder paste. Else, soldering balls will be possibly aroused due to solvent flashing or metallic particle oxidization.
Defects Caused in Placement Process
Placement, also called chip mounting, is regarded as the most complicated manufacturing step as far as SMT assembly is concerned so that the level of placement indicates the performance of SMT assembly manufacturing. Therefore, the quality of chip mounting stands for the level of SMT. However, defects tend to be caused mostly in this step, leading to the highest defect rate of manufacturing equipment. For example, missing components possibly occur due to badly-performed nuzzle; component’s wrong orientation may be aroused due to the errors taking place on component suppliers; or, misplacement may be caused due to incorrect alignment.
Defects Caused in Soldering Process
Soldering refers to a process during which devices are stuck to PCB board through melting metallic solder that will be cooled down and become hardened with adhesion completed. Soldering affects the performance of electronics products to the largest extent so soldering quality improvement lays a basis to product performance guarantee. When it comes to the overall process of soldering, all essential elements have to taken into serious considerations including surface cleanness, soldering temperature setting and solder quality. The top defect caused in reflow soldering process during SMT assembly manufacturing procedure is soldering balls that are small metallic particles caused on component surface through reflow soldering. Soldering balls possibly make ICs (Integrated Circuits) shorts, directly leading assembled circuits to failure. Furthermore, if smaller PCBs have to go through second soldering, shorts will be also aroused on circuit board due to rolling soldering balls or the whole board or product will be burned down.
Apart from the aforementioned defects causes during SMT assembly process, cost control or insufficient cleaning will also lead PCBs to be degraded in performance. Too many residues on the board surface will tend to make soldering joints far from being full and bright.
How to improve PCB assembly quality?
In accordance with the causes to affect SMT assembly product quality, some measures can be summarized to improve SMT assembly product quality during manufacturing process.
Solder Paste Printing Technology and Quality Management
In terms of SMT assembly, solder paste plays a key role in assembly manufacturing. After all, 70% of quality defects in electronic products with SMT assembly applied result from low-quality solder paste printing. Thus, improving solder paste printing performance is an important measure to increase SMT assembly manufacturing quality. Specifically speaking, measures can be made centering on solder paste quality, stencil placement and printing parameter setting.
Solder Paste Quality
Prior to its application, solder paste should be placed in the fridge with the temperature of 5℃ and it can’t be taken out until it is ready for its participation in manufacturing. Once solder paste contains too much moisture, splashing will easily be caused due to vaporation, which will further lead to the generation of soldering balls. Furthermore, before its application, solder paste container should be opened in room temperature and its temperature should go up naturally. The optimal application temperature of solder paste is approximately 20℃ and humidity 30% to 50%. Therefore, both temperature and humidity have to be rigorously controlled in SMT assembly manufacturing environment.
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