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SMT chip manufacturer talks about factors affecting the quality of SMT chip reflow soldering
Reflow soldering is one of the key processes in SMT processing, and the result of reflow soldering directly reflects the quality of surface assembly. Therefore, it is necessary to understand the factors that affect the quality of reflow soldering.
The soldering quality problem in reflow soldering is not entirely caused by the reflow soldering process, because the quality of reflow soldering is not only directly related to the soldering temperature (temperature profile), but also to the equipment conditions of the production line, PCB pads and design for productivity, and solderability of components, The quality of solder paste, the quality of printed circuit board processing, and the process parameters of each process of SMT are even closely related to the operation of the operator.
The assembly quality of SMT patches has a direct and important relationship with PCB pad design. If the PCB pad design is correct, due to the surface tension of the molten solder during reflow soldering (called self-positioning or self-correction effect), a small amount of tilt during mounting can be corrected. On the contrary, if the PCB pad design is not correct, even if the installation position is very accurate, there will be soldering defects such as component position deviation and suspension bridge after reflow soldering.
1. The key elements of PCB pad design:
According to the structural analysis of the solder joints of each component, in order to meet the reliability requirements of the solder joints, the PCB pad design should master the following key elements:
(1) Symmetry-The pads at both ends must be symmetrical to ensure that the surface tension of the molten solder is balanced.
(2) Pad pitch-ensure the overlap size between component ends or leads and pads. Too large or too small pad spacing will cause soldering defects.
(3) The remaining size of the pad-the remaining size of the component end or lead after overlapping the pad must ensure that the solder joint can form a meniscus.
(4) The width of the land-should be approximately the same as the width of the component end or lead.
2. Defects prone to reflow soldering:
If the design requirements are violated, soldering defects will occur during reflow soldering, and PCB pad design problems are difficult or even impossible to solve in the production process. Take rectangular chip assembly as an example:
(1) When the gap between the pads G is too large or too small, because the solder end of the component cannot overlap the pad during reflow soldering, suspension bridges and displacement will occur.
(2) When the size of the liner is asymmetric, or the ends of two parts are designed on the same liner, suspension bridges and displacements may also occur due to the asymmetric surface tension.
(3) Through holes are designed on the pads, and the solder will flow out from the through holes, resulting in insufficient solder paste.
Material change process in SMT patch production
During the processing of SMT chips, due to the small order quantity, frequent line changes, feeding and material changes are inevitable. Therefore, there must be a strict standardized process in this process to avoid incorrect material updates in the high-frequency replacement line during the material replacement process. So, what is the standard process to avoid replacing the wrong material? Let us analyze with you today:
1. Take out the paper feeder and take out the used paper tray.
2. SMT operators can take materials from the material racks according to their own stations.
3. The operator uses the job number and position table to check the removed material to confirm whether the specifications and models are the same.
4. The operator checks the new pallet and the old pallet, and checks whether the specifications and models of the two pallets are exactly the same.
5. The operator checks its materials to show whether the company is consistent with the pallet.
6. If there is any abnormality in the above inspection, the operator of the placement machine should immediately notify the delayed processing.
7. Take samples from the new pallet, and then park the material on the new and old pallets.
8. Paste the newly installed Feida materials on the "Refueling Process Record Sheet", and fill in relevant data and information such as fueling hours and operator management.
9. Put the feeder back into the placement machine according to the SMT placement machine station; you must effectively fill in your own supplementary information and record once.
10. The operator shall notify the project quality management personnel of material matching and testing, and must carry out material changes and inspections.
11. IPQC checks whether the data and information are correct according to the site number table, and whether the site is correct.
12. Before starting production, the operation inspection of the above-mentioned placement machine has been completed.
The above 12 steps are the standard process in the entire SMT refueling process, and the patch processing must be strictly followed. Each step of the process must have a description of each post. Only in this way can we avoid replacing the wrong material.
