PCB Virtual Welding Analysis

Wave automatic soldering technology has been used in the electronics industry for many years, but the late failure of solder joints is still a headache, which greatly affects the quality and reputation of electronic products.
This paper intends to make a preliminary study and discussion on the basis of practical experience.
The so-called "late failure of solder joints", refers to the surface appears to be the quality of the solder, there is no "lap welding", "Half welding", "pull the point", "copper" and other welding defects, in the workshop production, the machine is not defective, but to the user after a period of time, due to poor welding, poor conductivity caused by the failure occurred, is one of the reasons for the high early repair rate, which is "false welding".
After repeated research and experiment, it is concluded that the root causes are as follows:

1. Improper mating of PCB aperture and lead wire diameter
The difference between the aperture of the hand insert plate and the lead wire diameter should be 0.2-0.3mm. The difference between the aperture of the machine insert plate and the lead wire diameter should be 0.4-055mm. If the difference is too small, it will affect the plug-in, such as the difference is too large, there is a certain chance of "false welding" risk.
2. Post-weld point damage rate
If the pad is small, the amount of tin is insufficient, large, then the solder joint flat, will cause a small welding surface, poor conductivity, only appropriate, will get good quality solder joints, the reason is the solder joint formation process, the lead and solder pads between the "wetting force" and "balance" results.
And for the need to pass the large current of the solder, the pads need to be larger, after the wave soldering, but also the use of solder wire, and even add rivet welding, to obtain a reliable solder joints.
3. Component leads, PCB solder pads are not weldability
The solderability of the component leads is measured by means of the method specified in GB 2433.32-85 "the Solderability test method of wetting force weighing method", and its 0 time should not be less than 1 seconds, and the absolute value of the wetting force shall be no more than 35% of the theoretical humidity-adjusting force. However, the solderability of component leads is not uniform, and uneven is normal.
such as the CP line is not as good as tinned copper wire, silver plating line is not as long as the wire diameter is smaller than the line diameter, the plug is not as integrated block, storage period is not as short as the storage period, and so on, management of a slight negligence, will cause harm. For the solderability of the printed circuit board pad, it must comply with the GB L0244-88 "TV broadcast receiver printed circuit board specification" 1.6 of the technical conditions, that is, "when ... After dipping, the solder should be wetting the conductor, that is, the solder coating should be smooth, bright, pinhole, non-wetting or semi-wetting defects such as the area of 5% of the total area of coverage, and not concentrated in one area (or a pad) "; From another perspective, the solderability quality level of the PCB pads is not consistent. Therefore, in the actual production, the effect of the inconsistency is not surprising.
4. Solder flux soldering performance is poor
For flux soldering performance, should comply with the standard GB 9491-88, when using RA type, the expansion rate should be no less than 90%, the relative wetting force should not be less than 35%, moreover, in the production often its help welding performance with the use of time will gradually reduce or even failure. Therefore, when the flux performance is poor, it is very likely that the welding performance of the component, the pad on the production of "false welding."
5. Improper control of wave soldering process conditions
For the wave soldering process conditions, generally the following two aspects of control, according to the actual effect to determine the appropriate process parameters.
5. 1 tin pot temperature and welding time control for different wave soldering machine, due to the difference in the width of the crest surface, must adjust the transmission speed of the printed circuit board, so that the welding time is greater than 2.5 seconds, generally refer to the following relationship curve (see Figure 2): In the actual production, often can only evaluate the appearance of solder joints and defect rate, How the conductivity is unknown, "false welding" from this.
According to Wang Zhishing, vehicle Zhao Hua "SMT Wave soldering technology research", in the welding process, the gold phase of the welding point changes through the following three stages of change:
(1) The alloy layer is not fully formed, is only a semi-adhesion binding, low strength, poor conductivity:
(2) Complete formation of alloy layer, high solder joint strength, good conductivity;
(3) Alloy layer aggregation, coarsening, brittle phase generation, strength reduction, conductivity decreased.
In the actual production, we found that the setting of different pot temperature and welding time, and did not set a suitable angle of inclination, there is full of solder joints, change the book, and then full of solder joints and increase up to the "pull point" phenomenon, so I think, must be controlled in when the more lap solder delaval tip, the process conditions are reduced to less and no tip, " The maximum control of the virtual welding.
In addition, I think that the phenomenon in addition to the use of metallographic structure to explain, but also with the "wetting force" changes and solder at different temperatures of "fluidity".
5. 2 preheating temperature and flux specific gravity control
Control a certain proportion of flux and preheating temperature, so that the printed circuit board into the tin pot, the solvent volatile in the flux is similar, but not too dry, it can maximize the role of the help welding, even if the shortest 0 times, the maximum wetting force, such as not drying, the temperature is low, the welding time is prolonged, through the tin peak time is insufficient The flux performance is reduced, even adhesion to the lead, pad, can not be removed from the role of the oxide layer; both tendencies are highly prone to "false welds". (Force Feng recommended the use of LF-DW series of hot air wave soldering, to solve the above problems)

    According to the above five factors, we in the production, can be printed on the design of the board design requirements of the printed circuit board, the standard of the enterprise standards, the requirements of the components, printed circuit board solderability test and storage management system, flux, tin lead solder to set the brand use, the wave soldering process strictly in accordance with the requirements of the process file operation, Every day to record the process parameters, check the quality of solder joints, will produce a "false welding" of the various factors to compress to a minimum.
     with the development of production technology, automatic plug-in wire bending and two times welding process, so that the quality of welding has been considerably improved, but still inseparable from the above factors.
    Therefore, the control "virtual welding" still must from the printed circuit board design, components, printed boards, flux and other welding materials quality management, wave soldering process management aspects of comprehensive control, in order to minimize the "false welding", improve the reliability of electronic products.