Analysis of the main causes of solder joint failure due to wave of welding knowledge
Date：2022-11-18 14:27:21 Views：133
Wave soldering is a necessary equipment in the welding of electronic product plug-ins. However, faults caused by poor wave soldering, faulty solder joints and poor conductivity occur from time to time. To solve the problems, it is necessary to know the main causes of wave soldering defects. In order to help you understand, the following contents are organized by the Core Creation Testing Network for your reference.
False soldering of wave soldering point:
I Improper matching between printed board aperture and lead wire diameter
The difference between the diameter of the hand plugboard and the diameter of the lead wire should be 0.2-0.3mm. The difference between the hole diameter of the machine plug-in board and the lead wire diameter should be 0.4-055 mm. If the difference is too small, the plug-in will be affected. If the difference is too large, there will be a certain probability of "false soldering" risk.
II Solder joint damage rate of wave peak in the later period
If the solder pad is too small, the amount of tin is insufficient. If the solder pad is too large, the solder joint will be flat, which will result in a small welding surface and poor conductivity. Only appropriate solder joints can be obtained. The reason is that the "wetting force" and "balance" between the lead wire, the solder pad and the solder during the formation of the solder joint. For the solder joints that need to pass high current, the pad needs to be larger. After wave soldering, it needs to use tin wire for additional welding, and even add rivets for additional welding to obtain reliable solder joints.
III Poor solderability of component leads and PCB pads
The solderability of component leads shall be measured by the method specified in GB 2433.32-85 Test Method for Solderability of Wetting Force Weighing Method. The zero crossing time shall not be greater than 1 second, and the value of the wetting force shall not be less than 35% of the theoretical wetting force. However, the solderability of component leads is not always caused, and uneven is a normal phenomenon. For example, CP wire is not as good as tinned copper wire, silver wire is not as good as tinned wire, large wire diameter is not as good as small wire diameter, connector is not as good as integrated block, long storage period is not as good as short storage period, etc. A little neglect in management will cause harm. The solderability of solder pads for printed boards must meet the technical conditions specified in Article 1.6 of GB 10244-88 Specifications for Printed Boards for Television and Broadcasting Receivers, that is, "after... immersion welding, the solder should wet the conductor, that is, the solder coating should be smooth and bright, and the area of defects such as pinholes, non wetting or semi wetting should not exceed 5% of the total area, and should not be concentrated in areas (or pads)"; On the other hand, the solderability quality level of PCB pads is not always the same. Therefore, in actual production, it is not surprising that the effect will not be produced.
IV Poor performance of flux
The soldering performance of the flux shall comply with the standards specified in GB 9491-88. When RA type is used, the expansion rate shall not be less than 90%, and the relative wetting force shall not be less than 35%. In addition, its soldering performance will gradually decrease or even become invalid with the extension of service time. Therefore, when the performance of flux is poor, it is very likely to produce "false soldering" on components and pads with poor solderability.
V Improper control of wave soldering process conditions
For wave soldering process conditions, the following two aspects are generally controlled, and the appropriate process parameters are determined according to the actual effect.
1. Control of tin pot temperature and welding time
For different wave crest welders, due to the different width of the wave crest surface, the transmission speed of the printed circuit board must be adjusted so that the welding time is more than 2.5 seconds. In actual production, only the appearance quality and defect rate of the solder joints can be evaluated, and the welding strength and conductivity are unknown. Therefore, "faulty soldering" comes from this. During the welding process, the metallographic structure of the welding spot has changed in the following three stages:
(1) The alloy layer is not completely formed, which is only a semi adhesive bonding with low strength and poor conductivity:
(2) The alloy layer is completely formed, the solder joint has high strength and good conductivity;
(3) The alloy layer is aggregated and coarsened, brittle phase is formed, strength and conductivity are reduced.
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