R.S.F. Engineering - wave soldering production processing and how to control the weld?

Wave soldering process control solder wave automatic welding technology, in the electronics industry has been applied for many years, but later on solder joint failure is still a difficult problem, which greatly affects the quality of electronic products and reputation. This paper according to the practical experience to make preliminary research and discussion. The so-called" solder late failure", refers to a seemingly solder joint quality is passable, there is no " welded"," half spot"," sharp"," exposed copper welding defects", in the workshop production, loaded into the machine is one thing, but to the user after a period of time, as a result of poor welding, electric performance is poor and the failures have occurred, causing early high repair rate is one of the reasons, this is" cold solder joint". I think the reasons are as follows: printed circuit board aperture and the lead wire diameter mismatch
Hand inserting plate aperture and the lead wire diameter difference, should be in 0.2 - 0.3mm. Machine inserting plate aperture and the lead wire diameter difference, should be in 0.4 - 055mm. If the difference is too small, then the effect plug-ins, such as difference is too large, have a chance to" weld" risk. As the pad is small, then the amount of tin is insufficient, too large, then welding flat, can cause small welding surface, the conductive performance is poor, only proper, can get good quality solder joint, investigate its reason, is in the process of forming a solder joint, the lead and pad and solder between the " wetting" balance"" the results. And the need for high current to flow through the solder pads, need more, after soldering, also with a tin wire welding, even with rivets and welding, to get reliable solder joints. Component lead, PCB pad poor solderability
Component lead solderability, with GB 2433.32-85" wetting force weighing method Solderability Test Method" prescribed by the method of measurement, the zero cross time shall not be greater than 1 second, the wetting power of the absolute value of the theory of humidity stress should not be less than 35%. However, components lead solderability, are not consistent, uneven is a normal phenomenon. If the CP line as tinned copper wire, silver-plating line as tinned wire, wire diameter greater than the diameter of small, connectors as integrated block, long storage period as short shelf life and so on, the management of any negligence, will cause harm. PCB pad solderability, must conform to the GB l0244-88" TV receivers for printed boards" 1.6 under the technical conditions, namely" when ... ... After soldering, solder wetting conductor shall be, namely the solder coating shall be smooth, bright, pinhole, not wetting or half wetting defect area does not exceed 5% of the total area of coverage, and not concentrated in one area ( or a pad" ); from another point of view, PCB pad the welding quality level, also is not always consistent. Therefore, in the actual production, the effect will be not at all surprising inconsistency. Flux solder poor performance
The flux for soldering performance, should be consistent with the GB 9491-88 specified by the standard, when using the RA type, growth rate should be not less than 90%, the relative wetting force should not be less than 35%, moreover, in produce often the soldering performance with the use of time will gradually decrease or even failure. Therefore, the flux poor performance when, very likely in the welding property of differential element, produce" solder pads on the". Wave solder improper control of technical conditions
For wave soldering process conditions, generally for the following two aspects of control, according to the actual results to determine the appropriate process parameters.
A tin pot temperature and welding time control
For different wave soldering machine, wave crest surface due to the different width, must adjust the printed circuit board transmission speed, so that the welding time greater than 2.5 seconds, the general may refer to the following relation curve (see Figure 2 ): in actual production, often can only be evaluated by solder joint appearance quality and defect rate, the weld strength, conductivity how can make nothing of it" we," the resulting. According to Wang Ducheng," the car York SMT wave welding technology research", in the process of welding, weld microstructure changes through the following three stages of change: alloy layer is not complete generation, is only a half attached to binding, low intensity, poor conductivity of alloy layer; complete generation, welding high strength, conductivity; alloy layer aggregation, coarsening, brittle phase formation, reduces the strength, electrical conductivity decreased. In practice, we found, set different tin pot temperature and welding time, and not suitable for tilt angle, have the spot is full, book, and a solder joint spot is full and increased until the" tip" phenomenon, therefore I think, must be controlled in a when produce more lap welding Lila tip the process conditions, reduced to a solder less and no sharp pull," weld" to maximize the control. In addition, I think, the phenomenon can be explained in the metallographic structure, but also with the" wetting" changes and solder at different temperatures and the" liquidity".
B. Preheat temperature and flux density control
The control of certain flux and the proportion of preheating temperature, the printed circuit board into the tin pot, solder solvent almost, but not too dry, can play a maximum flux effect, even if the zero crossing time is the shortest, wetting force, such as drying, then the lower temperature, welding by the time prolonged, Hyun insufficient; such as drying too, then flux reduced performance, and even attached to the lead, pad, not to remove the oxide layer; the two tendencies are prone to" cold solder joint". According to the above five factors, we in the production of printed circuit board design, the provisions of the" PCB design technology requirements" enterprise standard, provides components, PCB solderability test and storage management system, on the flux, tin lead solder for plant licensing use, on the crest welder foreword in strict accordance with the requirements of operation process files, regular daily recording process parameters, inspection of solder joints quality, will have a" fault" factors compressed to a minimum. In recent years, the factory products" weld" reflect the very small, gained economic benefits. With the development of production technology, automatic plug-in wire bending and two welding process, the welding quality is significantly improved, but still can not be separated from the above aspects factors. Therefore, control" we" must still from the circuit board design, components, PCB, flux and welding consumables quality management, wave soldering process management aspects of comprehensive control, to minimize the" fault", improve the reliability of electronic products.