With the progress of science and technology, mobile phones, tablet computers and other electronic products are light, small, portable for the development trend, in the SMT processing of electronic components are also becoming smaller, the former 0402 capacitive parts are also a large number of 0201 size to replace. How to ensure the quality of the solder joints has become an important issue of high-precision SMD. Solder joints as a bridge for welding, its quality and reliability determines the quality of electronic products. In other words, in the production process, the quality of SMT is ultimately expressed in the quality of the solder joints.
At present, in the electronics industry, although the research of lead-free solder has made great progress and has begun to promote its application worldwide, and environmental issues have been widely concerned about, the use of Sn-Pb solder alloy soft brazing technology is now still the main connection technology for electronic circuits.
A good solder joint should be in the life cycle of the equipment, its mechanical and electrical properties are not failure. Its appearance is shown as:
(1) A complete and smooth shiny surface.
(2) The proper amount of solder and solder to completely cover the pads and leads of the soldered parts, the component height is moderate.
(3) good wettability; the edge of the soldering point should be thin, solder and pad surface wetting angle of 300 or less is good, the maximum does not exceed 600.
SMT processing appearance inspection content:
(1) whether the components are missing.
(2) Whether the components are wrongly affixed.
(3) There is no short circuit.
(4) whether the virtual welding; virtual welding is relatively complex reasons.
I. the judgement of false welding
1. The use of online tester special equipment for inspection.
2. Visual or AOI inspection. When the solder joints found to be too little solder solder wetting bad, or solder joints in the middle of the broken seam, or solder surface was convex ball, or solder and SMD do not kiss fusion, etc., we must pay attention to, even if the phenomenon of a slight hidden danger, should immediately determine whether there is a batch of soldering problems. Judgement is: see if more PCB on the same location of the solder joints have problems, such as just individual PCB problems, may be solder paste is scratched, pin deformation and other reasons, such as in many PCB on the same location have problems, at this time it is likely to be a bad component or a problem caused by the pad.
II. The causes and solutions to the virtual welding
1. Defective pad design. The existence of through-hole pad is a major flaw in the PCB design, do not have to, do not use, through-hole will make the loss of solder caused by insufficient solder; pad spacing, area also needs to be a standard match, or should be corrected as soon as possible to design.
2. PCB board has oxidation phenomenon, that is, the pad is not bright. If the phenomenon of oxidation, the rubber can be used to wipe off the oxide layer, so that its bright reappearance. pcb board moisture, such as suspected can be placed in the drying oven drying. pcb board has oil stains, sweat stains and other pollution, this time to use anhydrous ethanol to clean up.
3. Printed solder paste PCB, solder paste is scraped, rubbing, so that the amount of solder paste on the relevant pads to reduce the amount of solder, so that the solder is insufficient. Should be made up in a timely manner. Supplementary methods available dispenser or pick a little with a bamboo stick to make up for the full.
4. SMD (surface-mounted components) of poor quality, expiry, oxidation, deformation, resulting in false soldering. This is the more common reason.
Oxidised components are not bright. The melting point of the oxide increases.
At this time with more than three hundred degrees of degrees of electric chromium iron plus rosin-type flux can be welded, but with more than two hundred degrees of SMT reflow soldering plus the use of less corrosive no-clean solder paste will be difficult to melt. Therefore, oxidised SMD should not be soldered with reflow furnace. Buy components must see if there is oxidation, and buy back in time to use. Similarly, oxidised solder paste can not be used.
Post time: Aug-03-2023