Causes and solutions of virtual welding in SMT production

I. Judgment of virtual welding:

1, the use of online tester special equipment (commonly known as needle bed) for inspection.

2. Visual inspection (including magnifying glass and microscope). When the visual discovery of solder joint solder is too little solder infiltration is not good, or the middle of the solder joint is broken, or the solder surface is convex ball, or solder and SMD do not melt, it is necessary to pay attention to, even if the slight phenomenon will form a hidden danger, should immediately determine whether there is a batch of virtual welding problem. The method of discrimination is to see if there are many solder joints in the same direction on PCB. If the problem is only on a single PCB, it may be caused by scratching of solder paste and deformation of pins. If there are problems in the same direction on many PCB, it is likely to be caused by poor components or faulty pads.

Two, virtual welding causes and solutions

1. There are defects in pad planning. There should be no through hole on the pad, through hole will cause the loss of solder solder lack; Pad distance and area also need standard matching, otherwise the planning should be corrected as soon as possible.

2. PCB board has oxidation phenomenon, that is, the welding disk is dark and not bright. If there is oxidation, use eraser to remove the oxide layer, so that its bright light. PCB board damp, if suspected, can be placed in the drying box. PCB board with oil stains, sweat stains and other pollution, at the moment to clean with anhydrous ethanol.

3. For PCB printed with solder paste, the solder paste is scraped and rubbed, which reduces the solder paste quantity on the relevant pad and makes solder shortage. Should make up in time. The method of filling can be made up by dispensing machine or picking a small amount with bamboo stick.

4, SMD (surface attached components) quality is not good, expired, oxidation, deformation, the formation of virtual welding. This is a common reason.

(1) The oxidized element is not bright. The melting point of oxide increases, at the moment with more than three hundred degrees of electric ferrochrome and loose flux can be welded, but with more than two hundred degrees of SMT reflow welding and the use of less corrosive free cleaning solder paste is difficult to melt. Therefore, the oxidized SMD should not be welded by reflow welding furnace. When buying components, we must see whether there is oxidation, and after buying back to timely use. Likewise, oxidized solder paste cannot be used.

(2) The surface mount components of multiple legs, their legs subtle, easy to deformation under the effect of external force, once deformation, will certainly occur virtual welding or lack of welding phenomenon, so before welding to carefully check and repair in time.

(3) For PCB printed with solder paste, the solder paste is scraped and rubbed, which reduces the solder paste amount on the relevant pad and makes solder shortage. It should be made up in time.