PCB manufacturing solder joint appearance quality inspection


With the advancement of technology, the electronic components are getting smaller  in pcb manufacturing . The previous 0402 resistors are mostly replaced by 0201 size . How to ensure the quality of solder joints becomes an important issue for high-precision mounting. Solder joints serve as a bridge for welding, and its quality and reliability determine the quality of electronic products. That is to say, in the production process, the quality of SMT is ultimately expressed as the quality of the solder joint.

PCB manufacturing solder joint appearance quality inspection

PCB manufacturing solder joint appearance quality inspection

At present, in the electronics industry, although the research on lead-free solder has made great progress, it has been promoted and applied worldwide, and environmental protection issues have also received widespread attention. PCB manufacturing processing uses Sn-Pb solder alloy technology is still the main connection technology for pcb manufacturing electronic circuits.


A good solder joint should be within the life of the equipment, and its mechanical and electrical properties will not fail. Its appearance is as below:


(1) a complete, smooth, shiny surface;

(2) Appropriate amount of solder and solder completely cover the solder area of the pads and leads, and the component height is moderate;

(3) Good wettability; the edge of the solder joint should be thin, and the wetting angle of the solder and the pad surface should be 300 or less, and the maximum should not exceed 600.


SMT processing visual inspection content:

(1) Whether the components are missing;

(2) Whether the components are mislabeled;

(3) Whether there is a short circuit;

(4) Whether there is virtual welding; the cause of virtual welding is relatively complicated.


First, the judgment of the virtual welding

  1. Usingthe special equipment of online tester for inspection.
  2. Visual or AOI test. When it is found that theinsufficient solder joint solder and poorsolder infiltration, or there is a broken joint in the middle of the solder joint, or the solder surface is convex or spherical, or the solder does not blend with the SMD, it is necessary to pay attention, even a slight phenomenon may cause hidden dangers. It should be immediately judged whether there is a problem of batch soldering. The method of judging is to see if there are more solder joints in the same position on the PCB. For example, it is only a problem on the individual PCB manufacturing, which may be caused by scraping of the solder paste, deformation of the leads, etc., such as there are problems in same position on many PCBs. It is likely to be caused by poor components or problems with the pads.


Third, the cause and solution of the virtual welding

  1. The pad design is defective. The presence of vias in the pads is a major defect in PCB design. It should not be usedif not so necessary. The vias will cause solder loss and solder shortage. The pad pitch and area also need standard matching. Otherwise, the design should be corrected as soon as possible.
  2. The PCB board has an oxidation phenomenon, that is, the pad is not bright. If there is oxidation, use an eraser to remove the oxide layer to make it bright. The PCB manufacturing board is damp and can be dried in a dry box if suspected. The PCBmanufacturingboard is contaminated with oil stains and sweat stains. At this time, it should be cleaned with absolute ethanol.


  1. The PCB on which the solder paste is printed, the solder paste is scraped and rubbed, so that the amount of solder paste on the relevant pads is reduced, so that the solder is insufficient. It should be replenished in time. The method of supplementing can be done with a dispenser or with a bamboo stick.
  2. SMD (surface mount components) is of poor quality, expired, oxidized, deformed, resulting in virtual solderingduring pcb manufacturing. This is the reasonwhich it’s more common.

(1) The oxidized component is not bright. The melting point of the oxide is increased. At this time, more than 300 degrees of electric ferrochrome and rosin-type flux can be used for welding, butit’s difficult to melt solder paste by more than 200 degrees of SMT reflow is used together with the use of less corrosive no-cleaning.  Therefore, the oxidized SMD should not be welded by the reflow oven. When buying components, be sure to see if there is oxidation, and use it in time when you buy it back. Similarly, oxidized solder paste cannot be used.


(2) The surface mount components of multiple legs have small legs and are easily deformed under the action of external force. Once deformed, the phenomenon of virtual welding or lack of welding will occur. Therefore, it is necessary to carefully check and repair in time before mounting or after welding.

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