Today we are going to describe the types of defects that often occur in board mounting and the causes of their occurrence. Here, we will introduce the defects that are likely to occur in the “soldering appearance inspection” that checks the joints after reflow.
Not implemented (implementation confirmation)
Not mounted (mounting confirmation) is the basis of visual inspection of board mounting. Inspect that the correct electronic components are mounted in the correct positions and that there are no mounting omissions. Possible causes include forgetting to mount in the mounting process, unbonded due to transfer leakage of solder paste, inadequate supply of parts, and dropping after the mounting process.
Disconnection / short
It is in a state where it is not reproduced as designed due to disconnection due to defective solder and short circuit due to unnecessary solder. There are multiple other causes of disconnection and short circuit. So it is the most effective measure to perform a continuity inspection after manufacturing. Also, please note that there are cases where it is barely conducting during manufacturing.
The phenomenon in which glass fibers peel off from the resin is called “crazing.” In addition, the phenomenon that glass fibers peel off mainly is called “measling”.
It is in a state of being peeled from the resin of the glass fiber due to thermal shock, moisture of the substrate, imperfections in the laminating process, etc. Also called delamination, the substrate in this state cannot be used.
Void blowhole pinhole
Blow holes are holes that can be made into solder fillets. With the gas generated in the solder floating up to the surface, voids may be generated inside the through holes and may not be visible from the outside. A pinhole is a state in which a cavity is created under the solder and a small hole penetrates the surface. Both are more likely to occur if there is moisture, dirt, gas, etc. on the printed circuit board due to the release of gas.
Soldering uses the capillarity and wetting phenomena to join. “Wetness” is the familiarity of solder, and this property is expressed as “wetness”. Depending on the nature of the solder used, poor wetting may occur due to greasy stains at the soldering location, insufficient soldering temperature, insufficient flux amount, etc.
Solder balls are the ones in which the solder on the tip of the iron is hardened. As the name suggests, it becomes ball-shaped and usually peels off from the board, so it does not become a defect. However, be aware that if it gets caught in the gap between leads, it may cause a short circuit. The cause may be that the iron is pulled out too fast.
Bridge (bridge solder), icicles (tsuno)
The state in which the solder is connected like a bridge between pins such as ICs is called “bridge (bridge solder)”, and the state in which the solder pops out in a square shape without being completely connected like a bridge is called “icicle (icicle)”. I call it. Possible causes include low soldering temperature, short time, insufficient wetting, and flux problems. Both will cause a short circuit.
Standing parts / standing chips
There is a defect in the soldering on one side, and the parts come off and stand up. Possible causes include printing misalignment, mounting misalignment, pad design problems, and excessive soldering. To prevent parts and chips from standing, it is possible to reduce the land size, preheat, and reduce the amount of solder paste inspection applied.
Potato solder (cold joint)
“Imo solder” is a defect that occurs when the temperature of the iron is too low or the time to apply it is too short. Potato solder has a rounded fillet due to poor wetting. In addition, potato solder can cause voids and lead to poor continuity.
A defect that occurs when the amount of solder to be sent is small is “insufficient solder”. It also occurs when lands and leads are dirty.
Conventional inspection method
The appearance inspection of board mounting and soldering is one of the important inspections to guarantee the joining reliability. The inspection method is generally visual inspection. There is also a dedicated inspection device, but there are cases where it cannot be introduced because it is expensive. In recent years, due to the miniaturization and integration of electronic components. There are an increasing number of cases where it is difficult to distinguish by visual inspection and it is necessary to carry out microscopic inspection.