Image processing: Board inspection

Electronic boards are used in the electrical appliances and automobiles around us. The size of the electronic board is small enough to fit in one hand. For example, the functions of electronic boards used in smartphones go beyond the boards used in large computers in the past. You can see how small electronic components are packed with electronic components.

However, in the past and present, if one of the parts mounted on the board fails, the function of the machine will still stop. The big difference is that, for example, it was easy to visually identify a solder short circuit in a large electronic board component, but it is not easy to distinguish it with a small integrated board.

What is the purpose of board inspection?

Substrate manufacturing

SMT is the main method for manufacturing electronic boards, so an image of this flow is shown in Fig. 2. SMT is an abbreviation for surface mount technology, and is a method of joining electrodes of electronic components to pads on the surface of a printed circuit board.

① Printing works

This is the process of applying solder to the pad part of the printed circuit board.
Place the metal mask on the printed circuit board, apply solder paste (liquid solder) on the metal mask, and apply the solder on the pad.

② Parts mounting process

It is a process of arranging parts on the pad with an automatic machine called a mounter.

③ Reflow process

Put the board in a reflow oven and heat it to melt the solder and fix the parts on the pad.

In addition to the SMT process, electronic boards are manufactured following the SMT process, including the process of inserting electrodes into through holes and soldering, and the process of manually mounting parts by hand or using special equipment.

Types of board defects that occur in surface mounting

Site of occurrence	Defect name	feature	others
Parts	Uninstalled	Genuine parts are not mounted in the correct position on the board	Attention should be paid to visual inspection due to the malfunction of the board.
Parts	Parts misalignment	The component will disengage from the pad and will be mounted out of position. In addition to vertical and diagonal deviations, it is in a state of protruding from the land.
Solder	Disconnection	It is in a state without solder
Solder	Short	It is a state where the solder is excessive and adheres to the next pad.
Solder	Void	It is a fillet hole created by floating the gas generated inside the solder. Voids at the joint have an effect on the strength of the joint	We need a way to eliminate voids at the manufacturing stage
Solder	Blow hole	It is a hole where the generated gas comes out from the inside of the fillet.	It may be a large cavity inside and needs to be confirmed
Solder	Pinhole	It ’s a small hole at the base.	Those that penetrate the board are not acceptable
Solder	Poor wetting	The familiarity of the solder is wet, and if it is not wet, smooth soldering will not be possible.
Parts, solder	Parts float	Also known as the Manhattan phenomenon, parts are soldered on only one side and the other side stands up.	There is a possibility of poor heating of the pad
Solder	Solder ball	It is in a state where the solder is scattered and solidified. There is a risk of short circuit if there is a large ball between the leads	Usually a fine mass that will come off naturally
Solder	Bridge	Solder is connected between the pins of the IC.
Solder	Icicles	With icicle-like protrusions on the soldering surface, if it is large, it will be a bridge.
Solder	Not soldered	There is no soldering, or the pad is not familiar to the land at all.	Be careful not to overlook it by visual inspection
Solder	Crack	There are cracks on the surface of the solder.	Over time, there is a risk of poor continuity.
Solder	Immo Handa	It is called a cold joint. Occurs when the solder and lands are not completely wet, and the fillets are rounded and do not melt evenly.	One of the causes is insufficient heating

What is board inspection?

In SMT manufacturing, the process of finding defects on the board inspection is an inspection process centered on visual inspection after reflow, inspecting whether the reflowed board is good or bad, soldering is good, and the arrangement of parts is correct, and NG. The item will be repaired or discarded.

Depending on the manufacturing plant, inspections are also performed in the printing process and mounting process.
There are two major reasons.

• Early detection of defects to prevent delays in post-processes
• When a defect in the board is discovered in a later process, it is possible to immediately know where the defect occurred.

◆ Printing engineering inspection

Inspect whether the position and amount of solder paste printed on the printed circuit board matches the opening of the metal mask and the pad of the printed circuit board.

◆ Mounting process inspection

Inspect for misalignment of parts, incorrect combination, missing parts, forward / reverse polarity, etc.

◆ Reflow process inspection

In the visual inspection of the joint, we mainly inspect the welding condition of the solder. The test is done by visual inspection or by using AOI. AOI is classified into three types: a method of imaging with a camera, a method of using the reflected light of a laser, and a method of using the transmitted dose of X-rays, and there is also an inspection device that combines these three methods. The point is how to process the captured images.

What is a board inspection device?

(1) About board inspection equipment

An inspection device is an alternative to the manual inspection of boards. The electronic board inspection device is called AOI, and the device that inspects the quality of solder is called SPI. All the board inspection equipment needs to detect all the board defects introduced in Table 1. One of the important things about solder paste inspection, which accounts for most of the board defects in Table 1, is setting and adjusting thresholds.