PCBA Tech - SMT inspection and rework and cleaning requirements

PCBA inspection and rework

1. Assembly inspection tools:

5 to 10 times magnifying glass with halo light

Printing process standard, eliminate confusion

A 10 to 30X microscope may be needed to inspect difficult parts such as fine pitch components and solder paste quality.

For high-volume manufacturing automated visual inspections, such as 3D laser scanning or X-ray scanning may be reasonable.

2. Assembly inspection technology:

When checking for trapped contaminants or J-leads, you need to tilt the circuit board to check under the components. In addition, if rework and trimming are to be performed at the same time as the inspection, a circuit board positioning table may be useful.

When in doubt, toothpicks or pointed wooden sticks can be used to verify the solder joints of leaded components. Use a pick or stick to gently push the top edge of the lead to check if the connector is attached. This prevents damage to the circuit board and leads. An unsoldered lead or lead with a cold joint will move when pushed, which requires modification.

Use common vocabulary to write down or record all defect types. This information is useful for finding common defects and trends so that their source can be found and eliminated.

Good solder joints:

Smooth and shiny. There are no voids and no pores.

Good solder joint wettability

Concave rounded corner shape without excessive solder deposits.

3. Polish, rework and repair:

Requires rework or repair of components/circuit boards that do not meet process or performance standards.

Commonly used rework tools are soldering irons, desoldering stations that use suction, hot air jets, solder cores, etc.

Rework of SO and other high-lead quantity packages requires prior training, and some exercises on dummies will be helpful.

Since the leads and terminals of the SMT package are small, the thermal requirements are lower compared to through-hole component pads. Only after ensuring that all leads have been desoldered or the solder has been reflowed, can the component be removed. If the operation is improper, the pad area is likely to be damaged.

Before removing the component, gently push it to check if the solder is completely melted.

When the connection solder melts on each lead or terminal of the component, it can easily be removed and replaced with a new one.

Operator training is required because it requires new technology and new tools.

4. Tools required:

Alcohol in the dispensing bottle

Cotton swab

'R','RMA' flux in a small dispensing bottle

Toothpicks or pointed sticks

Slender tweezers

According to the required size of solder core

Provide fine tip temperature control soldering iron and spare parts as required

Desoldering station with proper position

Manual hot air nozzles and nozzles and hot air rework station

Solder paste and dispenser

No. 24 (0.015”) flux cored solder wire

Antistatic workstation with wrist strap and properly grounded

SMT cleaning and cleaning options requirements

1. Cleaning requirements

The printed board needs to be cleaned to remove flux residues and other contaminants left after soldering. Cleaning or cleaning the circuit board can prevent potential electrical failures due to electromigration. Cleaning operations can remove the following contaminants:

i) Ionic contaminants

ii) Non-ionic pollutants

iii) Particulate pollutants

Water-soluble fluxes usually produce ionic (also called polar) contaminants that need to be cleaned with water. The non-ionic (also called non-polar) contaminants produced by rosin flux require non-ionic solvents, such as trichloroethane.

2. Cleaning options

Note: Rosin and Rosin Mildly Activated (RMA) flux does not need to be cleaned. However, for high reliability applications and aesthetic reasons, these boards may need to be cleaned.

Due to the presence of corrosive elements in the flux residue, water-soluble flux needs to be thoroughly cleaned, and water-based cleaning is an ideal choice. Since rosin is insoluble in water, when using water-based cleaners for rosin flux, an alkaline chemical substance called a saponifier is added to the water. If ultrasonic vibration is used to assist cleaning, the efficiency of the cleaning treatment will be significantly improved. However, ultrasonic vibration is not limited to the cleaning fluid and the surface to be cleaned, but is also transmitted to electronic components that may be damaged. If the bonding wires are free and are not tightly encapsulated in plastic or any other packaging material, the bonding wires inside the active components between the chip and the bonding pads may break. High power and high frequency vibration can also cause the external lead to break. The following parameter sets are generally accepted:

Maximum frequency 40 kHz

Ultrasonic maximum loading time 1 to 5 minutes

Maximum power 10 W/L

The boards are placed on the shelf so that they cannot touch each other.

In all cases, the time interval between SMT soldering and cleaning should be shortened to the shortest (less than one hour) to obtain a good cleaning effect.