PCBA Tech - SMT patch precautions and CAF cause PCBA failure analysis

SMT patch processing precautions

SMT patch processing precautions, SMT production equipment is a high-precision mechatronics equipment, equipment and process materials have certain requirements for environmental cleanliness, humidity, and temperature. So what are the precautions for SMT patch processing?

1. Solder paste refrigerated

The solder paste has just been bought, and if it is not used immediately, it needs to be placed in the refrigerator for refrigeration. The temperature is best to be 5 degree Celsius-10 degree Celsius, not lower than 0 degree Celsius. There are many explanations on the mixing and use of solder paste on the Internet, so I won’t introduce them here.

2. Replace the wearable parts of the placement machine in time

In the placement process, due to the aging of the placement machine equipment and the damage of the suction nozzle and feeder, it is easy to cause the placement of the placement machine to be crooked, and cause high throwing, reducing production efficiency and increasing production costs. In the case that the placement machine equipment is not available, it is necessary to carefully check whether the suction nozzle is blocked or damaged, and whether the feeder is intact.

3. Measuring furnace temperature

The quality of PCB board soldering has a great relationship with the reasonable setting of process parameters of reflow soldering. Generally speaking, the furnace temperature test needs to be carried out twice a day, at least once a day, in order to continuously improve the temperature curve and set the temperature curve that best fits the welding product. Don't miss this link for the sake of production efficiency and cost savings.

CAF (conductive anode wire) causes PCBA failure analysis

As electronic products become lighter, thinner, and smaller, as well as the development of components and packaging technologies, PCB circuits have become denser, and the hole spacing and line spacing have become narrower. This has improved the electrical insulation of copper clad laminates. High demands.

The shortening of the distance between the conductors increases the voltage per unit distance between the conductors. Under the influence of the direct current electric field, the metal ions in the conductors (such as metallized holes or lines) produce an electrochemical reaction, and the metal is dissolved into ions. The insulating layer (or surface) between the conductors has the phenomenon of precipitation. In a high temperature and humid environment and voltage applied, copper metal ions from the wire circuit or metallized holes are precipitated from the electrode and migrate. The galvanic corrosion phenomenon that occurs by the migration of copper ions on the glass fiber is called CAF (Copdutive Anodic Filament). The occurrence of CAF will reduce the insulation resistance between conductors and even cause a short circuit, which seriously affects the reliability of PCBA.

1 Case analysis of LCD TV PCBA failure

1.1 Brief description of the problem

According to after-sales maintenance engineers, when repairing special LCD TVs, it is found that the cause of the failure is mostly caused by poor insulation between the vias and the grounding vias next to them. The proportion is relatively high and the reasons have common characteristics. Analyze and study this problem, find out the cause of the problem and formulate solutions.

1.2 Analysis process

1.2.1 Physical appearance of failed PCBA and bare board PCB

Description: The product board-FR-4; board thickness-1.6 mm; number of layers-double-sided board; single board size-200 mm*190 mm.