PCB Tech - Cause Analysis and Prevention of Explosion in PCBA Assembly--Analysis of Causes of Explosion

Cause Analysis and Prevention of Explosion in PCBA Assembly--Analysis of Causes of Explosion

1 What is a burst board
Burst is the common name for delamination or blistering of printed boards.

Delamination is a separation phenomenon that occurs between layers in the substrate, between the substrate and the conductive copper foil, or any other layer in the printed board.

Blistering is a delamination that manifests as local expansion and separation between any layers of the laminated substrate or between the substrate and the conductive copper foil or protective coating layer. Blistering is also a form of stratification.

2 Analysis of the cause of the board burst
Customers' products are used in industrial control inverters, and the design requirements are PCBs with CTI values. This 4-layer PCB has special requirements in the production and application process. Due to the particularity of the CTI>600 copper clad laminate material, it cannot be directly laminated with the inner layer circuit. This type of board must be laminated with two different types of interlayer insulating prepreg materials to meet the CTI and lamination bonding strength at the same time. The process requirements.

Due to the use of two prepreg insulating materials for pressing, the two materials have different resin types. Compared with the single insulating material of the conventional 4-layer board, the bonding force of the fusion interface of the insulating materials in 2 is relatively small. When the printed board absorbs moisture to a certain extent in its natural state, during wave soldering or manual plug-in soldering, the PCB instantly rises from room temperature to above 240 degrees Celsius. The moisture absorbed in the board is instantly heated and expands and vaporizes, creating huge pressure inside. When the pressure is greater than the bonding force of the insulating layer in 2, the plate will burst.

Under normal circumstances, the failure of the board is caused by congenital deficiencies in materials or processes. For the material, it is a copper clad laminate or PCB, and the process includes the production process of the copper clad laminate and the PCBA board, the production process of the PCB, and the PCBA assembly process.

(1) Moisture absorption during PCB manufacturing

The raw materials of PCB have a good affinity for water and are easy to absorb moisture. The following shows that the existence of water in the PCB, the way of water vapor diffusion, and the pressure change of water vapor pressure with temperature, to reveal that the existence of water vapor is the primary cause of PCB explosion.

The moisture in the PCB mainly exists in the resin molecules and the physical structure defects inside the PCB. The water absorption rate and equilibrium water absorption of the epoxy resin are mainly determined by the free volume and the concentration of polar groups. The greater the free volume, the faster the initial water absorption rate, and the polar groups have an affinity for water, which is the main reason why the epoxy resin has a higher moisture absorption capacity. The greater the content of the polar groups, the balanced water absorption The larger the amount. On the one hand, during PCB reflow soldering or wave soldering, as the temperature rises, the water in its volume and the water with the limiting group forming hydrogen bonds can obtain enough energy to diffuse in the resin. Water diffuses outwards and accumulates at defects in the physical mechanism, whose molar volume increases. On the other hand, as the welding temperature increases, the saturated vapor pressure of water also increases.

According to the data, as the temperature rises, the saturated vapor pressure of someone also rises sharply, reaching 400P/kPa at 250 degrees Celsius. When the degree of adhesion between the material layers is lower than the saturated vapor pressure generated by water vapor, the material will burst into delamination or blistering. Therefore, moisture absorption before soldering is the main cause of PCB explosion.

(2) Moisture absorption during PCB storage

PCBs with CTI>600 should be classified as moisture-sensitive devices. The presence of moisture in the PCB has an extremely important impact on its assembly and performance. If a PCB with a high CTI value is stored without moisture or poor moisture resistance, it is agreed to cause moisture absorption. Obviously, under static conditions, with time, the moisture content of PCB will gradually increase. The difference between the water absorption rate of vacuum packaging and the water absorption rate of non-vacuum packaging with the temporary storage time is shown in the figure below.

(3) Long-term moisture absorption during PCBA production

During the production process, due to materials or other factors that affect the PCBA in a long-term exposure during the production process, it may also cause the PCBA with CTI>600 to absorb moisture. If welding is carried out after absorbing moisture, there will also be a risk of plate bursting.

(4) PCBA lead-free process production welding curve is not good

For the PCBA lead-free process, Sn53/Pb87 solder has been replaced by SnAg-Cu lead-free solder, and its melting point has risen from 183 degrees Celsius to over 217 degrees Celsius, and the temperature of reflow soldering and wave soldering has increased from 230 to 235 degrees Celsius to 250 degrees Celsius. ~255 degrees Celsius, the peak temperature may be higher. During the welding process, if the welding time is long, the sharp increase in welding heat will magnify the factors due to poor PCB production, and the possibility of board explosion will increase.