PCBA Tech - Soldering method for PCBA tin penetration and LGA assembly

1. How much influence does the soldering method have on PCBA tin penetration?

What are the conditions in the PCBA patch that will affect the tin penetration rate? Why is tin penetration rate so important? In industries that are closely related to life, such as medical care and automobiles, many products have stringent quality requirements. The tin penetration rate, as a criterion for welding quality, also has its own importance. Among them, in addition to the problems of flux and solder wire/solder paste, the most influential one is the soldering method.

1. Manual welding is mainly an original method for processing some special-shaped parts and devices that need to be manually welded in the process sequence. This method is likely to cause plug-in welding when the soldering iron temperature setting is not appropriate and the welding time is unreasonably adjusted. The tin penetration rate of the spot is insufficient, and it leads to false soldering of the product. Therefore, this method can be used as plug-in welding under normal circumstances. If the product has very high requirements on the tin penetration rate, it is necessary to carefully consider whether this method is feasible.

2. Wave soldering is used as the main soldering equipment of DIP plug-in in PCBA processing. The influence on soldering quality in soldering is direct, such as the height setting of the wave crest, the setting of the temperature curve, the moving speed of the product and the length of the soldering time. And so on, so the process control of wave soldering directly determines the quality of tin penetration.

3. Selective wave soldering, as an "upgraded version" of wave soldering equipment, can apply solder paste to a single point, which greatly eliminates the influence of wave height and soldering time, and can greatly ensure the PCBA tin penetration rate. Therefore, in the processing of medical electronic PCBA, in order to ensure the tin penetration rate of the plug-in material, Jingbang Electronics will use selective wave soldering on the whole board to ensure the quality stability of the customer during use. This method is the most helpful welding method that has been verified for the solder penetration rate.

Second, the assembly process analysis of LGA in PCBA processing

In addition to the conventional components in PCB patch processing, we often encounter some special components, especially the iterative update of new products in recent years, and there are also many devices that have been updated, such as LGA.

1. Background

LGA, that is, no solder ball array package, similar to BGA, but without solder balls.

2. Process characteristics

For bottom surface-end package, the distance between the bottom of the package and the PCB surface (Stand-off) is very small after soldering, generally only 15-25um, and the flux residue will often bridge. At the same time, the solvent in the solder flux processed by PCBA is generally not easy to volatilize, forming a viscous shape instead of a general solid state. Since most of the solvents in the flux use alcoholic organic compounds, which have hydrophilic characteristics, if there is a bias voltage between adjacent pads, it may leak.

3. Assembly process

Generally, the center-to-center distance of the LGA pads is relatively large, and the 1.27mm design is mostly adopted. The key to the Smt soldering process is to ensure that the flux activator is completely volatilized and decomposed. Therefore, a longer preheating time, a higher soldering peak temperature, and a longer soldering time should be used.

Long heating time: designed to completely volatilize the solvent in the flux.

High welding peak temperature and long welding time: aiming to make the activator decompose completely or volatilize completely.

4. Design

It is recommended to use solder mask to define the pad design, which helps to ensure the separation of the solder around the LGA pad.