PCB Tech - What are the factors that cause bad tin on the tin surface of fast pcb circuit boards?

1. Tin surface quality defects of PCB circuit board factory

a).The operation is not in accordance with the operating specifications at the time of shipment:

The circuit industry has extremely strict requirements on the workshop environment and the standard operation of employees. In particular, the chemical reaction environment is required in the production of circuit boards. Therefore, no impurities are allowed to penetrate. After the board spraying process is completed, the subsequent one The series requires employees to wear anti-static gloves to operate, because finger sweat or stains directly contact the surface, which will cause surface oxidation. If it causes defects, it is extremely difficult to find, and it is irregular, and it is difficult to find out in tests and tinning experiments.

b).The tin furnace used for spraying tin is not cleaned up on time:

The on-time maintenance of the tin furnace is very important, because the tin spraying is a vertical cycle process. The circuit board surface will be under strong pressure. For those boards whose solder mask is not fully dried and the characters are not strong, impact will occur, causing them to fall off and deposit in the furnace., After high temperature evaporation, if it is not cleaned for too much time, it will cause surface adhesion.

c).The source of tin for incoming materials:

For material procurement, some circuit board factories blindly seek to reduce costs. When using tin-sprayed raw tin, the procurement industry recycles tin or sources of unstable content. Generally, circuit board factories with extremely low unit prices may have such a risk., It is recommended that you choose suppliers carefully.

d).Storage environment and transportation:

This is the link between the circuit board factory and the placement factory. Generally, there is little inventory of circuit boards, but the general inventory requires the storage environment to be dry and humid, and the packaging is complete. During transportation, it is required to be handled as lightly as possible, and no vacuum is allowed. The packaging is damaged and stored for a long time. The theoretical storage time of the tin-sprayed board is one month, but the best solderability time is within 48 hours. If the storage time exceeds one month, it is recommended to return to the circuit board factory to clean and bake the board with a special solution. Baking parameters 150°, 1 hour

2.Quality problems during soldering in the SMT factory

a).The solderability of circuit board holes and solder joints affects soldering quality

The solderability of the circuit board holes and the racket is not good, which will cause false soldering and false soldering, which will affect the unstable operation of the components in the circuit, resulting in poor conduction between the surface components of the multilayer board and the inner wire, causing the machine to fail or sometimes to run well. Time is bad. The so-called solderability is the property that the metal surface is wetted by molten solder, that is, a relatively uniform continuous smooth adhesion film is formed on the metal surface where the solder is located.

The main factors affecting the solderability of printed circuit boards are: (1) The composition of the solder and the nature of the solder. Solder is an important part of the welding chemical treatment process. It is composed of chemical materials containing flux. Commonly used low melting point eutectic metals are Sn-Pb or Sn-Pb-Ag. The impurity content must be controlled by a certain proportion, In order to prevent the oxides generated by impurities from being dissolved by the flux. The function of the flux is to help the solder wetting the surface of the circuit to be soldered by transferring heat and removing rust. White rosin and isopropanol solvents are generally used. (2) The welding temperature and the cleanliness of the metal plate surface will also affect the weldability. If the temperature is too high, the solder diffusion speed will increase. At this time, it will have a high activity, which will cause the circuit board and the molten surface of the solder to oxidize rapidly, resulting in soldering defects. Contamination on the surface of the circuit board will also affect the solderability and cause defects. These defects Including tin beads, tin balls, open circuits, poor gloss, etc.

b). Welding defects caused by warpage

Circuit boards and components warp during the welding process, and defects such as virtual welding and short circuit due to stress deformation. Warpage is often caused by the temperature imbalance of the upper and lower parts of the circuit board. The large PCB will also warp due to the drop of the board's own weight. The ordinary PBGA device is about 0.5mm away from the printed circuit board. If the device on the circuit board is large, the solder joint will be under stress for a long time as the circuit board cools down and the solder joint will be under stress. If the device is raised by 0.1mm, it is enough to cause For special products, the yin and yang jointing of the circuit board factory can be required to reduce the warpage, or to adopt the appropriate size of the imposition as far as possible, and it should not be too large or too small.

3. The design of the circuit board affects the welding quality

In the layout, when the circuit board size is too large, although the soldering is easier to control, the printed lines are long, the impedance increases, the anti-noise ability is reduced, and the cost increases; Mutual interference, such as electromagnetic interference of circuit boards. Therefore, the fast PCB design must be optimized:

a) Shorten the wiring between high-frequency components and reduce EMI interference.

b) Components with heavy weight (such as more than 20g) should be fixed with brackets and then welded.

c) Heat dissipation issues should be considered for heating components to prevent defects and rework caused by large ΔT on the surface of the components, and the thermal components should be far away from the heating source.

d) The arrangement of the components is as parallel as possible, which is not only beautiful but also easy to weld, and is suitable for mass production. The circuit board is best designed as a 4:3 rectangle. Do not change the wire width to avoid wiring discontinuities. When the circuit board is heated for a long time, the copper foil is easy to expand and fall off. Therefore, avoid using large-area copper foil.