1. Factors affecting the quality of PCB soldering
From PCB design to assemble of all components to a high-quality circuit board, there is a need for strict control of PCB design engineers, even welding processes and welding workers.
There are mainly the following factors:
PCB file, the quality of the board, the quality of the components, the degree of oxidation of components pins, the quality of the solder paste, the printing quality of the solder paste, the accuracy of the SMT machine, the mounting quality of SMT machine, the temperature of the reflow soldering etc.
Because people who do circuit design often do not solder circuit boards, they cannot obtain direct soldering experience, and do not know the various factors that affect soldering;
While the workers from assembly factory don't understand the PCB design. They only know finish the production task, and they have no thoughts and no ability to analyze the reason of poor welding.
2. Suggestion for PCB layout design
There are some suggestion for PCB layout, hope you can avoid all kinds of bad drawings that affect the quality of welding.
A. About tooling hole
There should be four holes (minimum diameter 2.5mm) at the corners of the PCB board for locating the board when printing solder paste. It requires the center of the X-axis or Y-axis to be on the same axis. As shown below:
B. About Fiducial mark: for placement machine positioning
Mark points should be marked on the PCB.
The specific position: in the diagonally opposite corners of the board. It can be a round or square pad, do not mix with other components. If there are components on both sides, then should mark both side.
When designing the PCB, please note the following item:
a. The shape of the Mark point is as follows;
b. The size of A is 2.0mm;
c. There should be no shape or color change that may cause erroneous recognition, within the range of 2.0 mm of the outer edge of the Mark point.
d. The color of the Mark point should be different from the color of the surrounding PCB.
e. To ensure recognition accuracy, the surface of the Mark point is plated with copper or tin to prevent surface reflection.
C. About 5mm Break away tab
When drawing a PCB, please leave a 5mm edge in the long-side direction for SMT machine to transport the board. The SMT machine cannot mount the components in this area.
A board with double side components should be noted that the second over-reflow will scratch the soldered components, and the pad will be removed and the board will be destroyed. Therefore, do not place any SMD components within 5 mm of the long side.
D. Do not put hole to into PAD
The defect is that the solder paste will flow into the via hole during reflow, resulting in lack of tin in components pads. This is the pseudo soldering.
E. About the polarity of diodes and tantalum capacitors
The polarity of the diode and tantalum capacitor should be marked according to the Industry rules, to prevent the worker welding the wrong direction
F. About silk screens
Please hide the part number. Especially for boards with many components. Otherwise, It feel dazzled and hard to find the location of the weld.
The font size of the silkscreen should not be too small to be seen, and stay away from holes to avoid misreading.
Nowadays, more and more engineers can draw, route and design PCBs with software. But the author thinks that it is necessary to pay attention to the above factors.