Understand the measures for short circuit in SMT processing

Understand the measures for short circuit in SMT processing
In the SMT patch processing, there will be short circuits, mainly between pins of small pitch integrated circuits, so it is also called "bridging" The occurrence of short circuit will directly affect the efficiency of the product, leading to the production of defective products, and the short circuit phenomenon of SMT patches The following Lingxinte technicians will introduce the cause and solution of the fault SMT patch
1、 Template
The bridging phenomenon in SMT chip processing is mainly due to the small IC pin spacing, which usually occurs when the pin spacing is 0.5mm or less. In this case, if the template is improperly designed or slightly omitted to be printed, it is easy to cause a short circuit. Phenomenon

Solution: For integrated circuits with spacing of 0 5mm and below, because their pitch is very small, it is easy to bridge Keep the length of mold opening mode unchanged, and the opening width is 0.5~0.75 pad width The thickness is 0.12~0.15 mm It is better to use laser cutting and polishing to ensure that the opening shape is inverted trapezoid and the inner wall is smooth, so that the solder paste can be effectively released and well formed during the printing process, and the number of filter screen cleaning can be reduced
B. Printed by PCBA
In SMT chip processing, printing is also a very important link. In order to avoid short circuit caused by improper printing, the following problems should be noted:
1. Wiper type: There are two kinds of wipers: plastic wiper and steel wiper. For integrated circuits with a spacing of 0.5mm, steel scraper shall be used for printing to facilitate the formation of solder paste after printing.
2. Adjustment of the wiper plate: the working angle of the wiper plate is printed in the 45 ° direction, which can significantly improve the imbalance of the opening directions of different solder paste templates, and can also reduce the damage to the opening of fine spacing templates; The pressure of the scraper is usually 30N/mm2.
3. Printing speed: The solder paste will roll forward on the template under the push of the squeegee.  Fast printing speed is conducive to the rebound of the template, but at the same time, it will block the solder paste from being printed If the speed is too slow, the solder paste will not roll on the template, resulting in low resolution of solder paste printed on the pad The printing speed range of asphalt is 10-20mm/s
3. PCBA solder paste
The correct selection of PCBA solder paste is also very important to solve the bridging problem. When using integrated circuit solder paste with a spacing of 0.5mm or less, the fineness should be 20~45um, and the viscosity should be about 800~1200pa. s. The activity of solder paste can be determined according to the cleanliness of PCB surface, and RMA grade is usually used.
4、 PCBA installation height
For integrated circuits with a spacing of – 0 5mm, 0 distance or 0-0. The installation height of 1mm shall be used during installation to avoid the collapse of solder paste forming due to too low installation height and short circuit during reflow
5、 PCBA reflow
During the backflow of PCBA processed by SMT chip, the following conditions may also cause short circuit, such as:
1. Heating speed is too fast; 2. The heating temperature is too high; 3. Solder paste heating speed is faster than circuit board; 4. The wetting speed of flux is too fast.
Ling Xinte will introduce the causes and solutions of PCB short circuit in SMT chip processing today. In SMT chip processing, there are many PCB processing programs, and operators need to carefully handle each link to reduce the occurrence of undesirable phenomena.