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PCB pads have certain requirements. A better pad design can lay a firmer and more efficient foundation for subsequent PCBA circuit board manufacturing.
PCB patch processing
1. For SMT components on the wave soldering surface, the pads of larger components (such as transistors, sockets, etc.) should be appropriately increased. For example, the pad of SOT23 can be lengthened by 0.8-1mm to avoid the "shadow effect" of the component causing empty soldering.
2. The size of the pad should be determined according to the size of the components. The width of the pad is equal to or slightly larger than the width of the welding rod of the component, and the welding effect is the best.
3. Generally, between two interconnected components, we will avoid using a single large pad. This is because the solder on the large pad will connect the two components to the middle. The correct method is usually to separate the pads of the two components and connect the two pads with a thinner PCB wire. If the wires are required to pass a larger current, several wires can be connected in parallel, and the wires are covered with green oil.
4. There should be no through holes on or near the pads of SMT components. Otherwise, during the REFLOW process, the solder on the pads will flow away along the through holes after melting, resulting in virtual soldering, less tin, or flow Causes a short circuit to the other side of the board.
SMT patch processing and assembly
Printed circuit boards must adapt to the current rapid development of SMT chip assembly technology. The use on PCBA circuit boards is already the mainstream product of SMT chip manufacturers. Nearly all circuit boards will be processed by SMT chips, which is enough to see its importance on the circuit board.
SMT patch processing.
1. High density: As the number of pins processed by SMT patch can reach hundreds or even thousands, and the pin center distance can reach 0.3mm, the high-speed BGA on the circuit board needs fine lines and fine pitch. The line width is reduced from 0.2~0.3mm to 0.1mm or even 0.05mm, and the double wires between 2.54mm grids have developed to 4, 5 or even 6 wires. The fine lines and fine pitch greatly increase the assembly density of SMT. In the case of high precision of the corresponding SMT chip processing equipment, the corresponding chip processing plant can complete it.
2. Small aperture: Most of the metallized holes in SMT are not used to insert component pins, and no soldering is performed in the metallized holes. The metallized holes are only used as electrical interconnections between layers, so It is necessary to reduce the aperture as much as possible to provide more space for the SMT patch. The aperture has changed from 0.5mm in the past to 0.2mm, 0.1mm or even 0.05mm.
3. Low thermal expansion coefficient: any material will expand after heating. Polymer materials are generally higher than inorganic materials. When the expansion stress exceeds the bearing limit of the material, the material will be damaged. Due to the many and short SMT pins, the CTE between the device body and the SMT is inconsistent, and device damage caused by thermal stress occurs from time to time. Therefore, the CTE of the SMT circuit board substrate should be as low as possible to adapt to the matching with the device.
4. Good high temperature resistance: Most of today's SMT circuit boards need to install PCB components on both sides. Therefore, the circuit board processed by SMT chip needs to be able to withstand two reflow soldering temperatures. At present, lead-free soldering is widely used, and the soldering temperature is relatively high. After PCB soldering, the SMT chip circuit board is required to have small deformation and no blistering, the pad still has good solderability, and the surface of the SMT chip circuit board still has a high flatness.
