PCB Fabrication

Basic introduction and basic features of surface mount printed boards (SMB)

by:A-TECH      2021-03-18
The printed circuit boards used for the mounting and interconnection of electronic components must adapt to the rapid development of the current surface mount technology (SMT). The printed boards for mounting surface mount components (SMD) are now commonly referred to as surface mount printed boards. Board-making (SMB), which includes simpler single-sided boards, more complex double-sided printed boards, and more difficult and more complex multilayer boards.  In the 1990s, the international PCB technology was reformed and developed with the goal of being able to produce higher and higher density SMBs. SMB has become the mainstream product of current advanced PCB manufacturers, and almost 100% of PCBs are SMB. The surface-mounted printed board has the following main features compared with the printed board where the lead components are inserted. After using surface-mounted components and devices, the metalized holes on the printed circuit board are no longer used for inserting components and device leads, and no soldering is performed in the metalized holes. The metalized holes are only used for electrical interconnection. Minimize the aperture as much as possible, from 0.5-1.0mm in the past to 0.3, 0.2 or 0.1mm. Foreign countries call the aperture of 0.3-0.5mm a small hole (Small Hole), which will be smaller than 0.3mm. The holes in the SMB are mainly micro-holes and small holes. It is estimated that the small holes with a diameter of less than 0.48mm accounted for only 5% in 1984 and increased to 31% in 1991. The number of pins for SMT ICs has been as high as 100 to 500, and MCM components are as high as 1000-2000. The pin center distance has been reduced from 2.54mm to 1.27, 0.635, 0.3175mm, and even 0.15mm. Therefore, SMB requires fine lines and narrow spacing, and the line width is reduced from 0.2-O.3mm to O.15mm, O.10mm, or even 0.05mm. From the past two wires placed between two pads increased to 3-5 wires. It is extremely difficult for such a very thin wire to visually check for gaps, short circuits and open circuits. It is estimated that thin wires with a line width less than 0.13mm accounted for only 4% in 1984, and increased to 28% in 1992. .  Since surface mount components are to be mounted on the SMB, high accuracy of the wire pattern is required, especially the land pattern accuracy requires +/-O.05mm, and the positioning accuracy requires +/-O.05-0.075mm. Due to fine lines and high precision, the requirements for the surface defects of the substrate are strict, especially for the flatness of the substrate. The warpage of SMB is required to be controlled within 0.5%, while the warpage of general non-SMB printed boards is required to be less than l-1.5%. The SMB is required to have good dimensional stability, and the thermal expansion coefficient of the mounted leadless chip carrier and the substrate material should be matched to avoid the lead breakage caused by the stress caused by the different thermal expansion value in the harsh environment. Aramid and quartz fiber with a smaller expansion coefficient should be used. Base, BT resin, PI resin copper clad substrate, copper/invar/copper metal core substrate material can be used in strict environment. SMB requires the metal plating (coating) coating on the pad to be smooth. The printed board with tin-lead alloy electroplated and hot-melted generally forms an arc-shaped surface due to the surface tension of the tin-lead alloy during the hot-melting process, which is not conducive to SMD is accurately positioned and mounted, and the solder-coated printed board is leveled by vertical hot air. Due to the effect of gravity, the lower part of the pad is generally more protruding than the upper part, which is not flat enough, and it is not conducive to mounting SMD, and the vertical hot air leveling The printed board is not evenly heated, and the lower part of the board is longer heated than the upper part, and it is prone to warping. Therefore, SMB should not use hot-melt tin-lead alloy coating and vertical hot-air leveling solder coating, which requires horizontal hot-air trimming. Flat technology or other plating (coating) technology. Water-soluble heat-resistant pre-solder has been used to replace the hot air leveling process.  Accuracy requirements, so most of the solder mask patterns on the SMB use liquid photosensitive solder resist. Since the SMD can be installed on both sides of the SMB, the SMB also requires the solder mask patterns and marking symbols to be printed on both sides of the board. Mounting machines are used when installing SMD on SMB. Most of them use large imposition for surface mounting. After infrared welding or vapor phase welding, the SMB is separated one by one. Therefore, SMB is required to be supplied in the form of large imposition, and V is used when producing SMB imposition. The method of groove milling or milling groove to leave nodes.   line. Buried vias are plated holes that connect two or more inner layers in a multilayer board. Blind vias are used to connect the outer layer of the multilayer board with one or more inner layer-oriented plating holes. The use of buried vias and blind vias is an effective method to improve the wiring density of the multilayer board, reduce the number of layers and the board size, and can greatly reduce The number of through plated holes.  SMBs with high-speed circuits require control of characteristic impedance, and use of thinner materials, which are becoming thinner.
Custom message
Chat Online 编辑模式下无法使用
Chat Online inputting...
Please hold on and we will get back to you soon