Surface Mounted Printed Board (SMB) Features

Printed circuit boards used as electronic component mounting and interconnections must be adapted to the rapid development of current surface mount technology (SMT). Printed board mounting surface mount components (SMD) is now commonly referred to as surface mounting. Boards (SMBs), which include simpler single-panel, more complex two-sided printed boards, also include difficult, more complex multilayer boards.

In the 1990s, the international PCB technology was changed and developed with the goal of increasing the density of SMBs. SMB has become the mainstream product of current advanced PCB manufacturers, and almost 100% of PCBs are SMBs. Surface mounted printed boards have the following major features compared to printed boards with inserted lead components.

After surface mounting elements and devices are used, the metallized holes on the printed board are no longer used as insertion elements or device leads, and soldering is no longer performed in the metalized holes. Metalized holes are used only as electrical interconnections. Minimize the aperture, from the past O.5 - 1.0mm into O.3, 0.2 or O.1mm, foreign aperture is called 0.3-0.5mm aperture is called Small Hole, will be less than O.3mm below The holes are called micro holes. The micro holes and holes are mainly on the SMB. It was estimated that the holes with a diameter less than 0.48 mm accounted for only 5% in 1984 and increased to 31% in 1991.

The number of pins used in SMT ICs has been as high as 100 to 500, MCM components as high as 1000 to 2000, and the pin center distance has been reduced from 2.54 mm to 1.27, 0.635, 0.375 mm, or even O.15 mm. Therefore, SMB requires fine lines, narrow pitches, line widths from 0.2-0.3mm to O.15mm, O.10mm, even 0.05mm, from laying two wires between the past two pads to laying 3-5 wires With such fine lines, it is extremely difficult to visually inspect the gaps, short circuits, and open circuits. It has been estimated that the line width is less than the 0.13mm thin line, which accounted for only 4% in 1984 and increased to 28% in 1992. .
Due to surface mount components mounted on the SMB, a high accuracy of the wire pattern is required. In particular, the accuracy of the land pattern requires +/- O.05 mm, and the positioning accuracy requires +/- O.05-0.075 mm.

Due to the stringent and high-precision requirements on the surface defects of the substrate, especially the more stringent requirements on the flatness of the substrate, the warpage of the SMB is required to be controlled within 0.5%, while the warpage of the non-SMB PCB is generally required to be less than L-1.5%.

It is required that the SMB has good dimensional stability, and the coefficient of thermal expansion of the leadless chip carrier and the substrate material to be mounted should be matched so that the stress caused by the difference in the thermal expansion values ​​in the harsh environment may lead to the breakage of the lead. Therefore, the aramid fiber and the quartz fiber having a smaller expansion coefficient need to be used. Base, BT resin, PI resin copper foil substrate, copper / invar / copper metal core substrate material can be used in a strict environment.

SMB requires plating on the metal plating (coating) layer on the pad, and the hot melt plated tin-lead alloy generally has a rounded surface due to the effect of surface tension after tinification in the hot melt process. SMD accurately positions and mounts. Vertical hot air leveling is applied to the soldered printed circuit board. Due to the effect of gravity, the lower part of the pad is generally more protruding than the upper part. It is not smooth enough, and it is not conducive to mounting SMD, and the vertical hot air is evenly leveled. The board is not heated uniformly, and the time for heating the board is longer than the upper part, and it is prone to warpage. Therefore, it is not appropriate for SMB to use hot-melt tin-lead alloy plating and vertical hot air leveling of the solder coating layer, requiring horizontal hot air conditioning. Flat technology or other plating (coating) technology. Water-soluble, heat-resistant pre-fluxing has been used as a replacement for hot-air leveling processes.

The Solder Mask on the SMB also requires high precision. The commonly used method of screen printing solder mask has been difficult to meet the high precision requirements, so most of the solder mask patterns on the SMB use liquid photosensitive solder resist, which is available on the SMB. SMD is installed on both sides, so SMB also requires solder mask patterns and markings on both sides of the board.

When SMD is installed on SMBs, placement machines are used, most of them are surface-mounted with large-splash editions, and SMBs are separated by infrared welding or vapor phase welding. Therefore, SMBs are required to be supplied in large-splash editions, and they are used when producing SMB impositions. Slot milling or milling slots to stay in place.

Source: SMT Forum