Printed Circuit Board Ink and Progress (8)

Eight, solder paste printing materials

Solder paste printing is used on SMT (Surface Mount Technology) printed boards and is also a special function print material. Its role is to precisely position the chipless components (SMD, SMC) with no leads or short leads, and to solder the SMD, SMC, and surface mount printed boards.

The main components of the solder paste are solder alloy powder (Sn/Pb, Sn/Pb/Au, Sn/Pb/Bi, etc.) and flux (synthetic resin) as well as active agents, additives and organic solvents, and have the following characteristics:

(1) Has a certain viscosity, no change in viscosity for a long time storage, and no separation phenomenon of flux and alloy powder, and good printability.
(2) After printing, the solder paste will not collapse when it is placed or warmed up.
(3) Good solderability. No solder beads are produced during soldering.
(4) For no-clean solder paste, a small amount of residue will not affect the electrical performance of the circuit.
There are two ways to print solder paste. One is screen printing, and the other is hollow stencil printing. The latter is widely used nowadays. The latter is divided into two types: contact printing and non-contact printing. print.

The material of the hollow stencil is mainly stainless steel, and the thickness is between 0.1 and 0.3 mm. There are two methods for making the stencil: one is for double-sided photochemical patterning and the other is for double-sided etching, similar to the double-panel manufacturing process; the other is Use computer-controlled laser cutting graphics.

The paste paste used for screen printing has a viscosity of 500-650 PS; while the paste paste for paste-holing stencil printing has a viscosity of 700-900 PS. The thickness of the paste after printing is basically equal to the thickness of the stencil, and the stencil printing The shape of the paste of the solder alloy powder is spherical particles.

The screen printing machine used for solder paste printing is a full-precision printing machine with a vision system. It usually has a precise PCB positioning system, image detection, image storage and processing, and an image display processing system to ensure solder paste. Overprint accuracy.

The precision of fine solder paste printing is related to the following process references:

(1) The production precision of the hollow template, such as the cross-sectional shape of the opening of the figure and the thickness of the stencil;
(2) Manufacturing accuracy of PCB patterns and precision of reference hole machining;
(3) The repeat positioning accuracy of the printing press and the offset of the relative positions of the printing plate and the stencil.
(4) The viscosity of the solder paste ink, the shape of the powder, and the surface shape formed.

Author: Yehong Xun

Source: Screen Printing