Sixth, conductive printing materials
Conductive printing is a functional printing material. Printed materials used on printed boards are carbon paste, copper paste and silver paste. Single- and double-sided printed boards (rigid) buttons, bridging wires, through-holes, notch breakage repairs, and flexible printed circuit wire patterns are all made using conductive ink.
(1) Carbon paste printing
Carbon paste printing is a one-component thermosetting printing material with the following characteristics after film formation:
a. The conductive printed material can protect the copper foil and conduct electric current after it is cured. It has good conductivity and low impedance. The surface resistance of carbon paste printing is 20-25 Ω/□. The surface resistance of the copper paste is 0.06 Ω/□.
b. It is not easy to oxidize, stable in performance and resistant to acid, alkali and chemical solvents after curing.
c. Strong adhesion, anti-stripping; good combination with copper foil or glass cloth.
d. Good abrasion resistance, pencil hardness of more than 6H after curing. Anti-wear, keyboard contact up to 1 million times.
e. Good thermal shock resistance, resistance to welding test 260°C, 50 s (seconds) × 5 times without influence, hot air leveling process will not stick tin.
During screen printing, the screen is selected according to the fineness of the carbon paste printing material and the width and spacing of the printed wire. For a screen size of 0.25 mm or more, a screen size of 100 to 150 mesh can be selected, and for a screen size of 0.25 mm or less, 150 to 200 can be selected. Mesh screen. Because the carbon paste printing is precisely aligned, an adjustable self-tensioning frame should be selected.
Carbon paste must be fully stirred before use, because the filler in the carbon slurry is mainly graphite carbon black, easy to produce sedimentation during storage, screen printing should be minimized with or without the addition of diluent, while within the scope of the curing process The higher the temperature the longer the better, so that the film color bright, full, film thickness can be guaranteed, the usual curing conditions: temperature 120 ~ 150 °C, time 30 ~ 40 min.
(2) Silver paste printing
Silver paste is a one-component type printing ink consisting mainly of ultra-fine silver powder and thermoplastic resin. It is mainly used for flexible printed circuits and film contact switches, electromagnetic shielding, pre-coating of heating elements, etc., in PET, It can be used on PC and PVC sheet, with strong adhesion and hiding power, low temperature curing, controllable conductivity and low resistance value. The surface square resistance is 0.007 Ω/□.
The main technical specifications of silver paste are as follows:
Solid content 65% to 70%
Viscosity 600～900 PS
Storage period 180 d (days) (below 5°C)
200-250 mesh stainless steel or polyester screen is used for printing, and the curing conditions should be selected according to the type of substrate. In principle, the lower the curing temperature, the longer the curing time; otherwise, the curing time The shorter. If the temperature is 70 °C, the curing time is 30 min, at 90 °C, it takes 15 min, and 120 °C, only 5 min.
After the printed material has cured, the circuit has good performance, strong adhesion, can't be stripped with 3 M tape, has good bending resistance, and has little change in resistance.
