Printed circuit board printing materials and operation technology (3)

(3) The liquid photosensitive liquid photosensitive resisting printing material is a printing material matched with a liquid photosensitive plating resist printing material and is suitable for the production of fine wires and ultrafine wires, mainly composed of a photosensitive resin and a thermosetting resin, and a photosensitive material. Agents, heat curing agents, thermal polymerization inhibitors, fillers, additives, pigments, solvents, etc., divided into two parts: the main agent (green) and additives (white). Before screen printing, weigh and mix according to the ratio requirements, add a small amount of diluent, stir evenly and stand still for more than 30 minutes to eliminate air bubbles and fully cure. After curing, a cross-polymerized network structure results.
The brief process flow of the liquid-sensitive solder mask is as follows:
Substrate processing  coating  standing  pre-baking  cooling  exposure  standing  development  inspection  after baking a. substrate pre-treatment dipping acid, brush brushing light, washing and drying.
b. Apply 100 to 150 mesh screens and hand-print screens to print A and B sides.
c. Allow to stand for 10-15 minutes.
d. Pre-drying hot air circulation oven, double-sided baking, temperature 70 ~ 80 °C, time 30 ~ 40 min, cooled to room temperature.
e. Exposure cooling exposure machine, 5 ~ 7 kW, light energy control 300 ~ 500 mJ/cm2, the temperature inside the exposure box should not exceed 40 °C.
f. Developing automatic spray developing machine, 1% concentration of Na2CO3 solution, spraying pressure of 0.2-0.3 MP/cm2, temperature of 28-32°C, time of 60-80 s, showing a control point of 50%-60%.
g. After baking hot air circulation oven, 150 °C, 40 ~ 50 min.
If the post-baking process is not performed, the liquid photo-sensitive solder resist printing material can also be used as an anti-etching printing material.

Fourth, characters, marking materials

Characters and markings are used to print on one or both sides of a printed board. These symbols are used for component assembly and repair. After screen printing, it requires clear marking, beautiful color, three-dimensional, chemical solvent and chemical resistance, good heat resistance, adhesion and insulation, and resistance to tin-lead alloy solder. The color is mainly white, divided into two major categories of thermosetting and photosetting.

As with solder masks, most of the single-panels use photo-curing. The double-sided and multilayer boards are thermoset. The main technical indicators and operating processes such as the ratio of the main agent and auxiliary agent, the curing time and temperature, and the film forming characteristics, the storage period of the printing material, etc., are basically the same as the corresponding type of the solder resist printing material.

Fifth, plug hole printing material

Plug-hole printing material is a one-liquid dilute alkali-soluble printing material, which is used to block through-holes (metallized holes) when it is used for double-sided or multi-layered plate positive plate making, so that it is not destroyed by the etching liquid and masked with dry film. There are similarities, which are used in the manufacture of double-sided or multi-layer printed boards by the plugging method. It eliminates the need for tin-plated lead alloys and tin-reduced lead alloys, reducing the number of processing steps and making them economical and practical.

The plug hole printing method of plug hole has roller coating, dip coating, and also can use a special plugging device to press the printing material into the hole, and then it can be cured. As a plug printing material, as long as the curing thickness reaches 0.7 mm on one side, whether the solidification in the middle does not affect the anti-corrosion ability, the plug hole printing material has two kinds of thermosetting and photo-curing.

The thermosetting plug printing material is dried at room temperature for 4-6 h (hours) or baked at a temperature of 80°C for 1 h. It is required that the plug hole is flat, has good adhesion, does not damage the coating, and is resistant to acid or alkaline etching liquid when worn. It can be removed by 2% to 4% NaOH solution. Since the thermosetting printing material has a solid content of about 75% and contains a solvent, the solution volatilizes during curing, and the printing material tends to produce concave phenomenon. Since the light-curing type contains no solvent, the plug hole printing material is smooth and full, not Concavity will occur (Figure 3).

Figure 3 Thermosetting and photosetting plug printing

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.

Seven, peelable printing materials

Peelable printing material is a kind of printing material that provides temporary protection and masking during production and processing. It usually requires toughness of the film layer, resistance to acid, alkali, chemicals, impregnation of tin-lead solder, and appropriate mechanical impact resistance. It has good stability and is removed by hand or simple tools after finishing.

The peelable printing material used in printed boards is mainly used to protect conductive printed carbon paste printing buttons, gold-plated keys, and plug parts, and is used as a solder resist for electronic component assembly, partial soldering, and damage resistance for shape machining. Its printing process conditions are as follows:

60 to 100 mesh screen, photosensitive film thickness 100 ~ 120μm, scraper hardness selection of 60 ~ 65 degrees, heat curing temperature 150 °C, 5 ~ 8 min. For some UV-curable UV curable lamps, three 80 W/cm2 lamps with a distance of 10 cm and a conveying speed of 2 to 3 m/min, the curing time is much longer due to the thick film layer. Clean the screen with benzene alcohol solution or special wash screen solvent.

After the assembly or processing is completed, the removed film layer can be peeled off by hand or tweezers without any adverse effect on the masking substrate.

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.

Source: Jiangxi Huasheng Communications Corporation Author: Yehong Xun