1 Overview
1.1 Introduction
The screen printing process has the following three applications in double-sided and multi-layer printed boards.
Solder Mask: A protective layer that is applied to a printed circuit board that does not require soldering. Purpose: To prevent bridging during soldering, provide a long-term electrical environment and chemical protection, and to form a beautiful "clothing" for printed boards, including thermosetting epoxy green oil (including UV uv green oil) and liquid photosensitive resister ink. II. Big system. Usually green, but also black, yellow, white, blue solder.
Component Characters: Provides yellow, white, or black markings to provide information for component installation and future repair of printed boards.
1.2 Scope
Describes the entire process of silk screen printing, including: safety production, use of equipment, required materials, process flow and control parameters, manufacturing process (principle, working conditions, screen preparation, screen making, ink mixing, squeegee use, screen printing positioning Ways, board inspection, brush board, screen printing, pre-baking, exposure, development, curing), document and process review, inspection and testing projects. Finally, outline the curtain coating. The content of this chapter is mainly based on liquid photo-resistance solder masks and characters, applicable to double-sided and multi-layer printed circuit boards.
1.3 Importance
Solder mask is the “coat†of the printed circuit board. The most intuitive quality for the user to see the printed board is the solder mask. Therefore, the solder mask has always been critical. In addition, the silk screen solder mask and the characters are long manufacturing of the printed board. In the post process of the process, the unfinished value of the soon-to-be completed printed board has made mistakes in the post process. The loss is too great and it is not worth it. Furthermore, solder mask and character are one of the most scrapped processes. Therefore, it is important to stabilize the process of screen printing solder resist and character and to straighten out the process management, document control and equipment maintenance.
Since the 1990s, the use of traditional silk-screen thermosetting epoxy green oils has become less and less common in printed board factories. This is because
As follows: the density of double-sided and printed boards is increasing, and the high density of small holes SMT is an irreversible trend in the development of printed boards.
12~0.20mm narrow welding ring is already the majority, screen printing thermosetting green oil has been powerless, appearance is not neat, oil leakage on the pad, jump printing, due to the silkscreen staff's proficiency and emotional influence is too large, so the current majority Both double-sided and multi-layer boards have eliminated thermosetting epoxy green oils and switched to liquid photosensitive resist inks. $Page break $
2, safe production
Chemicals commonly used in screen printing processes are sodium carbonate (Na2Co3), sodium hydroxide (NaoH), sulfuric acid (H2SO4) and various acetone and toluene.
Washing net water and other organic solvents will burn the skin and eyes when used improperly; there are ultraviolet light sources in the screen printing room, 3~7 kW exposure lamps, and several ovens, and the heating temperature reaches 150°C when the printed board solder resist heat-cures. Hours, improper use will also injure the skin of the eyes and hands and feet; screen printing room needs to use the film, printing plate manufacturing process is often used as a negative film. Store the film, use the film room to maintain the same temperature and humidity, and dust, otherwise it will cause the film thermal expansion and contraction, dust, hair out of the film and solder mask swelling, will lead to PCB scrap, finger print It will also be rejected on the board. Based on the above reasons, doing a good job in the safe production of screen printing is an important part of ensuring the normal work order and product quality of employees. Therefore, the following requirements are required for the safe production of the silk screen printing process:
(1) Into the screen printing room to put on overalls, wearing a cap, put on hairless work shoes. All hair should be placed in a working cap, the shoe should be clean, dirty shoes, cleaning shoes do not mix to a pile.
(2) Wear protective goggles and rubber gloves when adding drugs. Especially when dealing with the rework board de-resistor film, the printed board is hot at the high temperature of sodium hydroxide.
Soak in alkaline, wear gloves and glasses when adding drugs and dispensing plates. One employee of a factory was inadvertently withdrawing the solder mask, and the concentrated hot alkali splashed on the eyes of the skin. He was hospitalized for more than one month and spent 5-6 million yuan on medical expenses. After medical treatment, his eyes quickly collapsed. This is a painful lesson.
(3) Chemicals are toxic and should not be taken orally. Note that the skin is not in direct contact with solvents. PCB Solder Resist Ink in an oven at 150°C
When you open the oven, a large amount of solvent vapor will emerge. Do not inhale. If you find vomiting, dizziness, etc., report to the foreman immediately.
(4) equipment failure, chemical spillage should be promptly dealt with, clean the floor, repair equipment.
(5) When the printed board is removed from the hot oven, wear gloves to prevent burns.
(6) Do not touch the cover of the device. Hats, switches, UV light sources should not be exposed, and human eyes should not be exposed to ultraviolet light. Oven ventilation is very important, a company has an employee moving an oven ventilation valve, the amount of ventilation is reduced, the solvent on the board can not be volatilized, more than the old glue, check for a month, because some people have moved a convulsions Switch, ruined a lot of board, profound lesson. $Page break $
3, equipment
To build a new factory or to manage screen printing operations, you must know the bottom of your home, and you must be familiar with how many devices you need to know to start a normal job. The following outlines the equipment required for the silkscreening process.
No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Equipment name Screen printing machine (automatic, semi-automatic machine, manual screen printing station) Screen exposure frame exposure light source (2 ~ 4KW) Oven, multiple (conveyor type, vertical can, there is enough ventilation, temperature tolerance ± 2 °C) Water gun ( High pressure, 1000psl) Net Box Flush Developer Cabinet Grinding Scraper UV Dryer Screen Printing Mesh Shelf (Self-adjustable, Tensionable Type) Tensiometer Stirrer or Oscillator Exposure Machine (5~7KW, Water-cooled) Developer (Conveyor type, punching pressure 2.5kg/cm2,) brushing machine (pickling - washing - brush plate [two pairs of brush roller, or pumice powder type] - water burning - drying) Huang Guangguang table, magnifying glass viscometer
Functionality Screen printing silk screen stencil manufacturing (exposure) Screen printing stencil manufacturing (exposure) Cable stencil manufacturing, solder resist, character pre-baking and post-curing. Removal of stencils, spray cleaning Scrubbing screen squeegee UV curing screen printing Screen tensioning Ink agitation Exposure development Printed plates Screen printing ink Front brush inspection Printed board inspection Ink viscosity
All the above equipment should be clean, tidy, clean, regularly maintained according to the document, and complete the equipment maintenance card. $Page break $
4, materials
The screen printing process involves screen printing room, stencil making room, and materials used for exposure and development room.
4.1 silk screen
(1) Thermosetting solder resist. Usually two-component, solder mask ink and hardener, mixed in proportions (eg 19:1, 20:1, 2:1), printed on a circuit board using a stencil, exposing pads and holes, and then in an oven 140~150°C 40~60 minutes ink curing, in the 1970s and 1980s, Taiyos?22, Pck501, 401, Coates X2?5, HYSOl?R1000, Gin HNa SM?02, many domestic and foreign professional PCB manufacturers use more After these models, in the 1990s, the factories gradually phased out this difficult-to-operate control. They were apt to seep through oil and had a poor-looking thermosetting solder resist. The uv solder-resisting green oil was mostly used on single-sided boards and double-layered. Board does not apply.
(2) Liquid photoimage æ¢ ble sdlder mask ink, currently used in a large number of printed board factories to promote. Major brands include Tamura DsR?200, TaiyoPSR?000, Cotes XV?00, ToYo K?000, Gin Hwa DSR?000 and Lea Ronal, Danachm and other companies' products, two-component, UL models are UL. Certification. through
Often green, yellow, black, white and other colors. Asian products are bright, and European and American products are mostly matt.
(3) Character ink, thermosetting type is two-component, such as British Coateskspl (curing agent SP1606), Taiwan refined (Gin Hwa) ML411
(hardener SM 502), etc. are white, Coates Ksp4, HYsol50?02BR, etc. are yellow, and the main agent and hardener are mixed at a ratio of 100:8, 19:1, 100:6. Carefully provided by the supplier before use. The data is mixed, and the photohardness characters are passed through a UV curing machine (usually three 80w lamps) and can be hardened within a few seconds. Due to the slight yellowing of the light solid ink, some of the fussy client characters must use thermosetting white ink.
(4) Diluent, the special diluent provided by the ink supplier should be used, and dilute the solder mask resist ink with wash water instead of diluent, otherwise
Product quality is difficult to stabilize, there is uneven competition, there are problems such as foaming of dense lines.
(5) conductive ink, usually printed on the board after the completion of the solder mask on the keyboard circuit printed with conductive ink, such as coates X2302, Gin
Hwa LR6007 and other models constantly stirring, conductive ink gradually diluted, using a good screen printed on the corresponding circuit board, curing at 140 ~ 150 °C for about 40 minutes, the ink curing.
(6) Wash the net water to wash the screen.
(7) Acetone, xylene, ethyl solution, one of the washing solution.
(8) Paper tape, PVC tape, commonly used 1.5 inches wide, used to seal the net.
(9) Rubber scraping, 65 to 75 degrees.
(10) Newsprint used for screen printing.
(11) Blue adhesive ink is a kind of peelable ink, blue color, which is convenient for customers to weld assembly components in different ways.
4.2 template
(1) Screen: 18~36T screen, blue adhesive printing; 43T liquid photo-resist welding ink printing (special condition will be used depending on the line thickness
36T and 77T); 100T, character printing; 43? 7T carbon ink printing.
(2) Film film: used to make screen templates; 25μm thickness, for character screen; 35μm, solder mask or graphic screen; 50μm, carbon ink screen.
(3) Grinding paste (Autoprep): It is used to lightly grind the new and old screens, roughen the surface of the screens, and strengthen the film adhesion.
(4) Auto Gel; used to remove the old film version.
(5) Universal Mesh prep; degreasing and degreasing of the screen surface.
(6) Autosolve: Used to remove stubborn old stains and ghosting on old mesh.
(7) Sealing glue: used to seal the net.
(8) Kraft glue: used for pulling nets.
(9) Strong glue: used as a binder for stretch nets.
4.3 Development and Others
(1) Anhydrous sodium carbonate (or anhydrous potassium carbonate): Industrial or cp grades can be used for the liquid photoimageable solder mask developer.
(2) Antifoaming agent: used as a defoamer for developer.
(3) Sulfuric acid: It can be used in industrial grade.
(4) Protopine: Industrial grade, ie, sodium hydroxide, used as a desoldering film.
(5) Alcohol: cp grade, cleaning supplies.
(6) Scotch tape: The exposed film is fixed with a window tape around its periphery, and the film and the printed board are positioned and fixed before exposure.
(7) Optical wedge (foot): 17 or 21 cells (grade), test exposure energy, Dupont, supplied by Danachem.
(8) 3M tape: Test solder mask and character adhesion.
(9) Exposure pins: Fixed solder masks and printed boards for exposure positioning. $Page break $
5, process flow and process parameters.
5.1 Liquid Light Imaging Solder Mask Ink Process
Screen preparation → screen manufacturing → ink mixing → grinding scraper → board inspection → brush board → screen printing A surface → stay → pre-baking → stay → silk screen B surface → stay → pre-baking → stay → double facing → exposure → Stop → Develop → Check → Solidify → Check... → Send the next process
5.2 characters, carbon ink and other processes
First check the work card, which is a single-sided or double-faced character, character color, carbon black ink to be printed, blue plastic sheet is double-sided or single-sided, and the through-hole is all or part of the hole blocking plate.
Prepare the net, screen manufacture, stir the ink, prepare the scraper → check the board → (ink to be printed, plug the hole, blue plastic sheet) → screen (carbon ink to be printed) pickling → washing → drying printing → check →Drying... →Whether the second side is printed (Yes) → Screen printing → Drying → Checking... → Sending the next process
5.3 Control Parameters
(1) Liquid photoimageable solder mask ink.
Brush plate: pickling → washing → brush plate (two pairs of brush roller or pumice) → washing → dry → drying. Transfer or brushing machine. Pickling section