Printed circuit screen printing process (2)

6, the manufacturing process.

6.1 Principles and Working Conditions

The main components of the liquid photoimageable solder resist ink include: photosensitivity epoxy (EPOXY), and Acrylic resin, such as C

Diacid epoxy resin, phenolic epoxy resin, cresol epoxy resin, carbamoylamine; photoinitiator, such as Xanthone, amine organometallic compound; filler, such as silica powder: hardener, such as aromatic Fats, anhydrides, imidazoles; solvents, such as ether esters; defoamers, that is, surfactants, and so on more than 10 kinds of ingredients. The photosensitive resist ink forms a network polymer structure after curing.

Working principle: Liquid photo-imaging solder resist ink is called photosensitive adhesive, and it is also called wet film. Photosensitive adhesive is screen-printed and pre-baked, and then subjected to photo-imagewise exposure. Ultraviolet radiation causes photopolymerization of the epoxy resin system with free radicals. The unexposed part (pad is blocked by the film) is developed in dilute alkali solution. Wash and remove. The photosensitive part of the printed board is heat-cured, so that the resin is further branched into a permanent hardened layer. This is the imaging principle of liquid photo-resist soldering.

Cross-linked structure of propanediol epoxy resin:

After phenolic epoxy cross-linking:

After urethane resin cross-linking:

Working conditions: Environment, 21±1°C, Humidity 55±8~5%, Dust-proof; Exposure and screen printing sensitive areas, Lights are yellow; Workers should change shoes and wear caps Wear work clothes: Brush plate and developing plate Put on the gauzeless gloves on the plate; take the plate with both hands when the plate is placed, the fingers do not touch the pattern and the solder mask area, gently lightly, and the plate corners are protected from bumps; take the plate from the oven after thick Yarn gloves. More importantly, any employee must develop habits. Before the silk screen printing work, he must first read the work card and work according to the requirements of the work card.

6.2 Screen preparation.

(1) to find the required screen, for liquid photo-imaging solder resist ink, usually with 43T (special circuit board with 36T or 55T, 77T);

For character inks, the screen is 100T.

(2) Prepare the required Internet film. Typically, a 25 μm (micron) thick film is used for the character network, a 35 μm film is used for the solder mask, and a 50 or 80 μm thick film is used for the carbon ink system.

(3) Check the frame. Check whether the net of the frame is in good condition, whether there is a hole, and whether it can be used again; it is important to check the tension of the center of the frame and the surrounding position with a tension gauge. The tension of the screen of 43~77T is 18~20 Newton/. Cm, 100T screen, 14~16 Newtons/cm.

(4) stretch net.

A. Self-expanding frame: This type of frame is aluminum, bulky, expensive, but fast adjustment, durable, do not need to use stretcher, but the screen should be in accordance with the frame before sewing around in the metal can be punched On the other hand, printed boards are used at home and abroad. The network changing process is:

• Take out the cap screws on the old frame and remove the metal bead and the old net around the perimeter of the net. The batten is a round metal rod.

• Wash the remaining solder mask around the frame with washing water and cotton cloth.

• The frame is placed on the table and the number of screen meshes is selected. The bead is inserted into the edge of the screen. The nut is inserted into the bead and the screw is screwed on.

• Insert the screw into the notch of the frame and tighten the screws from the center of each edge of the four sides. Note that the tension must be balanced. Otherwise, the screen will be pulled.

· Gradually tighten the screws in a diagonal sequence. After tightening, measure the tension of the four corners and the center with a tension gauge to 18 N/cm or more.

The 100T screen tension is at least 14 N/cm. If not enough, the tightness of each screw can be adjusted until it is satisfactory.

B. Pneumatic stretch net: This type of net frame is also aluminum, simple and lightweight, and is used by most printing plants. But need to be equipped with a pneumatic stretching machine to use for stretching, see Figure 1, the stretching process is as follows:

• If the new frame is used, wash the surface with acetone before stretching, and use it. If it is an old frame, remove the mesh and seal the tape to remove the oil before use.

• Adjust the frame on the frame and open the airflow valve so that the net clamp of the frame is in close proximity to the frame and adjust the support screws of the frame so that the frame remains horizontal and is about 5 mm above the chuck, so that the tension is uniform.

Fixed mesh. Cut the desired size mesh yarns, straighten the edges in the order, and fix them with a clamp. Note that the direction of the mesh wires should be the same. The warpwise weft mesh wires should be perpendicular to each other, and the direction of force should be parallel to the mesh wires.

·Pressure tension net. Slowly increase the air pressure to around 6kg/cm2, and the tension meter shows up to 20N/cm or more. Segmentation time sharing

Sub-net can achieve satisfactory results. Each time the net is opened for at least 5 minutes, the time is slightly longer, so that the tension is uniformly conducted and the tension is stable.

·Glue. After the pressure was stabilized for half an hour, a strong glue (ratio 5:1) was evenly coated on the gauze and the frame with a squeegee.

· Protective tape. After gluing, allow it to stand for more than 8 hours. After the adhesive is fully dried, loosen the fixture, remove the frame from the frame, remove the extra gauze outside the frame, and seal the four sides of the frame with the adhesive. In order to protect the adhesive from being dissolved by the solvent, the life of the frame is prolonged.

·Test tension. Tension meter is used to measure the four corners and the center tension, which is required to reach 18~20 N/cm, and the 100T screen tension reaches 14~16/cm Newton. The stretched mesh frame needs to be stopped for more than 1 day, and the tension meter is shown in Figure 2. $Page break $

6.3 Screen Production

A. Screen mesh

(1) check the original negative film (net map, character map, solder mask). See if there are any stains, scratches, manufacturing date, company logo, uL mark, work card number, etc.

(2) Select online film. Cut a piece of film of the same size as the original image (Autotype). The selected film thickness, resistance welding pass

Commonly used 35μm, character 25μm, carbon ink 50μ0μm, blue plastic 80μm.

(3) pre-network processing. The new wire mesh, wet both sides of the mesh, brush back and forth with Autoprep for a few minutes, thoroughly rinse both sides with running water, then brush with a degreaser (Unirersal Mesh prep) for several minutes, rinse both sides with running water. Old screen, first go to screen powder (Auto

(Gel) coat the two sides of the screen, and wait for three or five minutes. Brush the softened gelatin on the back of the screen with a brush. Rinse thoroughly with running water. Then apply evenly to the two sides of the screen with an ink remover (Auto solver) and leave for two or three minutes. Dissolve the stain, thoroughly wash it, wash it with a high-pressure water jet, and then clean it with an auto prep followed by a Univrsal Mesh prep as in the new screen treatment. Note the operator. Wear protective gloves.

(4) film film Internet access. Wet the screen with water and dry the excess drops of water. The dry film is applied to the bottom surface of the screen so that the dry film is completely attached to the screen.

(5) Drying. Put the frame in the oven and dry it. Remove the film from the film at 40°C for 10-15 minutes.

(6) stick negatives. After the frame is cooled, the film is pasted on the dry film, and the back emulsion faces the dry film of the screen. The four corners of the film are fixed by transparent tape.

(7) Exposure. The entire frame was placed on a vacuum rack with the film facing the glass, a vacuum rope was placed on the frame, and the vacuum pump was started. The frame is vacuumed and the frame is placed in a vertical position. Iodine tungsten lamp 1.2 meters from the frame, open the exposure lamp, exposure. 4kw power source, for reference exposure time: 25μm film, 120~150 seconds; 35μm film, 250~350 seconds; 80μm film, 150~200 seconds. The automatic timer controls the exposure, and after exposure, the light source and the vacuum pump are turned off.

(8) Development. Remove the frame from the exposure frame, remove the film, and first rinse the screen with a small stream of water to remove unexposed and unexposed parts of the dry film. If tap water is cold, use warm water for development. After the graphic is displayed, rinse thoroughly with high-pressure water and drip dry excess water. Put the frame into the oven and dry it completely (40°C).

(9) Closure. After the screen is cooled, use a brush and sealer to repair the plate, make up the pinholes, and pits. Use plastic sheet to evenly seal the sealant on the outside of the figure and seal it. After closing the net, place the net frame in an oven and dry it.

(10) Make a mark. On the net frame, stick the work card number, the version number, the production date, the number of screen meshes, the producer and other marks, and wait.

B. Aluminum Foil Screen (Applicable to Via Hole Plugging Process)

(1) Choose a 0.2-0.3 thick aluminum foil. The surface of this foil should be flat, without creases, and cut to the same size as the frame size.

(2) Drill holes that need to be plugged in the CNC drilling machine. The diameter of the drill is the same as that of the holes on the printed board. The drilling points are distributed as far as possible in the center of the aluminum foil.

(3) Cut the edges to remove the burrs and cut the excess edges of the drilled aluminum foil. Each side frame is about 25mm thicker than the hole blocking pattern. Remove drilling burrs and sand them with fine sandpaper.

(4) Fix the aluminum foil. The aluminum foil is fixed on the back of the 36~43T blank net frame with Kraft glue, that is, when the hole is blocked, the screen is used to add aluminum foil.

Block holes to avoid oil seepage. Special Note: For the hole on the two sides of the PCB after plugging holes, the hot-air leveling (spraying) is required, or both sides of the PCB must be covered by solder mask and the lead-tin-free board is not required. The hole near the hole and the SMT pad are not dense and the circuit is sparse; the printed surface is blocked; for the through hole after the hole is plugged, the via hole of the component surface allows the cover to be soldered and the solder surface is blocked. The hole-welding ring requires a lead-tin plate, and the component surface should be selected as the plug-hole printing surface for screen fixing. During the entire screen production process, it is necessary to ensure that the surface of the aluminum foil is flat without creases. Otherwise, the inaccuracy of drilling and screen printing alignment will be caused. Figure 3 shows the BGA (Ball Grid Display) guide that the customer requires on the printed board. Through hole plugging graphics

6.4 ink stirring

· Reading card, according to the technical specifications of the work card, the correct choice of ink color, type, model.

• Determine the proportional relationship between the selected ink base and hardener.

• Stir the hardener into a running water.

• The main agent and hardener are weighed out in proportion. Suppliers do not need to weigh the main ink and hardener in one or more cans.

• Pour the hardener into the main agent ink for about 5 minutes until the hardener diffuses completely into the main agent. Then put it into the ink

Stir in the machine (or oscillate in the ink shaker) for about 15 minutes to ensure that the ink is fully mixed.

• Before use, the agitated ink needs to stay for 15 minutes to allow air bubbles to escape.

• Use a mixing knife to bring up a small amount of well-stirred ink and observe if the viscosity is appropriate. Necessity Use a viscometer to measure the viscosity of the ink at this time, and use a thermometer to measure the temperature in the ink. The normally mixed two-component ink should be used up within 24 hours.

Special attention: Liquid photo-imaging solder mask inks are generally strictly thinner to ensure that the solder mask is uniform in color and enhances the gold-plated and thermal shock resistance of the solder mask. Under very special circumstances, such as printed copper 70μm printed board, when printing is difficult to allow about 1Oml/

Kg of thinner. However, when the printing of the character ink is difficult, a small amount of diluent can be added.

6.5 Scraper use

(1) The choice of scraper.

Requires abrasion resistance, solvent resistance, polyurethane rubber is usually used, blade angle 70 ± 5 °. The hardness is 70±10°, and the blade is generally exposed to about 2mm on the squeegee frame. Shenzhen Sanhe Company, Siweiya, Xindongbang, Tianjin Rubber Products Factory, and Shanghai Rubber Research Institute all have silk screen scrapers for sale.

(2) Use of scraper.

Scraper length selection. According to the size of the printed image, a suitable rubber scraper is generally required to have a longer scraper than the length of each side of the figure.

15 mm or more.

Scraper installation. Select the sharp edge of the straight edge of the scraper rubber and install it on the scraper handle on the outside to facilitate the use. Tighten the screws to fix the scraper.

·Scraper. If the edge of the squeegee is scratched, notched, and the cutting edge is not sharp, it should be ground on the grinding squeegee to keep the squeegee end face in good condition.

6.6 Screen Positioning

The purpose of silk screen positioning is to ensure that there is no misalignment between the printed board and the screen board to be printed at the time of each printing, so as to ensure the consistent accuracy of the screen printing. The number of printing plates is not large, the developing pressure of the developing machine is large enough, the screen printing techniques are used properly, and the liquid photosensitive resister printing ink is often used for blank screens. At this time, the screen printing positioning requirements are not strict, but the screen printing character inks may need to be printed with dot patterns. The hole of the double-sided multi-layered board is not clean in order to prevent the ink from leaking into the hole. At this time, the positioning problem should be fully considered. Solder plug vias