What are The Factors That Affect The Quality of PCB Steel Mesh

PCB stencil mainly has the following factors that will affect the quality of PCB stencil:

1. We have discussed the production process of the stencil before. We can know that the best process should be electropolishing after cutting the laser stencil. Both chemical etching and electroforming have processes such as film making, exposure, and development that are prone to errors, and electroforming is also affected by uneven substrates.

2. The materials used include PCB frame, silk screen, steel sheet, adhesive glue, etc. The PCB screen frame must be able to withstand a certain program of relay and have good levelness; it is best to use polyester mesh for the screen, which can maintain stable tension for a long time; it is best to use 304 for the steel sheet, and the matte one will be better than the mirror surface. It is more conducive to the rolling of solder paste (glue); the adhesive must be strong enough and resistant to certain corrosion.

3. The quality of the opening design has the greatest impact on the quality of the PCB stencil. As discussed above, the design of the opening should consider the manufacturing process, aspect ratio, area ratio, experience value, etc.

4. Whether the production materials are complete or not will also affect the quality of the PCB stencil. The more complete the information, the better. At the same time, when data coexist, it should be clear which one prevails. Also, generally speaking, making stencils with data files can reduce errors as much as possible.

5. The quality of the stencil can be maintained by using the correct printing method. On the contrary, the incorrect printing method such as excessive pressure, PCB stencil or PCB is not level when printing, etc. will damage the stencil.

6. Cleaning the solder paste (adhesive) is relatively easy to solidify. If it is not cleaned in time, the opening of the stencil will be blocked, and the next printing will be difficult. Therefore, after the PCB stencil is removed from the machine or the solder paste is not printed on the printing machine for 1 hour, it should be cleaned in time.

7. Store the stencil in a specific storage place, and do not place it randomly, so as to avoid accidental damage to the stencil. At the same time, do not stack the PCB stencils together, which is not easy to handle and may bend the screen frame.

Stencils are SMT templates (SMT Stencil): it is a special mold for SMT; its main function is to help the deposition of solder paste; the purpose is to transfer an accurate amount of solder paste to the exact position on the empty PCB.

Development of SMT process: SMT stencil (SMT template) is also used in glue process. [1] 2 Stencil Evolution Stencil was originally made of wire mesh, so it was called mask at that time. At first it was nylon (polyester) mesh, and later due to durability, barbed wire mesh, copper wire mesh appeared, and finally stainless steel mesh. But no matter what material the screen is made of, it has the disadvantages of poor molding and low precision.

With the development of SMT, the requirements for stencils have increased, and stencils have been produced. Affected by the cost of materials and the difficulty of production, the original stencils were made of iron/copper plates, but also because they were easy to rust, stainless steel stencils replaced them, which is the current SMT stencil.

According to the manufacturing process of SMT steel mesh, it can be divided into: laser template, electropolishing template, electroforming template, step template, bonding template, nickel plating template, etching template. Laser Stencil (Laser Stencil) Laser stencil is the most commonly used template in the SMT stencil industry. Its characteristics are: directly using data files to reduce the production error link; SMT template opening position is extremely accurate: the whole process error ≤ ± 4μm ; The opening of the SMT template has a geometric figure, which is conducive to the printing and molding of solder paste. The following materials are required to make laser steel mesh:

Data files must be: PCB: correct version, no deformation, damage, or breakage; data files: Weixin steel mesh (SMT template) processing group can accept multiple CAD data formats: GERBER, HPGL, *.JOB, *.PCB , *.GWK, *.CWK, *.PWK, *.DXF, *.PDF; and data designed by the following software: PAD2000, POWERPCB, GCCAM4. PROTEL, AUTOCADR14(2000), CLIENT98, CAW350W, V2001. When the data is too large, it should be compressed and transmitted, and any compression format such as *.ZIP, *.ARJ, *.LZH can be used; the data must contain SMT solder paste layer (including Fiducial Mark data and PCB shape data), and must also contain a character layer data in order to check the front and back of the data, component category, etc.

Next, we will focus on the GERBER file; the GERBER file is a data format proposed by the American GERBER company; it converts PCB information into electronic data that can be recognized by various photoplotters, also known as photoplotting files. The GERBER file is a software structure with X, Y coordinates and additional commands. The official scientific name of the GERBER format is "RS274 format". It has become a standard format file in the PCB industry. The GERBER file is combined with the Aperture list (also known as D-Code) file to define the starting point of the graphic and the shape and size of the graphic. D-Code defines the size and shape of lines, holes, pads or other graphics in the circuit. There are two types of GERBER files: RS274D and RS274X RS274D contains X, Y DATA, but does not contain D-Code files; RS274X contains X, Y DATA, and D-Code files are also defined in this file. Electropolishing stencil (E.P.Stencil) Electropolishing stencil is to post-treat the steel sheet by electrochemical method after laser cutting to improve the opening wall. Its characteristics are: 1. The hole wall is smooth, especially suitable for ultra-fine pitch QFP/BGA/CSP 2. Reduce the number of times of wiping SMT templates, greatly improving work efficiency. Electroforming template (E.F.Stencil) In order to meet the requirements of short, small, light and thin electronic products, ultra-fine volume (such as 0201) and ultra-fine spacing (such as ūBGA, CSP) are widely used, so that the SMT steel mesh industry Higher requirements have also been put forward for printing templates, and electroforming templates have emerged as the times require. The characteristics of the electroforming template produced by our company are: different thicknesses can be made on the same template.

Step stencil (Step Stencil) Due to the different requirements for the amount of solder paste when soldering various components on the same PCB, it is required that the thickness of some areas of the same SMT stencil is different, which results in a STEP-DOWN&STEP-UP process template. STEP-DOWN Stencil Local thinning of the stencil to reduce the amount of tin when soldering specific components. Bonding Stencil (BondingStencil) The COB device has been fixed on the PCB, but the SMT process of tin printing is still required, which requires the use of a Bonding Stencil. The bonding template is to add a small cover to the PCB bonding position corresponding to the template to avoid the COB device and achieve the purpose of flat printing. Nickel-plated stencil (Ni.P.Stencil) In order to reduce the friction between the solder paste and the hole wall, facilitate demoulding, and further improve the release effect of the solder paste, on the basis of the traditional subtractive process "electropolishing" stencil, an additional The special post-treatment addition process - "nickel plating" has been patented. Nickel-plated stencils combine the advantages of laser stencils and electroformed stencils. The etching template is made of 301 steel sheet imported from the United States. The etching steel mesh is suitable for PCB board printing with corners and spacing greater than or equal to 0.4MM. It is suitable for copying boards and film use. CAD/CAM and exposure methods can be used at the same time, depending on the difference. Parts can be scaled, and there is no need to calculate the price according to the number of parts. The production time is fast. The price is cheaper than laser templates. It is convenient for customers' film filing.

The production process of steel mesh includes: chemical etching method (chemical etch), laser cutting method (laser cutTIng), electroforming method (electroform).

1. Chemical etch process: data file PCB→film production→exposure→development→etching→steel cleaning→sheet Features: one-time molding, faster speed; cheaper price. Disadvantages: It is easy to form an hourglass shape (not enough etching) or the opening size becomes larger (over-etching); objective factors (experience, medicine, film) are greatly affected, there are many production links, and the cumulative error is large, so it is not suitable for fine pitch stencil production method ; The production process has pollution, which is not conducive to environmental protection.

2. Laser cutting method (laser cutTIng) process flow: film making PCB→coordinate taking→data file→data processing→laser cutting→polishing→netting Features: high data production accuracy, little influence of objective factors; trapezoidal opening is conducive to demoulding; Can do precision cutting; moderate price. Disadvantages: cutting one by one, the production speed is slow.

3. Electroform Process flow: Coating photosensitive film on the substrate→exposure→developing→electroforming nickel→forming→steel sheet cleaning→netting Features: the hole wall is smooth, especially suitable for the ultra-fine pitch steel mesh production method. Disadvantages: the process is difficult to control, the production process is polluted, which is not conducive to environmental protection; the production cycle is long and the price is too high. The opening design of the stencil should consider the release property of the solder paste, which is determined by three factors: ①The aspect ratio/area ratio of the opening; ②The geometric shape of the side wall of the opening; ③The smoothness of the hole wall. Among the three factors, the last two factors are determined by the manufacturing technology of the steel mesh, and we consider more about the former one. Because the laser stencil is very cost-effective, here we focus on the opening design of the laser stencil. First of all, we know aspect ratio and area ratio: Aspect ratio: the ratio of the opening width to the thickness of the stencil. Area ratio: The ratio of the opening area to the cross-sectional area of the hole wall, as shown in the figure below: Generally speaking, to obtain a good demoulding effect, the width-thickness ratio should be greater than 1.5, and the area ratio should be greater than 0.66. When to consider the aspect ratio and when to consider the area ratio? Usually, if the opening length does not reach 5 times the width, the area ratio should be considered to predict the release of the solder paste, and the aspect ratio should be considered in other cases. Here are some examples of openings for components:

Of course, when designing the opening of the stencil, one cannot blindly pursue the aspect ratio or area ratio while ignoring other process issues, such as tinning and tinning. In addition, for chip components above 0603 (1608), we should pay more attention to how to prevent tin balls. The above mainly talked about the opening design of the solder paste process stencil. Let’s briefly introduce the opening design of the glue process stencil (SMT template): Due to the characteristics of the glue, the experience value of the opening design is very important. The opening of the printed rubber stencil is generally a long strip or a round hole; two positioning holes should be opened for non-MARK point positioning. Remarks: The width W of the strip should be: 0.3mm≤W≤2.0mm

The thickness of the printed rubber stencil is generally selected from 0.15 mm to 0.2 mm. Tips for opening design of the stencil (SMT template):

1. For fine-pitch IC/QFP, in order to prevent stress concentration, it is best to have rounded corners at both ends; the same is true for BGA with square holes and 0402 and 0201 pieces.

2. The anti-tin ball opening method of chip components is best to choose the concave opening method, which can effectively prevent component tombstones.

3. When designing the stencil, the opening width should at least ensure that 4 largest solder balls can pass through smoothly.

4. Post-processing Etched and electroformed stencils generally do not undergo post-processing. The post-processing of stencils mentioned here is mainly for laser stencils. Because metal slag will adhere to the wall and opening after laser cutting, surface grinding is generally required; of course, grinding is not only to remove slag (burrs), but also to roughen the surface of the steel sheet. Increase the surface friction to facilitate the rolling of the solder paste to achieve a good tinning effect. If necessary, you can also choose "electropolishing" to completely remove slag (burr) and improve the hole wall.

5. Cleaning SMT stencils should be cleaned before, during, and after use (usually with SMT stencil cleaners): Wipe before use; regularly wipe the bottom of the stencil during use, To keep the stencil demolding smoothly; after use, it is necessary to clean the stencil in time, so that the same good demoulding effect can be obtained next time. Stencil cleaning methods generally include wiping and ultrasonic cleaning: Wiping Wipe the stencil with a lint-free cloth (or special stencil wiping paper) pre-soaked with detergent to remove cured solder paste or glue. It is characterized by convenience, no time limit, and low cost; the disadvantage is that it can not clean Hao stencils thoroughly, especially fine-pitch stencils. In addition, some printing machines have an automatic wiping function, which can be set to automatically wipe the bottom of the stencil after printing several times. This process also uses a special steel mesh to wipe the paper, and the machine will spray the cleaning agent on the paper before the action. Ultrasonic cleaning Ultrasonic cleaning mainly includes immersion type and spray type, and some manufacturers use a semi-automatic ultrasonic cleaning machine to clean the steel mesh. Choice of cleaning agent The ideal stencil cleaning agent must be practical, effective, and safe for people and the environment, and it must also be able to remove solder paste (glue) on the stencil well. Now there is a special stencil cleaning agent, but it may elute the stencil, so it should be used with caution. If there is no special requirement, alcohol or deionized water can be used instead of special cleaning agent for stencil.

The use of SMT stencil is a "delicate" precision mold, so when using it, you should pay attention to:

1. Handle with care; 2. Clean (wipe) the steel mesh before use to remove the dirt carried during transportation; 3. Stir the solder paste or red glue evenly to avoid blocking the opening; 4. Printing pressure Adjust to the best: the pressure when the scraper can just scrape off the solder paste (red glue) on the stencil is the best. 5. It is best to use board printing when printing; Stop for 2-3 seconds before demoulding, and the demoulding speed should not be too fast; 7. Do not hit the steel mesh with hard objects or sharp knives; 8. After the steel mesh is used up, it should be cleaned in time, and returned to the packing box, placed On a dedicated storage shelf.

There are mainly the following factors that will affect the quality of steel mesh:

1. Manufacturing process We have discussed the manufacturing process of stencil earlier, we can know that the best process should be electropolishing after laser cutting. Both chemical etching and electroforming have processes such as film making, exposure, and development that are prone to errors, and electroforming is also affected by uneven substrates.

2. The materials used include screen frame, wire mesh, steel sheet, adhesive glue, etc. The screen frame must be able to withstand a certain program of relay and have good levelness; it is best to use polyester mesh for the wire mesh, which can keep the tension stable for a long time; it is best to use 304 steel sheets, and the matte ones are better than the mirror ones. It is more conducive to the rolling of solder paste (glue); the adhesive must be strong enough and resistant to certain corrosion.

3. Opening design The quality of the opening design has the greatest impact on the quality of the steel mesh. As discussed above, the design of the opening should consider the manufacturing process, aspect ratio, area ratio, experience value, etc.

4. Whether the production materials are complete or not will also affect the quality of the stencil. The more complete the information, the better. At the same time, when data coexist, it should be clear which one prevails.

5. The quality of the stencil can be maintained by using the correct printing method. On the contrary, incorrect printing methods such as excessive pressure, uneven stencil or PCB during printing, etc. will damage the stencil.

6. Cleaning the solder paste (adhesive) is relatively easy to solidify. If it is not cleaned in time, the opening of the stencil will be blocked, and the next printing will be difficult. Therefore, after the stencil is removed from the machine or the solder paste is not printed on the printing machine for 1 hour, it should be cleaned in time.

7. Store the stencil in a specific storage place, and do not place it randomly, so as to avoid accidental damage to the stencil. At the same time, the stencils should not be stacked together, which is not easy to handle and may bend the screen frame.