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How much do you know about the factors that affect the quality of PCB_SMT laser steel mesh?
2021-08-20
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Author:IPCB

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


1. Production process We have discussed the production process of steel mesh before, we can know that the best process should be electropolishing after laser stencil cutting. Both chemical etching and electroforming have processes that are prone to errors such as filming, exposure, and development, and electroforming is also affected by the unevenness of the substrate.


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


3. Opening design The quality of the opening design has the greatest impact on the quality of the PCB stencil. As discussed earlier, the design of the opening should consider the manufacturing process, the aspect ratio, the area ratio, and the empirical value.


4. Production data The completeness of production data will also affect the quality of PCB steel mesh. The more information, the better. At the same time, when the data coexist, it should be clear which shall prevail. Also, generally speaking, making stencils with data files can reduce errors as much as possible.


5. How to use The correct printing method can maintain the quality of the stencil. On the contrary, incorrect printing methods such as excessive pressure, PCB stencil or PCB not level during printing, etc., will damage the stencil.


6. Cleaning The solder paste (glue) is easier to solidify. If it is not cleaned in time, it will block the opening of the stencil, 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. The storage of steel mesh should be in a specific storage place and should not be placed randomly, so as to avoid accidental damage to the steel mesh. At the same time, the PCB steel mesh should not be stacked together, so that it is not easy to handle and may bend the screen frame.


Introduction to Steel Mesh:


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


The development of SMT process: SMT steel mesh (SMT template) is also used in glue process. [1] 2 Steel mesh evolution Steel mesh was originally made of wire mesh, so it was called a mask at that time. It started with nylon (polyester) mesh, later due to durability, there were wire meshes, copper meshes, and finally stainless steel meshes. However, no matter what kind of wire mesh is, it has the disadvantages of poor molding and low precision.


With the development of SMT, the requirements for stencils increase, and stencils are produced. Affected by the cost of materials and the difficulty and ease of production procedures, the original steel mesh was made of iron/copper plate, but also because of easy corrosion, stainless steel mesh replaced them, which is the current steel mesh (SMT Stencil).


Steel mesh classification


According to the production process of SMT steel mesh, it can be divided into: laser template, electropolishing template, electroforming template, step template, bonding template, nickel-plating template, and etching template. Laser template (LaserStencil) Laser stencil template is currently the most commonly used template in the SMT steel mesh industry. Its characteristics are: Direct use of data files for production, reducing production errors; SMT template opening position accuracy is extremely high: the overall error ≤±4μm ; The opening of the SMT template has a geometric pattern, which is conducive to the printing and forming of the solder paste. The following materials are required to make laser stencil:


1. PCB 


2. Data file information must: PCB: the correct version, no deformation, damage, or fracture; Data file: Wei Chuangxin Steel Mesh (SMT template) processing group can accept a variety of CAD data formats: GERBER, HPGL, *.JOB, * .PCB, *.GWK, *.CWK, *.PWK, *.DXF, *.PDF; and the following software design data: PAD2000, POWERPCB, GCCAM 4.14, 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 it must also contain a character layer Data in order to check the front and back of the data, component category, etc. Below we focus on the GERBER file; the GERBER file is a data format proposed by the US GERBER company; it converts PCB information into electronic data that can be recognized by a variety of light drawing machines, also known as light drawing files. GERBER file is a software structure with X, Y coordinates and additional commands. The official scientific name of GERBER format is "RS274 format". It has become a standard format file in the PCB industry. The GERBER file combined with the Aperture list (also known as D-Code) file defines the starting point of the graph and the shape and size of the graph. D-Code defines the size and shape of lines, holes, pads or other patterns 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 template (E.P.Stencil) Electropolishing template is after laser cutting, through electrochemical method, the steel sheet is post-processed to improve the wall of the opening hole. Its characteristics are: 1. The hole wall is smooth, which is especially suitable for the ultra-fine pitch QFP/BGA/CSP 2. The number of wiping of the SMT template is reduced, and the work efficiency is greatly improved. Electroforming template (EFStencil) In order to meet the requirements of short, small, light and thin electronic products, ultra-fine volume (such as 0201) and ultra-fine pitch (such as ūBGA, CSP) are widely used. As a result, the SMT steel mesh industry Higher requirements were also put forward for printing templates, and electroforming templates came into being. The characteristics of the electroforming template produced by our company are: Different thicknesses can be made on the same template. Step Stencil (StepStencil) Due to the different requirements for the amount of solder paste when soldering various components on the same PCB, the thickness of some areas of the same SMT template is required to be different, which leads to the STEP-DOWN&STEP-UP process template. STEP-DOWN template Partially thin the template to reduce the amount of tin when soldering specific components. Bonding Stencil (BondingStencil) The COB device has been fixed on the PCB, but the patch process of tin printing is still required, which requires the use of the Bonding template. The bonding template is to add a small cover at the PCB bonding position corresponding to the template to avoid the COB device and achieve the purpose of flat printing. Nickel-plated template (Ni.P.Stencil) In order to reduce the friction between the solder paste and the hole wall, facilitate demolding, and further improve the release effect of the solder paste, at the beginning of 2004, Wei Chuangxin Company used the traditional subtractive process "Electropolishing". "On the basis of the template, a special post-processing addition process-"nickel plating" has been added, which has been patented. The nickel-plated template combines the advantages of the laser template and the electroforming template. The etching template is made of 301 type steel sheet imported from the United States. The etching steel mesh is suitable for PCB printing with corners and spacing greater than or equal to 0.4MM. It is suitable for copying and film use. CAD/CAM and exposure methods can be used at the same time, depending on the difference. The parts can be scaled, no need to calculate the price according to the number of parts. The production time is fast. The price is cheaper than the laser template. It is convenient for customers to file the film.


 Production process


The production process of steel mesh includes: chemical etch, laser cutting, and electroform.


 1. Chemical etch process: data file PCB→film production→exposure→development→etching→steel cleaning→sheet Features: one-time molding, faster speed; low price. Disadvantages: easy to form an hourglass shape (not enough etching) or larger opening size (excessive etching); objective factors (experience, medicine, film) have a large impact, many production links, large cumulative errors, not suitable for fine pitch steel mesh production method ; The production process is polluted, which is not conducive to environmental protection.


2. Laser cutting process: film making PCB→acquisition of coordinates→data file→data processing→laser cutting→grinding→netting. Features: high precision of data production, small influence of objective factors; trapezoidal opening is good for demolding; Can be used for precision cutting; the price is moderate. Disadvantages: cutting one by one, the production speed is slow.


3. Electroform process: coating photosensitive film on the substrate → exposure → development → electroforming nickel → forming → steel sheet cleaning → netting. Features: smooth hole walls, especially suitable for 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. 5 Opening design The opening design of the stencil should consider the mold release of the solder paste, which is determined by three factors: ①The width to thickness 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 latter two are determined by the manufacturing technology of the steel mesh, and we consider the first one more. Because the laser steel mesh is very cost-effective, here we focus on the opening design of the laser steel mesh. First of all, we understand the aspect ratio and area ratio: Width-thickness ratio: the ratio of the opening width to the thickness of the steel mesh. Area ratio: The ratio of the opening area to the cross-sectional area of the hole wall, as shown in the following figure: Generally speaking, to obtain a good demolding effect, the width to 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? Generally, if the length of the opening does not reach 5 times the width, the area ratio should be considered to predict the release of the solder paste. In other cases, the aspect ratio should be considered. The following are some examples of component openings:

ATL

ATL

Of course, when designing the opening of the steel mesh, one should not blindly pursue the aspect ratio or the area ratio while ignoring other process issues, such as continuous tin, multiple tin, and so on. In addition, for chip components above 0603 (1608), we should think more about how to prevent tin beading. The above mainly talks 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 its characteristics, the experience value of the opening design is very important. Generally, the openings of the rubber-printed steel mesh have long strips or round holes; when positioning non-MARK points, two positioning holes should be opened. Remarks: 1. The width W of the long strip should be: 0.3mm≤W≤2.0mm 2. The diameter of the circular hole is:

ATL

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


 1. 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 and 0402, 0201 pieces with square holes.


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


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


4. Post-processing Etching and electroforming steel mesh generally do not do post-processing. The steel mesh post-processing mentioned here is mainly for laser steel mesh. Because metal slag will adhere to the walls and openings after laser cutting, surface polishing is generally required; of course, polishing 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 and achieve a good soldering effect. If necessary, you can also choose "Electropolishing" to completely remove the slag (burr) and improve the hole wall.


5. Cleaning SMT steel mesh should be cleaned before, during, and after use (usually cleaning with SMT steel mesh cleaning machine): Wipe before use; wipe the bottom of the steel mesh regularly during use, In order to keep the stencil demoulding smooth; clean the stencil in time after use, so that the same good demoulding effect can be obtained next time. The stencil cleaning methods generally include wiping and ultrasonic cleaning: Wiping Wipe the stencil with a lint-free cloth (or special stencil wipe paper) pre-soaked with detergent to remove the solidified solder paste or glue. It is characterized by convenience, no time limit, and low cost; the disadvantage is that it can incompletely clean the steel mesh, especially the densely spaced steel mesh. In addition, some printers have an automatic wipe function, which can be set to wipe the bottom of the stencil automatically after a few times of printing. This process also uses a special steel mesh to wipe the paper, and the machine will spray cleaning agent on the paper before the action. Ultrasonic cleaning There are two main types of ultrasonic cleaning: immersion and spray. Some manufacturers use a semi-automatic ultrasonic cleaning machine to clean the steel mesh. The choice of cleaning agent The ideal stencil cleaning agent must be practical, effective, and safe for people and the environment. At the same time, it must be able to remove the solder paste (glue) on the stencil well. There is a special stencil cleaning agent, but it may wash out the stencil, so you should be cautious when using it. If there is no special requirement, alcohol or deionized water can be used instead of special cleaner for steel mesh.


The use of SMT steel mesh is a "squeaky" precision mold. Therefore, you should pay attention to:


1. Handle with care; 2. Clean (wipe) the stencil before use to remove the dirt carried during transportation; 3. Stir the solder paste or red glue evenly to avoid clogging the openings; 4. Printing pressure Adjust to the best: The pressure when the squeegee can just scrape off the solder paste (red glue) on the stencil is best. 5. It is best to use board printing when printing; 6. After the squeegee stroke is completed, it is best if possible. Stop for 2 to 3 seconds before demolding, and the demolding speed should not be too fast; 7. Do not hit the steel mesh with hard objects or sharp knives; 8. Clean the steel mesh in time after using it, return it to the packing box, and place it in the box. On a dedicated storage shelf.


Quality impact


The following factors will affect the quality of steel mesh:


1. Production process We discussed the production process of steel mesh. We can know that the best process should be electropolishing after laser cutting. Both chemical etching and electroforming have processes that are prone to errors such as filming, exposure, and development, and electroforming is also affected by the unevenness of the substrate.


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


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


4. Production data The completeness of production data will also affect the quality of the steel mesh. The more information, the better. At the same time, when the data coexist, it should be clear which shall prevail. Also, generally speaking, making stencils with data files can reduce errors as much as possible.


5. How to use The correct printing method can maintain the quality of the stencil. On the contrary, the incorrect printing method, such as excessive pressure, the stencil or PCB is not level during printing, etc., will damage the stencil.


6. Cleaning The solder paste (glue) is easier to solidify. If it is not cleaned in time, it will block the opening of the stencil, 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. Storage The steel mesh should be used in a specific storage place and should not be placed randomly, so as to avoid accidental damage to the steel mesh. At the same time, the steel mesh should not be stacked together, so that it is difficult to handle and may bend the mesh frame.