I. Introduction

In the field of electronics manufacturing, solder paste printing is a critical step in surface mount technology (SMT), and its quality largely determines the quality of printed circuit board (PCB) products. According to relevant research, the solder paste printing process has a decisive impact on approximately 70% of PCB product quality in SMT production. The selection of a fully automatic solder paste printing machine is crucial, as it directly affects printing quality, efficiency, and cost, among other factors.

II. Key Points Related to Printing Process Parameters

(I) Pattern Alignment

Pattern alignment is a critical process in fully automatic solder paste printing machines. During this process, the printing machine camera centers the optical positioning points (MARK points) on the substrate and stencil on the worktable.  Subsequently, fine adjustments are made to the substrate and stencil in the X, Y, and Θ directions to achieve complete overlap between the substrate pad pattern and the stencil opening pattern. Accurate pattern alignment is considered to have a profound impact on printing quality. In the context of printing high-density integrated circuit boards, if there is a deviation in pattern alignment, serious problems such as short circuits will be directly triggered in subsequent soldering. This phenomenon is based on the close relationship between the printing process and subsequent processing steps of the circuit board; the accuracy of printing is fundamental to the smooth progress of subsequent steps.
 

(II) Squeegee and Stencil Angle

The angle between the squeegee and the stencil is of great importance to the injection of solder paste into the stencil openings. A smaller angle increases the downward pressure, which helps the solder paste to be injected into the openings, but it also has the disadvantage of causing the solder paste to be squeezed to the bottom surface of the stencil, leading to solder paste bridging. The common angle range is 45-60°, and most fully automatic and semi-automatic printing machines usually use 60°. This angle has been determined through long-term practice and effectively balances the effect of solder paste injection into the openings and avoids bridging problems.

(III) Solder Paste Volume (Rolling Diameter)

A solder paste rolling diameter ∮h of 13-23mm is generally considered suitable. When this value is too small, problems such as insufficient solder paste and incomplete printing are likely to occur. Conversely, when the solder paste volume is too large, at a constant printing speed, the excessive solder paste cannot form an effective rolling motion and cannot be completely scraped off by the squeegee. This can lead to poor demolding, excessively thick solder paste after printing, and other printing defects.  Furthermore, excessive solder paste exposed to the air for extended periods can negatively affect the solder paste quality. In production, to ensure the appropriate amount of solder paste, operators need to check the height of the solder paste strip on the stencil every half hour.  Any solder paste exceeding the length of the squeegee on the stencil should be moved to the front of the stencil and evenly distributed using a plastic squeegee.

(IV) Squeegee Pressure

Squeegee pressure is actually the depth of the squeegee's descent, and it is a key factor affecting printing quality. When the pressure is too low, the squeegee does not tightly contact the stencil surface. This not only increases the printing thickness but also leaves a layer of solder paste on the stencil surface, easily leading to printing defects such as bridging. Appropriate squeegee pressure ensures that the solder paste is evenly transferred through the stencil openings.  Excessive or insufficient pressure is detrimental to good solder paste printing.

(V) Printing Speed

Since the squeegee speed is inversely proportional to the viscosity of the solder paste, the speed needs to be reduced when printing narrow-pitch, high-density patterns. If the speed is too fast, the time the squeegee passes through the stencil openings will be too short, preventing the solder paste from fully penetrating the openings, easily resulting in incomplete solder paste formation or missed prints. Printing speed and squeegee pressure are related; reducing the speed is approximately equivalent to increasing the pressure, and appropriately reducing the pressure can improve the printing speed.

(VI) Printing Gap

The printing gap refers to the distance between the stencil and the PCB. Its size should be considered in conjunction with the amount of solder paste remaining on the PCB after printing. An appropriate printing gap ensures that the solder paste is accurately transferred to the PCB pads.  Too large or too small a gap will affect the amount and effect of solder paste printing.

(VII) Stencil and PCB Separation Speed

After solder paste printing, the instantaneous speed at which the stencil separates from the PCB is the separation speed, which is especially important in close-pitch, high-density printing. Advanced printing machines feature one (or more) small dwell processes when the stencil separates from the solder paste pattern, i.e., multi-stage demolding. This ensures optimal printing results. If the separation speed is too high, the solder paste's adhesion decreases, reducing the cohesion between the solder paste and the pad, causing some solder paste to stick to the bottom surface and opening walls of the stencil, resulting in printing defects such as insufficient printing and solder collapse.

(VIII) Cleaning Method and Frequency

Cleaning the bottom surface of the stencil is essential for ensuring printing quality. The cleaning method and frequency should be determined based on the solder paste, stencil material, thickness, and opening size (e.g., setting dry cleaning, wet cleaning, single pass, wiping speed, etc.). Because solder paste overflowing from the opening edges can contaminate the stencil, a reasonable cleaning method and frequency can promptly remove residual solder paste from the stencil, ensuring the quality of each print.

III. Equipment-Related Factors

(I) Image Recognition Method

The image recognition method of the positioning system is an important consideration, affecting printing accuracy and adaptability to different PCB boards. For example, when dealing with PCB boards with special surface materials or unclear MARK points, an excellent image recognition method can accurately identify and locate them, thus ensuring accurate printing.

(II) PCB Size and Thickness Adaptation Range

An ideal printing machine should be able to adapt to a wide range of PCB sizes and thicknesses, which is crucial for meeting the printing needs of various types of PCB boards. Whether it's a small, precision PCB board or a large industrial control PCB board, the printing machine needs to be able to operate normally.

(III) Self-Diagnosis System

A self-diagnosis system allows users to easily troubleshoot simple problems. For example, if a printing defect suddenly occurs during the printing process, the self-diagnosis system can quickly indicate possible problems such as scraper failure, vision system failure, or other mechanical component issues, thus reducing downtime.

(IV) Nitrogen Interface and Machine Sealing

The machine having a nitrogen interface is useful for printing environments with special requirements. Nitrogen can prevent solder paste oxidation during the printing process. Good sealing performance helps control some interfering factors during printing and is also a necessary condition under some special process requirements (such as preventing impurities from entering).

(V) Ease of Equipment Maintenance

From a machine structure perspective, ease of equipment maintenance is crucial. During the production process, if equipment malfunctions or requires regular maintenance, an unreasonable machine structure design may lead to prolonged maintenance time and increased maintenance costs. For example, if a large printing press has a complex internal structure and its parts are difficult to disassemble and install, it will create significant difficulties for maintenance work. The components within the equipment should have a reasonable spatial layout to facilitate cleaning, maintenance, repair, and replacement operations.

(Six) Overall Machine Structure Stability

Structures such as integrated workbenches can enhance the overall stability of the machine. A stable structure helps maintain accuracy during the printing process, especially during high-speed printing or long-term continuous production, preventing fluctuations in printing quality due to machine vibration and other factors.

(Seven) Stencil Cleaning Function

A variety of optional stencil cleaning methods can meet different production needs. For example, for production processes using different types or viscosities of solder paste, the appropriate cleaning method can efficiently clean the stencil, preparing it for the next printing cycle. Different cleaning methods, such as dry cleaning, wet cleaning, and ultrasonic cleaning, have different effects in dealing with different types of stencil contamination.

(Eight) MARK Recognition Vision System

An excellent MARK recognition vision system should be able to automatically search and locate, improving the efficiency of automated printing processes, especially when dealing with large-scale printing tasks of different types of PCB boards. This system requires high-quality visual acquisition and image recognition algorithms and should be able to adapt to different marking conditions on various PCB board surfaces.

(Nine) Ease of Operation

Simple and easy-to-use interactive operation can reduce operator training costs and the rate of operating errors. Printing presses with a concise operation interface and convenient and intuitive operation procedures can improve production efficiency and reduce the possibility of errors caused by cumbersome operations.

(Ten) Appearance Adaptability

Although the appearance of the printing press does not directly affect printing production, it is still a factor to consider. The appearance of the printing press should match the entire production line, achieving a harmonious and aesthetically pleasing look. Furthermore, if the appearance color can be selected according to the actual situation of the production line, it will be more conducive to the unified layout of the entire production environment.

(Eleven) Power and Air Supply Requirements

If the power and air supply requirements of the printing press are too stringent, it will place higher demands on the production environment, increasing production limitations and costs. For example, some machines require power supplies with special voltage and extremely high frequency stability, or require air sources with special pressure and extremely low moisture content. This may require the additional purchase of voltage stabilizers or gas purification equipment, causing unnecessary trouble for the company.

(XII) Cost-effectiveness Factors

Overall cost-effectiveness is an important factor for companies to consider. Companies should not only focus on the price level of the equipment at the time of purchase, but also consider the stability, maintenance costs, and replacement costs of vulnerable parts during long-term use. For example, some inexpensive printing machines may frequently malfunction during their service life, which will increase the total cost of ownership.

IV. Solder Paste Printing Machine Evaluation Indicators

(I) Printing Quality Related

1. Solder Paste Forming

The forming effect of solder paste is the primary indicator for evaluating printing quality. The finished solder paste should form an ideal shape on the PCB pads, being full, without collapse, and without bridging. For example, for the pads of electronic components with closely spaced pins, if the solder paste forming is poor, serious problems such as bridging (short circuit) between pins or insufficient solder paste leading to cold solder joints may occur, which directly affects the success rate and quality of subsequent electronic component placement and reflow soldering.

Many factors influence this, such as excessive squeegee pressure which may cause the solder paste to flatten, while excessive printing speed may cause incomplete solder paste formation or localized missing prints.

2. Solder Paste Thickness Uniformity

The thickness of the solder paste printed on the PCB pads should be uniform. Different electronic component pins or chip pins may have different requirements for solder paste thickness. For example, some high-power components may require a thicker solder paste layer to ensure good electrical connection, but if the solder paste thickness is uneven across the entire PCB board, it will not only affect the electrical performance during soldering, but may also lead to unreliable mechanical connections. Uniformity is affected by many factors, such as the performance of the squeegee (e.g., flatness of the squeegee material, uniformity of squeegee pressure, etc.), printing gap, and stencil aperture.

3. Solder Paste Positioning Accuracy

This indicator requires that the solder paste be printed accurately on its corresponding PCB pad, and the error must be within the allowable range. For chips or miniature electronic components with extremely small pin spacing, even a slight displacement of the solder paste can lead to incomplete coverage of the pins during soldering, resulting in soldering failure. This is closely related to the image recognition system, MARK point positioning accuracy, and the mechanical structure accuracy of the printing machine. For example, the higher the accuracy of the image recognition system, the more accurately it can obtain the positioning information between the PCB board and the stencil, thus ensuring the accuracy of solder paste positioning.

(II) Equipment Performance Related

1. Printing Speed ​​and Efficiency

The actual printing cycle of a fully automatic solder paste printing machine is an important indicator of its speed. Generally, the actual printing cycle of an automatic printing machine is usually (15-20) seconds, while high-speed production lines require less than 10 seconds. A shorter printing cycle means that more PCB boards can be printed per unit time, improving production efficiency. For example, in factories that mass-produce electronic products, the length of the printing cycle directly affects production capacity and delivery time.

Printing speed is limited by various factors, such as squeegee speed, separation speed between the PCB and the stencil, and image positioning correction speed. Increasing the squeegee speed may be limited by the viscosity of the solder paste, and if the positioning correction speed cannot keep up, excessively fast printing speed may lead to poor printing quality.

2. Repeat Printing Accuracy

The printing machine needs to maintain the same printing quality during multiple printing processes. This tests the number of prints under conditions of good printing consistency; the more prints, the higher the machine's repeatability. For example, in long-term continuous printing tasks or when printing the same type of PCB board multiple times, high-precision repeat printing requires the coordinated work of multiple parts of the printing machine, such as the visual positioning system, squeegee control system, and mechanical motion system. The accumulation of errors in any one link may reduce repeatability.

3. Equipment Stability

This includes mechanical structure stability and electrical system stability. Stable designs such as integrated workbenches in the mechanical structure can reduce vibration and shaking during the printing process, which is very important for ensuring printing accuracy. For example, during high-speed printing, if the machine structure is unstable, it may cause "ghosting" or displacement of the solder paste printing pattern.

Electrical system stability is reflected in stable power supply and normal and stable operation of the control system logic. If the electrical system fails, it may cause the printing machine to suddenly stop working, printing parameters to be incorrect, and other problems, affecting the continuity of production and product quality.

(III) Equipment Applicability

1. Applicability to Different PCB Types

With the continuous changes in PCB materials, shapes, thicknesses, and complexity, the printing machine should be able to adapt to various types of PCBs. For example, ceramic substrates and flexible substrates have different physical properties (such as flexibility, hardness, and surface finish) compared to traditional FR-4 substrates.  The requirements for printing parameters such as positioning and squeegee pressure also differ during the printing process. Therefore, the printing machine needs sufficient adaptability to handle these various types of PCBs.

This includes applicability to different printable elements on the PCB, including compatibility with printing large component pads to extremely small 0201 and 01005 chip components. This requires the printing machine's visual positioning system, squeegee adjustment system, and printing control system to have strong adaptability, enabling precise printing operations based on different printing elements.

2. Applicability to Different Solder Paste Types

Different types of solder paste (such as lead-free solder paste, silver-containing solder paste, etc.) may have different viscosities, particle sizes, flux compositions, etc. Therefore, the printing machine should be able to adapt to different solder pastes. For example, lead-free solder paste may have a higher viscosity, which requires the printing machine's squeegee system to provide sufficient pressure, and the printing speed and other parameters need to be adjusted accordingly to ensure smooth solder paste printing.

V. Comparison of Different Brands of Fully Automatic Solder Paste Printing Machines

(I) Well-known Foreign Brands

1. DEK (UK)

1) Printing Technology Advantages: It possesses advanced printing technology and performs exceptionally well in high-precision printing, especially for solder paste printing of ultra-small components. Its printing accuracy and repeatability are very high, meeting the solder paste printing requirements of tiny parts such as 01005, thanks to its high-precision mechanical structure and advanced visual positioning system.

2) Equipment Process and Design Level: The equipment is at a high-end level in terms of process and design. For example, it uses a precise casting structure, enhancing the stability of the equipment, and the structural layout is more compact and reasonable, facilitating maintenance. It also has a unique design in stencil release technology, enabling multi-stage release from the solder paste pattern, greatly improving the molding effect of solder paste printing.

3) User application feedback and evaluation: The equipment is widely used in large-scale electronics manufacturing companies and high-end electronics product manufacturing fields. Users report high equipment stability and reliable printing quality. However, the equipment price is generally high, posing a significant barrier for some companies with limited budgets, and the operation is relatively complex.