Eliminate fluctuations in solder paste printing

Sealed solder paste roller

working principle

Pump structure

RheoPump System Benefits

The biggest cause of fluctuations in the solder paste printing process is the constant change of the solder paste floating on the stencil. When the solder paste is just dispensed or printed, the low-boiling solvent in the solder starts to evaporate, and the solder balls start to oxidize in the air. During shutdown, the solder paste becomes less fluid. In addition, with the use of solder paste, solder balls will be reduced. Insufficient solder paste will interrupt the printing process and cause blanks; excess paste on the blade will not fully fill the mesh.

The use of automated paste dispensing and programming of the quantity and frequency can minimize operator errors. However, this method is time-consuming and sensitive to air conditions. At the same time, there is still a considerable amount of solder paste exposed to the air.

Sealed solder paste roller

A new distribution technology, called RheoPump technology, includes a dedicated seal head that holds the entire solder paste roller. In this way, the solder paste will not roll on the printing stencil that is exposed to the air, and only those solder pastes that are in contact with the stencil will be coated on the coating point.

The standard solder paste cartridge continuously fills the solder paste with the solder paste roller, which pressurizes the print head and provides the power to fill the mesh. This rheology technology virtually eliminates the largest source of fluctuations in the solder paste printing process, allowing it to be printed continuously, either during downtime or during operation.

Since the 1970s, the basic rheological technology has been used in many other fields. It was only in the early 1990s that it began to study the use of this technology for solder paste printing on printed boards.

working principle

Through the study of the technology and subsequent data testing, equipment that can be used on the production site has recently been made. Consider a machine that uses a patented circular cavity design to eliminate "dead zones" and allow the paste to mix thoroughly before being fed by a computer-controlled solder paste. This allows 100% utilization of solder paste and easy cleaning of the cavity.

The squeeze theory states that when pressure and friction act on the solder paste ball, it will start rolling (see Figure 1). The rheological pump system applies pressure directly at the top of the printing chamber and, with the rolling of the solder paste, pumps the solder into the holes of the printing stencil (Figure 2). As the pump moves over the stencil, friction will cause the solder to roll into the solder paste cavity. This rolling action helps to mix the paste and maintain its working viscosity. The pressure applied in the solder paste cavity is directly transmitted to the surface of the printing stencil and the mesh.

Since the rolling action of the solder paste is not the only factor for forming the mesh filling, a faster printing speed can be obtained. When the solder paste passes through the mesh, the pump's cutting blades are always in contact with the stencil. Once the mesh is filled, the thin edge of the cutting blade cuts it away from the paste. The surface area of ​​the pad, the surface tension of the solder paste between the stencil and the printed board, the structure of the mesh, and the ratio of the sizes thereof determine the quality of the solder paste coating.

Pump structure

The rheological pump head is connected to a balance pressure control pylon with two quick disconnects for easy insertion and removal. This hanger structure is based on a balance-controlled printhead mechanism that allows for floating mounting on the stencil so that it can continuously adapt to changing surfaces on the PCB, assuring a consistent seal with the stencil throughout printing. The solder paste chamber provides a layered flow for the solder paste, which allows the solder paste to roll and mix and maintain the viscosity of the solder paste. Two standard Pyles paste cartridges deliver the solder paste into the chamber and maintain the solder paste level of the print chamber while providing pressure to help fill the mesh.

The printing stencil and the pump head are in contact with each other at the tips of the two shear blades so that the blades must maintain a constant thin edge. According to long-term life test data, nickel-plated spring steel blades were used. The solder spacers at the ends of the shear blades keep the solder paste in the print area, eliminating waste.

The position of the top of the printing plate is determined by a touch sensor. With programmable pressure, the pump head is loaded against the stencil so that the blade rests firmly on the stencil and maintains a proper seal. The downstop holds the pump head down through the edge of the board for best print clarity.

A closed-loop air pressure regulator is used to regulate the pressure of the solder paste pumped into the stencil aperture. These regulators apply a constant pressure to the two paste cartridges, which then pressurize the solder paste chamber. The pump is lowered to its printing position and the solder paste contacts the stencil between the two shearing blades. Between the two printings, the solder paste is held in contact with the stencil at a position slightly higher than the lower stopper, so that the solder paste is sealed in the solder paste chamber to prevent leakage.

This pump head design has many advantages over conventional trailing edge extrusion blades and existing "rheological" printing techniques. Rheological technology is the next stage in the development of solder paste printing. The reduction of process fluctuations opens up a larger process window. This will further reduce process fluctuations and improve productivity and machine utilization because it no longer needs to be accompanied with process fluctuations. The system also improves efficiency and reduces costs.

RheoPump System Benefits

Rheological pump technology not only provides consistent, consistent printing, but also helps cut costs. For example, Speedline Technology's RheoPump system uses a unique touch sensor to improve stencil surface measurements, coupled with its balanced console system, to ensure a consistent seal on the surface of the stencil. The system has the following advantages:

Excellent Solder Paste Management The solder paste is completely enclosed and is not exposed to air flow, extending the solder paste's stencil life. The low-boiling solvent in the solder paste does not evaporate, so that the performance of the solder paste remains unchanged.

Save a lot of solder paste costs

Assume that the solder paste has a stencil life of six hours, an average of 250 grams of solder paste floating on the stencil, three shifts per day, and removal of 1 kilogram of solder paste. For each gram of solder paste at $0.1, you can save $100 per printer per day and $36,500 per year.

Reduced hazardous waste

The improved solder paste management reduces the amount of solder paste waste. For example, waste generated when the solder paste emerges from the edge of the press roll. As mentioned above, the cost of removing these wastes is very high. Reduced downtime

In the printing operation, since the pump system adopts a continuous feeding method, the shutdown due to the distribution or artificial addition of the solder paste is eliminated. With more than one paste cartridge, the number of cartridge changes will be reduced.

Accelerate product conversion

Since the template does not require cleaning of the solder paste, the conversion of the product is accelerated. The shear action of the pump eliminates the excess paste that may normally remain on the stencil.

Minimize lead pollution

Due to the reduction of cleaning and the reduction in the number of solder paste cartridge replacements (over the entire service life), the contamination of the human body by the solder paste and the lead it contains is reduced.

Adaptability to solder paste/container

Because the standard Pyles solder paste cartridge is used in the RheoPump system, your solder paste can be directly adapted to the pump system. No special packaging or changing of materials is required. The cartridges make maximum use of the solder paste in the container, and the solder paste contained in the substitute container may waste 30% to 50%. This saves the cost of confirming a new solder paste or specifying a different type of solder paste container than is currently required.

Improves solder paste's dynamic characteristics

The RheoPump system uses a unique circular solder paste cavity to enhance solder paste roll. Closed-loop programmable solder paste pressure applied to the stencil and adjustable lower stopper parameters allow the system to be flexibly adapted to a particular printing process.

Reduced cycle time

This system essentially allows for higher speeds than current standard rolling blades. Faster blade speeds mean less cycle time, plus reduced downtime and increased solder paste management performance for higher productivity.

Fewer manual operations due to the seal formed between the cutting blade and the stencil do not require supplementing the overflowed paste along with a conventional rolling blade.