10 Things to Consider When Buying SMT Printer Machine

SMT Quick-Tips: Selecting a Stencil Printer

Robert Voigt, DDM Novastar

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How To Select A Stencil Printer

What is a Stencil Printer?

The first step in a paste, place, reflow assembly operation is the Stencil Printer, which can be manual, semi-automatic, or automatic. This machine dispenses solder paste, using a squeegee, forced over the openings in a stencil onto a printed circuit board.

PRODUCTION VOLUME

The first question to answer is: What production range are you dealing with? This will help you to decide what level of automation you’ll need.

• Up to 150 boards/day indicates a manual system, and will likely run about $2,000-$5,000 for a decent new machine
• Up to 500 boards/day is in the semi-automatic range, and will command in the range of $8,000-$14,000
• Over 500 boards/day is in the fully-automatic range, and can cost $30,000 or more depending on the bells and whistles that come with it.

Manual systems

Here, speed is typically not an issue. Fine pitch and accuracy are the most critical factors. Accuracy is determined by how securely aligned the circuitboard is in the machine. There are 4 dimensions to consider for the control method: X, Y, Z and Θ.

Once the board is aligned in the machine, paste is applied using a squeegee. While there are typically a couple of options on all systems, most assemblers will be using a manual squeegee. Some machines have a built-in guide that aids in applying the right amount of solder paste. There are several key points to applying paste properly:

• Angle of attack
• Squeegee pressure
• Squeegee speed

Once the paste is applied, the stencil is lifted off the circuit board, either vertically, or pivoted. There is no practical difference in either of the two lift methods.

Semi-automatic systems

In a moderately high volume environment, several of the functions can be automated to some extent, making the process more efficient, more repeatable, and allowing a higher yield at lower cost.

The following are typically found in semi-automatic stencil printers:

• Auto open/close for board loading and unloading, reducing operator fatigue
• Controlling the squeegee pressure
• Controlling the movement and speed of the squeegee
• Visual alignment/assist options to facilitate positioning

Fully automatic systems

Fig 1: Adjustable camera on vision machine

Here, everything is managed without operator intervention. Systems may include automatic board loading/unloading and auto fiducial alignment for X, Y, Z and Θ positioning. Again, you get what you pay for. If you’re running 2,000 boards/day, more automation may provide significant dividends in terms of quality yield, reduced labor costs, and more control over your process. If you’re spending the kind of money a fully automatic system commands, you will need to perform some serious due diligence on special capabilities that can’t be covered in a short article.

CONSTRUCTION

Regardless of the volume, you’re looking for the best accuracy and repeatability you can get, so once fixturing is set up, you don’t want to waste time recalibrating.

Fig 2: Machine ground holding plate

Accuracy and repeatability are directly related to fine pitch capability, and pitch accuracy is directly related to machine stability. For this reason, construction of the holding plates is critical.

Machines constructed with sheet metal are not nearly as stable as those made using a machine ground holding plate with welded frames. This assures that nothing moves from one board to the next so you can expect repeatable accuracy from board to board.

Another factor affecting print quality is the squeegee type. Squeegee blades are made from several materials, and come in a variety of sizes. Blade types are commonly urethane, metal (most popular), and plastic. Metal squeegee blades typically enable a more controlled print height across the board, and work well with a mix of component sizes.

Some users consider the stencil printer to be the most important purchase of all – consider this quote from the Adafruit forum:

“Spending money on a good stencil printer is a really good idea. You live and die by the quality of the solder paste deposit on your PCB.”

Check out DDM Novastar's wide range of Stencil Printer Machines.

Have questions or need help specifying a machine? Use our contact form for assistance.

VENDOR SUPPORT

Fig 3: View of squeegee

One of the most important aspects of this evaluation (for any SMT machine, frankly) will be support, and the best way to learn how a company treats its customers is by word of mouth. Talk to several customers to find out how happy they are with the machine, the seller, and the support they provide. Where is the manufacturing plant? Can they help troubleshoot software or alignment issues over the ? Do they offer field service? Do they have spare parts in stock for immediate shipment? Remember to ask your supplier about their older machines in the field, and if down the road, spare parts are available, and about their capability to customize a spare part if the machine becomes obsolescent.

Ask what the expected life-cycle of the product is. The industry standard is 7 years. Remember, there is a difference between a true manufacturer and an equipment supplier or distributor.

Double-sided boards or flexible circuits

Do you need a method to print both sides of the board? Check to see that the model you’re considering offers a double-sided nesting fixture, or vacuum fixture to hold down flexible circuits.

Ease of use

Every stencil manufacturer makes different size frames. The stencil printer you buy should be able to accommodate various size mounting frames and/or tubular frames. The manufacturer should also be able to provide adapters for frameless stencils.

Don’t overlook electrical requirements. Be sure the machine you buy will plug-and-play in your environment without new wiring or plan on an adapter/transformer.

Remember: The application of solder paste is the start of your process. It must be right because the rest of the process can’t make up for improper solder placement.

The first step in the PCB smt assembly process is to use an SMT printer machine for solder paste printing on pcb stencil. Usually, SMT printer refers to solder paste printer machine or solder printer. It is also called an SMT stencil printer. The SMT printer machine generally has three crucial elements: mesh/steel plate, printing material, and squeegee.  

1. Selection of steel plate for SMT printer machine

Thickness: The thickness of the metal sheet determines the thickness of the solder paste printed by the SMT stencil printer

Link to Ling Yueyang

Etched steel plate: you must pay attention to whether there is over-etching or under-etching

Laser opening: The roughness and burrs of the hole wall will affect the problem of the solder paste staying on the hole wall during the solder paste printer is demolding. More suitable for parts below 0.65mm pitch.

Tension: Check whether the tension of the steel plate on the frame is sufficient and flat.

Opening size and shape: Well-planned opening shape and size control will help welding quality after solder paste printer printing. For example the avoidance of empty soldering, short circuit, tin bead, and other problems.

Steel plate surface: Moderate roughness will help the solder paste roll effectively on the steel plate.

2. Selection of printing materials for SMT printer

1. Solder paste: Most flow soldering operations use a solder paste printing machine. The solder paste is a material mainly made of tin-lead particles mixed with flux in a certain proportion. When the solder printer heats the temperature to higher than the tin-lead alloy point, the solder particles melt to form a solder joint. 

2. Adhesive material: It is mainly used for wave welding, but it is also used to prevent parts from wandering with Reflow or fixing when the pick-and-place machine is shaking at high speed. The curing method and different materials are divided into solder paste printer printing and dispensing.

3. Scraper: The squeegee is the executor of the SMT printer machine. It pushes the printing material on the surface of the pcb stencil to complete the printing operation through the opening of the pcb stencil by rolling and squeezing.

3. The printing method of smt printer machine

Manual printing: 

In the PCBA manufacturing process,Manual printing is done by holding the scraper of the solder paste printer machine by hand, which is only suitable for less precise products and is used early when the equipment investment cost is high.

Semi-automatic printing: offline smt printer machine, which requires manual picking and placing of each printed PCB, and the main type of mechanical positioning.

Fully automatic printing: online operation, the solder paste stencil printer can automatically take and send printed PCB and has a visual alignment system and 2D/3D inspection function after printing. And solder paste stencil printer has more choices in various printing parameters. More advanced smt stencil printer still have functions, such as the automatic erection of steel plates.

4. Parameters of SMT printer machine

1. 1. Squeegee pressure: The main function is to closely combine the stencil of the smt printer machine with the PCB to achieve better printing results.

1. 2. Printing speed: Generally, the SMT stencil printer more slower, the solder printing effect more better. Therefore, the speed of the smt stencil printer can be increased when the solder paste is normally rolling, and the pressure can be adjusted. Because the speed of the SMT stencil printer is high, the pressure is low. On the contrary, the speed is slow, and the pressure is high.

1. 3. Printing angle: The angle of the SMT stencil printer will determine the pressure that flows into the opening of the stencil and the amount of solder printing.

1. 4. Gap: For the smt screen printer, a fixed gap is needed to make the force of its rebound leave the printed material on the substrate. But in terms of steel plate, the flatter the better, so as not to lose control of the thickness and solder printing volume.

5. several inspection key points of smt screen printer operations

• 1. Accuracy: The smt screen printer must be aligned with the center of the PAD without offset because the offset will cause problems such as inaccurate alignment and offset of tin ball parts.

• 2. Resolution: After smt printer machine printing, the shape must be similar to the structure of the tofu block so as not to be short with the adjacent PAD.

• 3. Solder printing thickness: Solder printing must be consistent to control the quality level of each solder joint

• 4. Inspection tools:

A. After smt printer machine prints, you can use a magnifying glass to check the resolution and accuracy
B. A microbalance can be used to measure the total amount of solder printing materials on the same PCB
C. The thickness of the solder paste after the solder paste printer printing can be measured with a Laser thickness gauge
D. AOI can be used to detect

E. Can use 2D/3D function detection on SMT stencil printer

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