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Jan 29, 2021

What are Stencils and How to Use Them?

Stencils are implements that operators use when assembling printed circuit boards (PCBs) with surface mount technology (SMT) components. Stencils help by disseminating the proper quantity of solder paste accurately on the PCB pads for soldering surface mount devices (SMD).

Earlier, when SMDs were larger, operators could easily solder them manually. With the increase in circuit density, and the corresponding reduction in the physical size of SMDs, it was more efficient to mount the tiny components with the help of automated machines. To solder these components effectively, assemblers now used the reflow process, which required mounting the components on PCB pads with a prior deposition of solder paste. Using stencils allowed depositing the right amount of solder paste accurately on the pads. This also means each PCB must have its dedicated SMT stencil.

Stencil and Component Assembly

For assembling a PCB with SMDs, solder paste must be present only on the metal landing pads where the component leads will make an electrically conductive contact. Manufacturers make PCB stencils usually from a thin stainless-steel sheet with specially designed openings to let the solder paste through.

The openings in the stencil correspond to and match the location of solder pads on the PCB. The operator aligns the stencil on the PCB and uses a runner to force the solder paste through the openings, depositing only the appropriate amount accurately onto the pads, with the stencil acting as a guiding template. After the solder paste deposition process, the operator removes the stencil and moves the board to the pick-and-place machine for component mounting.

An outer frame usually holds the stencil under tension, allowing the operator to lower the stencil onto a blank PCB and raise it up to remove the board after deposition of solder paste.

Characteristics of SMT Stencils

The size and shape of the stencil aperture determines the volume, precision, and uniformity of the solder paste that it deposits on the PCB substrate—requiring strict quality control of the opening for a successful design of the stencil. This is especially true when the operator must deposit only a small volume of solder paste with high precision. Two parameters of the stencil are most important here—the area ratio and the apparent ratio.

The area ratio is the ratio of the open area to the surface area of the aperture wall. The apparent ratio is the ratio of the width of the aperture to the thickness of the stencil. The measurements of the two ratios allow designers to determine the approximate size of the hole.

For proper solder paste release, designers keep the area ratio greater than 0.66 and the apparent ratio greater than 1.5.

Stencil Technology

Manufacturers make high-accuracy stencils out of thin sheets of stainless steel or nickel. This is usually an expensive process, and for low-accuracy boards, stencils made of polyester mesh and stencil foil are common. The stencil foil bonds to the polyester mesh and an aluminum frame holds the polyester mesh under tension.

For stencils made of stainless steel or nickel, the process of forming the apertures or openings helps to categorize them. Stencil manufacturers can use additive or subtractive processes for making the stencils. Most common process are:

  • Electroforming - additive process
  • Chemical Etching - subtractive process
  • Laser Cutting - subtractive process

Electroforming : is an additive process, and the manufacturer builds up the stencil by electroforming nickel. It is easier for form smaller apertures with finer pitch on electroformed nickel stencils. The aperture walls are very smooth and mirror-like, allowing good solder paste release as it does not cling to the sides.

The electroforming process allows nickel to form around a photo resist template of apertures matching the board design. The processing time is high and the initial cost of manufacturing is also high. However, the cost of the stencil is independent of the number of apertures it contains. Therefore, for stencils with large aperture counts, the electroforming process allows quick turnaround times.

Chemical Etching : is a subtractive process, where aperture formation takes place because of chemical etching. Although a relatively low-cost process, the results are rather inferior as the etching forms rough aperture sidewalls.

Chemical etching creates apertures into the metal sheet using acid. The manufacturing speed is usually high, and cost of manufacturing low. However, the method is suitable only for forming large openings, and there is a tendency to form sand clock shape on the walls. Chemical etching is unsuitable for forming stencils with fine pitch openings.

Laser Cutting : is also a subtractive process, where aperture formation takes place as a laser beam removes unwanted material from the stencil foil, usually stainless steel. Laser cutting helps form stencils with high quality and with a high degree of precision. Unlike the other two methods, laser cutting allows making the openings trapezoidal rather than vertical, thereby allowing good solder paste release.

Laser cutting helps achieve more consistent results, and the greater accuracy of the process allows creation of apertures with very fine pitch at reasonable costs. The relatively low manufacturing speed is its only disadvantage.

Manufacturers make laser cut stencils directly from the Gerber data of the PCB. This reduces the chances of image registration errors. The Gerber data directly drives the computational CNC laser cutter to form precise, repeatable holes. The laser beam creates apertures with a tolerance of ±0.05 mm, which remains constant irrespective of aperture size.

Framed and Frameless Stencils

Two types of stencils are in general use—framed types and frameless types—the choice largely dependent on the upfront costs and the storage space available.

Framed SMT stencils : these are mostly ready to use. The manufacturer mounts the stencil on a piece of mesh on one of its sides before fitting the combination to an aluminum frame. The mesh provides the stencil with the necessary tension. Frames may vary in thickness and style, dependent mostly on the type of printer in use.

Framed SMT stencils have a higher upfront cost as they do not require additional attachments and the operator can start using them as they are. The setup is simple, as the operator must only load it up in the printer and it is operational. The stencils are recyclable, especially the frame part.

Frameless SMT stencils :these are frameless only during storage. For use, operators must fit the thin foil stencil into a reusable frame. Therefore, the operator can use multiple stencils with a single frame. Frameless stencils are good for low volume production, where the operator must changeover quickly to another PCB within a short duration.

As storing requires handling only a flat foil, operators can store about four times the number of frameless SMT stencils compared to framed stencils in the same storage space. When working with several types of PCBs, frameless stencils are advantageous as the operator can changeover to another stencil easily and quickly. The operating and storage costs of frameless stencils is lower, even with the extra operational step of removing one stencil and fitting another.

Using SMT Stencils

Manual Operation

For low volume production, the operator uses a flat surface table to position the PCB conveniently. They use two L-shaped metal plates to bracket the board on two of its opposite corners to immobilize it. The operator then places the stencil and its frame on the PCB, aligning the fiducial marks on the stencil with those on the PCB. Once the operator has registered the stencil accurately on the PCB, he/she clamps the frame of the stencil to the table. The frame usually has a hinge to allow the operator to lift the stencil and replace the PCB underneath, without disturbing the registration. When the stencil is lying on the PCB, there is a minute gap between the bottom surface of the stencil and the top surface of the PCB.

StencilPrinting

Fig 1: StencilPrinting

With a fresh PCB under the stencil, the operator places some solder paste on the top edge of the stencil and uses a squeegee (a flexible stainless-steel blade) to drag the paste down the entire length of the stencil. The squeegee pressure stretches the stencil making it touch the top surface of the board. While moving down the stencil, the solder paste drops through the aperture onto the pads of the board. As the squeegee passes, the pressure on the stencil reduces, and it regains its position above the board—leaving the solder paste deposit on the board surface. Two factors are of importance here. The first is the angle the squeegee blade makes with the stencil and second is the pressure the operator applies while dragging the solder paste. Typically, the angle between the squeegee blade and the stencil is 15-20 degrees towards the direction of the blade movement, while the pressure must be light to medium and constant. The operator may also use a stencil printer for holding the PCB and the stencil.

Automated SMT Stencil Printing

For high volume production, PCB assemblers use automated SMT stencil printers. A conveyor mechanism passes through the printer to help in transporting PCBs. As a PCB enters the machine, optical detectors lock it into position. The stencil then drops down on the PCB and the machine uses optical sensors to register the fiducial marks on the stencil and the PCB.

A solder paste dispenser then delivers the optimum amount for the PCB and an automatic squeegee or runner drags the solder paste across the stencil. The machine automatically lifts the stencil away from the board, and even cleans the underside of the stencil of any leftover solder paste. The conveyor moves the PCB out of the machine, while bringing in the next blank board.

As the entire operation is automated, the angle of the squeegee and the pressure the machine applies while dragging the solder paste remain constant and repeatable. This ensures consistent performance.

Advantages and Disadvantages of SMT Stencils

Stainless steel stencils have the advantage of a long life. However, they have a disadvantage because of the printing being on-contact—there is a possibility of solder smear while lifting the stencil from the PCB. To prevent this, manufacturers offer stencils with a special coating on the underside and the aperture walls. The durable coating makes the stencil both hydrophobic and oleophobic, enabling it to repel solder paste. Such coatings improve solder paste transfer efficiency, especially for small apertures, and this results in higher yields while reducing rework.

Conclusion

A properly designed SMT stencil can make the process of solder paste deposition on the PCB consistent and repeatable. This in turn, allows greater consistency in the reflow soldering process, making solder joints more reliable.