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SMT Stencil: The Ultimate Guide Is Here

Posted:03:01 PM January 03, 2023 writer: NextPCB

Stencil PCB is a stainless steel foil on which laser-cut holes correspond to component pins on the PCB's surface. It plays a vital role in providing solutions in manufacturing industries. After the proper alignment of the board on the PCB stencil, you can pull solder paste through the Stencil to fill in the Stencil's holes. The PCB stencil's sole purpose is to transfer wet lead to the bare board. The amount of damp information entering the board determines by the thickness of the stainless steel foil and the opening size. This SMT stencil technology has high scope due to its speed, reliability, and low cost compared to other primitive methods.

What is SMT Stencil?

Surface Mount Technology, also known as SMT, produces electrical components that involve mounting the components directly on the surface of a Printed Circuit Board (PCB). SMT's relevance has gone a long way toward replacing the previous electronic component production format, Through-Hole Technology.

With an SMT or PCB stencil, you can evenly spread solder paste on a circuit board before selling the PCB components. Previously, the board's stainless steel foil created an opening for each part attached to it. When you remove the stainless-steel foil from the board, the solder paste remains, and you can adhere it to the circuit board using the SMT stencil. Surface Mount Devices are devices manufactured by SMT technology.

SMT Stencil

The function of a Surface Mount Machine

Pick-and-place machines or SMT Component Placement Systems are other names for Surface Mount Machines. Surface Mount Devices (SMDs) places on the Printed Circuit Boards by these robotic machines (PCBs).

Because of their high precision, numerous use cases/applications, and wide range of electronic components that imply on them, these machines are widely used.

Types of SMT Stencil

1. Based on Manufacturing

Based on the manufacturing, it can be chemical etching, laser cutting, and electroforming. These include the below-mentioned terms:

Chemical Etching

This procedure entails first masking a specific area with a protectant before removing materials (metal). You can immerse the cloth in acid or etchant. As a result, it will aid in extracting any material the photoresistor protectant did not cover. It also has tight tolerances, making it ideal for creating apertures for hybrid and step stencils.

Laser Cutting

It is a thermal process that melts material in a specific area using a focused laser beam. You can create an aperture by focusing the laser machine on a stencil foil. It is, without a doubt, a subtractive procedure, but the result is always precise and produces a high-quality cut. That is why it is one of the most commonly used procedures.

Electroforming

It entails creating stencil parts by depositing material in bits. As a result, you will have a replica. Nonetheless, this process is notable for its complexity and high repeatability. As a result, electroformed stencils produce thicknesses ranging from 2 to 7 mils and increments of 0.01 mil. Because there is no burr, there is no need for polishing or buffing with this method. It also has a low coefficient, which reduces squeegee wear.

Furthermore, compared to chemical-etched and laser-cut processes, this one has the best paste release. Again, large apertures for stencils electroformed without metal distortion or heat transfer.

It features ultrafine pitch apertures and pastes transfer efficiencies of 95% or higher. As a result, you can use this precise and controlled process to achieve custom thickness and smooth walls.

2. Based on the Solder Paste Application

Based on the solder paste application, you can find the mentioned types as explained below:

Frameless Stencils

This solder paste SMT stencil is ideal for use with stencil tensioning systems.

Some of the benefits of this tool are:

  • You can use this tool for hand printing, paste printing, and prototyping.
  • However, it is important to note that you cannot permanently attach this Stencil to a frame Since it has relatively smooth aperture walls.
  • Furthermore, the frameless stencils have small grain structures that improve solder paste transfer from the aperture to the printed circuit board.
  • This tool is also less expensive than the framed Stencil.

Prototype Stencils

If you use manual solder paste, you should think about using prototype stencils. Moreover, this type of Stencil works according to the style you specify with a Gerber or CAD file. Then you can place it over a PCB and hand-print your design.

Some of its benefits can be:

  • The Stencil has alignment tools that allow for a simple printing process.
  • It is hand-printed precisely. As a result, the possibility of errors gets reduced.
  • The device is inexpensive and produces effective design results.
  • It requires less time to assemble prototypes.
  • The squeegee blade allows for accurate application.
  • Its temperature marker allows for precise heating.
  • This Stencil is useful if you print a temporary test product before creating a permanent design.

Framed Stencils

This SMT stencil also functions as a glue-in stencil with a permanent frame. Furthermore, the frame or mesh border provides maximum precision by stretching the foil.

The following advantages come with this framed Stencil:

  • It has double-bonding and long-lasting engineering. As a result, the tool can be used for high-wear and high-volume printing applications.
  • Because of its taut and smooth aperture, the tool produces accurate prints.
  • Framed stencils are extremely durable. As a result, it can withstand multiple uses.
  • The framed stencils are ideal for high-volume production. It can also assemble a large number of PCBs over a long period.
  • It provides a long-lasting fiducial layout.
  • It includes a pre-assembled product that uses in a screen printer immediately.

3. Based on the Material of Construction

Based on material constructions, some terms are as below:

Mylar and Kapton SMT Stencils

Laser etching Mylar and Kapton materials make these stencils.

  • You can use this Stencil can for prototyping.
  • It also provides optimal performance when hand soldering.
  • Furthermore, the stencils are less expensive than stainless steel stencils.

Stainless Steel SMT Stencils

This PCB stencil with stainless steel is useful for prototyping. Furthermore, the tool allows users to meet soldering requirements while reducing errors. It is also of higher quality than Mylar and Kapton stencils.

The SMT Stencil Printing Method

Manual Printing

Manual printing can produce better results in low-volume production. When manually printing with an SMT Stencil, there are three major steps.

  • Initially, the aperture-fill process comes into practice, which fills the aperture with solder paste.
  • Secondly, the paste transfer process is responsible for transferring the paste accumulated in the aperture to the PCB surface.
  • Finally, the position of the deposited paste of the aperture concerning the squeegee blade can affect the filling process.

Automatic Printing

An automated SMT stencil printer is more efficient than others when producing large quantities. PCBs through the printer using automation via a conveyor belt system transports.

  • After that, just the right amount of solder paste dispenses for the PCB, and drags across the Stencil with an automatic squeegee.
  • Following that, the machine automatically lifts the Stencil away from the board and removes any excess paste from the Stencil's underside.
  • The PCB removes from the machine, so the process repeats with the next blank board.

Advantages of SMT Stencils

The primary advantages of SMT over through-hole are as follows:

  • Reduce the size of the board's components. You can install the smallest detail with SMT technology is 0.1 x 0.1mm.
  • The ability to connect components is relatively high per component. Furthermore, the connection density is higher because the holes do not obstruct the routing space on the inner layers.
  • Circuit board components have mounted on both sides.
  • Compared to the other stencils, its error problem is when the surface tension of the solder melts and minor errors in component placement with automatic correction. Align the components with the solder pad by dragging them.
  • Mechanical performance improves in shock and vibration environments.
  • Resistance and inductance versus weld are lower in SMT stencils than in others, which results in fewer unwanted RF signal effects and improved high-frequency performance.
  • The number of holes drilled is lower than the other.
  • Due to fully automated execution, the degree of specialization archives at a 100% rate.
  • Simpler, automated assembly.

Disadvantages of SMT

SMT technology requires much greater attention to detail than through-hole assembly.

  • Even though the process automates, solder paste is always a major issue as solder holes become smaller and require less solder. It leads to the next issue: welds damages due to the thermal cycling of the potting compounds.
  • Furthermore, when deploying SMT technology, enterprises will incur significant investment costs.
  • Because of the SMT's small size and narrow spacing, assembling/repairing components with many components is more difficult, necessitating skilled operators and more expensive tools.
  • For each prototype, SMTs don’t use directly with plug-in breadboards, necessitating either a custom PCB or the mounting of the SMD on a pin-leaded carrier.

Design Considerations for Stencil

Stencil Thickness

Stencil Thickness

This factor contributes to a high-quality solder joint because the stencil thickness determines the quality of solder paste released from apertures.

To determine the appropriate stencil thickness, use the formulae below:

$$Aspect\;Ratio = \frac {Aperture\;Width (W)} {Stencil\;Thickness(T)} > 1.5$$

$$R = L \times \frac{W}{2 \times (L + W) \times T} > 0.66$$

Where,

R is the Area ratio

T is the thickness of the stencil foil

W - the width of the pad

L - length of the pad

You can also calculate the theoretical solder paste volume by multiplying the aperture area by the thickness of the Stencil.

Aperture Design

The general rule is that your stencil opening should be smaller than your PCB pad size. In addition, your PCB pad area should be greater than two-thirds the size of your internal aperture wall.

Stencil Materials 

The solder paste will exit the apertures depending on the stencil material you use.

Typically, you can use the most common material—stainless steel. However, if your project requires delicate pitch devices, you can use Nickel, a more expensive option. Furthermore, place your apertures away from the step edge if you want a smooth print.

Stencil Alignment

If you want your PCB pads to have a perfect solder paste print, add registration marks to your Stencil and PCB. As a result, the fiducial patterns will direct your alignment process.

PCB Design Specific

You can modify your PCB design to address any issues that may arise.

Create a window effect, for example, if your board design has a large copper pad underneath and you cannot apply solder paste.

Create stencil apertures if you have vias in your home that could affect your solder paste. Solder paste will not act on your vias this way.

Foil Thickness of SMT Stencil

The foil thickness explains by the components installed on the PCB circuit. Smaller packaged components, such as 0603 capacitors or 0.020 pitch SOICs, will necessitate a thinner stencil than larger pickings, such as the 1206 capacitor or 0.050 pitch SOIC. The thickness of PCB stencils ranges from 0.0254mm to 0.762mm. Most boards have foil thicknesses ranging from 0.1016mm to 0.1778mm.

Manual soldering was the norm back when through-hole components dominated electronic design. Attaching the component pins to the board is simple, with a soldering iron and flux. Through-hole parts push aside by smaller competitors, SMD components when devices can fit in a pocket or on the wrist.

These small components are more difficult to solder by hand, so if you have many models to assemble, you will need a faster way to get the details onto the board. To put all the SMD components on the board faster, you should learn about solder paste stenciling.

Making of SMT Stencils

Manufacturing Processes

Precision Etching Technology

  • Accurate and cost-effective solution locally by reducing stencil thickness to create recesses or raised areas.
  • The photo etching method, also known as 'photo chemical,' is a subtractive process using chemical action to selectively remove metal with a highly precise and cost-effective method of producing burr- and stress-free parts and tools in virtually any metal.
  • Almost any metal is useful in Tecan's Photo Chemical Machining (PCM) process. The following metals are the most commonly used for stencil production:
  • Stainless steel alloys 302, 304, and 430
  • Steel with a fine grain
  • Steel with a semi-fine grain
  • Nickel

You can see multi-level stencils under the precision method.

Multi-level stencils

  • The Photo Chemical Machining technique uses to create multi-level stencil features. Stepped stencils make by removing an isolated depth from the squeegee side of the Stencil while leaving the general thickness alone.
  • To make stencils with raised areas on the squeegee surface, remove most of the top surface to leave raised islands.

Laser Cut Stencils

It is useful for component pitches as small as 0.3mm due to improved dimensional accuracy achieving the results intended.

A 1.5° - 2° taper from the squeegee side to the release side provides the necessary trapezoidal aperture geometry to achieve consistent printed deposits for finer-pitched components.

Because the stencil apertures create sequentially using laser technology, stencils with higher gaps take longer to complete. A fine beam is used to cut stencil apertures, first from inside the crack (1) to its boundary (2), then tracing around (3 - 5) until the beam passes the point where it first met the limit. The resulting metal shape places the vacuum tray below.

The definition of the aperture wall affects the paste release and smoothness; cutting speed is critical.

Laser-formed Stencils

  • The stencil material is Nickel electroformed, and the apertures are laser cut. Fine-grain steel uses as a base material to improve the smoothness of the aperture wall.
  • Laser-formed stencils are a hybrid technology that combines the precision of laser-cut apertures with Nickel's enhanced paste release properties. To achieve optimal print deposit consistency, you can combine improved paste roll activation and multi-level technology with exceptionally quick turnarounds.
  • Fine-grain steel is a foundation for stencils with thicknesses up to 0.250mm.
  • When laser cutting stainless steel materials, trace elements liberate to the aperture walls to ensure that the relative wall roughness does not impede solder paste transfer.

It results in significant improvements to the smoothness of the aperture wall, allowing for better-printed deposits.

Design Rules of SMT Stencils

  • The size and shape of SMT stencil apertures determine the volume, uniformity, and definition of the material deposited onto substrates.
  • Aperture sizes can be determined using measures such as area ratio.
  • Each Stencil manufacturing technique considers its merits when designing apertures following these rules.
  • When the Stencil separates from the substrate during printing, competing surface tension forces determine whether the solder paste transfers to the pad on which it was printed or remained adhered to the stencil aperture walls.
  • Laser-cut apertures that have been electro-polished and electroplated during manufacturing promote improved paste transfer efficiency.
  • The final aperture dimensions determine how much solder paste prints on the PCB Stencil, which has three dimensions: length, width, and height.
  • The dimensional accuracy of stencils and printing positional accuracy determines the quality of the transferred cad data, the methods and technology used to manufacture the Stencil, and the operating conditions.

Stencil Design Considerations

Some terms to consider are Aperture Size vs. Pad Size, Aperture Shape, Stencil Thickness, Adhesives Printing, SMT Stencil Manufacturing Method, Stencil Thickness, Aperture Design, and other factors.

Aperture and pad dimension

  • The finer-pitch aperture openings should be slightly smaller than the size of the landing pad.

Aperture Shapes

  • The size of the stencil aperture determines the shape of the SMT.

The thickness of the Stencil

  • The thickness of the Stencil, or "foil," is an important aspect of the design. The relationship between pad size, aperture opening, and foil thickness influences optimal paste deposition onto a PCB.
  • While the aperture for a pad may be of proper size, a stencil that is either too thin or too thick may result in suboptimal solder paste deposition.

Cleaning Process of SMT Stencil

Professional cleaning equipment uses typically to clean SMT stencils. The smart stencil cleaning machine is currently the market's mainstream, aimed at the electronic industry's SMT stencil, copper stencil, wire stencil, microporous Stencil, and wafer trays. Similar stencil cleaning equipment uses compressed air as energy, does not require electricity, and does not pose a fire hazard.

It has a human-like design and one-button operation to assist with cleaning and drying; it operates with high-performance automatic air pressure. Cleaning fluid is recycled with minimal loss in cleaning equipment. In more detail, you can find some techniques for cleaning below:

a) First Step

Cleaning requirements under normal production conditions:

1) Gently scrape the solder paste from the scraper and the steel mesh into the labeled solder paste box.

2) Take two pieces of clean wiping net/cleaning cloth and place the left hand at the lower end of the steel mesh and the right hand on the steel net end, both moving at the same speed in the same direction.

3) If the observation is not clean, the left-hand rubs net paper/cleaning cloth on the lower end of the steel mesh. In contrast, the right-hand holds the air gun perpendicular to the steel net at the muzzle, moving synchronously in the same direction and blowing the steel mesh at a constant speed.

4) Examine the template for cleanliness (make sure there is no solder paste in the small hole gap).

5) Clean the steel mesh after every 15 PCS.

b) Second Step

After changing or stopping the line for more than 2 hours, you can disconnect the steel mesh immediately:

1) Using a mixing knife, scrape the tin on the steel mesh, scraper into the solder paste bottle, and cover the inner and outer covers.

2) Insert the steel mesh into the Stencil to clean and repair the grid inside the car.

3) Using an alcohol/washing water brush, remove any remaining solder paste from the stencil opening.

4) The left-hand places the rubbing net paper/cleaning cloth at the lower end of the steel mesh, and the right-hand holds the air gun perpendicular to the steel mesh at the muzzle and moves at the same speed to blow the steel mesh.

5) Place the steel mesh into the corresponding steel mesh frame after confirming that the cleaning is clean (there is no solder paste in the steel mesh).

Conclusions

NextPCB has seen the SMT stencil's effectiveness—how important it is to save time and be more consistent during manufacturing. Please contact us if you still have problems or questions. We will gladly assist you.

Tag: SMT Stencil
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