Printed circuit boards are vital elements of electronics. But to have them, an intricate production process must happen first. A vital element of the design, fabrication and assembly process of electronic circuit boards entails the development of a PCB stencil. It is instrumental in determining where the solder paste gets located on bare PCB boards before component mounting. However, most PCB enthusiasts pay little to no attention to PCB stencils. So what does it imply, what is its importance to the PCB production process?
PCB Stencil
By definition, it infers to the stainless steel sheet comprising of laser-cut openings deployed in placing solder paste on printed circuit boards. It is instrumental for SMT or surface-mount technology, especially on component placement. The stencil’s primary use entails the deposition of solder paste to designated spots on a bare circuit board. Solder paste deposits then allow components to get placed and aligned on the circuit board.
A PCB stencil needs to deposit the exact amount of solder paste precisely on the designated spot. It is vital as it ensures the pad-to-component solder joint provides the required mechanical strength and electrical connection. The use of a relevant PCB stencil in applying solder paste ensures a faster PCB assembly process. It also makes it simpler and precise.
Printed circuit board stencils get classified according to the mounting style and aperture type. As a result, the diverse PCB stencils include the following. Laser-cut stencils framed printed circuit board stencils, hybrid PCB stencils, and chemical-etch stencils. Other stencil types encompass frameless PCB stencils, etc. Such PCB stencils get offered by most PCB houses, including RayMing PCB and Assembly. You can also get customized stencils based on your PCB design needs.
Types of the PCB Stencil
The Framed PCB/SMT Stencil
It comes as a solder paste stencil that is laser-cut and gets permanently mounted on the stencil frame. Mounting the solder paste stencil deploys the mesh border, which tightly stretches your stencil foil within a frame. Your framed stencils by design apply to high volume PCB screen printing. The stencil also has a smooth aperture, which can get applied for 16-Mil pitches or below.
Features of Framed SMT Stencil
- It has a distinct process for the smooth aperture walls
- An incredible print performance
- Clean laser-cut openings or apertures
- It is perfect for stencil printing targeting high-volume on PCBs
- The unique process develops permanent (non-fading and non-removal) fiducial marks
- All the framed surface-mount technology stencils get double bonded with intention to withstand extreme wear
- It has a standard twenty-four hour turnaround period
Frameless SMT Stencil (Foil)
Alternatively referred to as foil, this type of stencil is laser-cut stencils designed to function with reusable stencil frames renowned as the stencil tensioning systems. It is an SMT stencil type that does not require permanent gluing into the frame. Frameless SMT stencils prove less expensive compared to framed stencils. It also saves money in terms of storage besides delivering advanced quality and superior performance.
Features of Frameless SMT Stencil
- Limited storage space needs
- It comes significantly less costly compared to framed stencils
- Seamless aperture walls, which can get deployed for the 16-mil pitch or below, especially for the micro BGA
- A twenty-four-hour turnaround standard
Prototype SMT Stencil
It forms a type of laser-cut stencils that get custom-made from the CAD or Gerber files you submit. The stencil, in this case, has to match the PCB prototype designed. The frameless solder paste stencils get designed specifically for manual printing by the PCB stencil manufacturer. It has to get printed manually because of its custom nature.
Features of the Prototype SMT Stencil
- It abolishes the error-prone and tedious manual process of soldering the prototype PCB by hand.
- It reduces the assembly period of the printed circuit board prototype
- Enhances the quality of your PCB prototype
- It features a lower cost compared to the production stencil counterparts
- It encompasses a complete “how-to” manual
- The prototype MT stencil a twenty-four-hour turnaround standard period
Electroformed SMT Stencil
It comes as a nickel-based stencil. The electroform SMT stencil or electroformed foils get permanently mounted on a stencil frame by a mesh border, which tightly stretches the foil taught within the frame. Electroformed stencils provide among the top paste release attributes available. Consequently, it gets deployed in fine pitch SMT applications to the PCB boards (12 mil to 20 mil pitch). It also applies in flip chips, wafers bumping, and µBGA‘s (6 mil to 12 mil pitch).
Features of the Electroformed SMT Stencil
- Its seamless trapezoidal sidewalls of the SMT stencil permits for a better solder paste release
- Nickel, a key component of the electroformed SMT stencil, possesses a low friction coefficient compared to other materials like stainless steel.
- Electroform foils prove more brutal than the hard and full stainless steel materials of similar thickness. It, therefore, offers a prolonged stencil shelf-life.
- It has a twenty-four-hour turnaround standard period.
Manufacture of PCB Stencils
All printed circuit board stencils get manufactured by creating openings on a stainless steel sheet (foil). The process gets aided by lasers, and the positions designate the spots SMT components will occupy. The stencil gets placed atop the PCB’s surface to align through the fiducial marks perfectly.
Post the alignment step; the solder paste gets applied or placed using a metal squeegee blade. It is on such circuit spots that hold the electronic components immediately after the stencil gets removed.
Aspects to Consider when Making a PCB Stencil Order
Printed circuit board stencils come varied based on the type and component layout desired. If you, therefore, want to develop or order a printed circuit board stencil, you have to consider the following aspects.
The Stainless Steel Sheet’s Thickness
It is prudent to get the right or precise stencil sheet thickness. The thickness of the stencil determines the release quantities of the solder paste via the opening. Because of this, the aperture size and sheet thickness play a fundamental role in the solder paste quantity deposited. However, more than the necessary solder deposits results in solder bridging. Conversely, fewer solder deposits create weak joints (solder) that influence the functionality of the final printed circuit board.
Size of the Aperture Created (Opening)
In most cases, the stencil aperture or opening gets designed smaller in relation to the printed circuit board size. The reason behind this concept entails the importance of avoiding solder defects like solder beads and bridging. Additionally, post the solder deposition, and upon removing the stencil, solder paste can get transferred or stick to the PCB or the stencil’s aperture walls. It becomes prudent to design the pad’s area as two-thirds of the stencil’s inner aperture to avoid solder defects.
Stencil Material
The stencil material can impact the stencil’s capability when it comes to the delivery of solder paste from the aperture or opening to the printed circuit board. The most predominant material used to develop stencils entails stainless steel.
Stencil Alignment
One popular way of getting a perfect solder paste print entails having registration marks (fiducial marks) on the stencil and printed circuit board. The marks ensure the proper alignment of the stencil and the printed circuit board.
The Process of Making a PCB Stencil
The predominant material when it comes to PCB stencils entails nickel or stainless steel. As a result, stencils can get classified based on the process followed during their manufacture. Please note that the manufacturing process mainly involves the stencil’s aperture formation process. The process can entail either of the following.
Electroforming
It encompasses an additive procedure in which the stencil foil gets developed through nickel electroforming. The process also gets referred to as E-FAB. While the process features a high initial cost, the price often proves independent of the aperture number. Additionally, a higher processing period gets considered a negative in this manufacturing or fabrication process.
Benefits of (ES) Electroformed Stencils
- Availability in all types of thickness
- Tapered slots
- High precision
Drawbacks of (ES) Electroformed Stencils
- A higher processing time
- High initial costs
- It requires film tooling
Chemical Etching
It is a naturally subtractive process that features the realization of apertures through a chemical etching procedure. It is relatively cheap as a process, though it mostly leads to inferior results (as the aperture sidewalls end up rough).
Benefits of a Chemical Etched Stencil
- It has the capability of withstanding any established and standard method of cleaning.
- It is relatively cheap
Drawbacks of a Chemical Etched Stencil
- It is very slow
- The process can result in apertures with rough sidewalls
- It also proves unfriendly to the environment
Laser Cutting
It also comes as a subtractive procedure where the stencil foil material gets removed through laser cutting. Laser cutting by default often leads to a finer output and assists in realizing greater accuracy. It also leads to a more consistent output or results compared to the process of chemical etching. However, it is prudent to note that the process is independent of the chemical environment and thus does not need moisture protection.
Benefits of a Laser Cut Stencil
- Tapered slots
- It is quicker compared to other stencil-making processes like chemical etching.
- Absence of film tooling
Drawbacks of the Laser-Cut Stencil
- The aperture’s sidewalls come rough
- It requires an extra deburring process
Guidelines of Designing a PCB Stencil
If you want to realize reliable and sturdy solder joints in your printed circuit board, it becomes necessary to print a stencil design. The aperture or opening size when it comes to the adjacent pad stencil has a direct correlation to the stencil’s thickness. But it is possible to design thicker stencils with a smaller opening to that of the pad size. On the other hand, thinner stencils can also get designed with aperture size to a stencil thickness ratio of 1:1. The recommendation, however, entails deploying a laser to ensure the steel mesh possessing apertures and electro-polishing.
Some of the design guidelines to consider, however, include the following.
The Adjacent Pad’s Stencil Design
The stencil’s thickness determines the quantity of solder paste that gets printed onto the printed circuit board. Plenty of solder paste results in bridging during the process of reflow soldering, and this often proves undesirable. Because of this, industry recommendations entail the QFN package of 0.5mm pitch to deploy a 0.12mm steel mesh thickness. On the other hand, the QFN package with the 0.65mm pitch needs to deploy a 0.15mm steel mesh thickness. However, it is possible to design the stencil’s aperture relatively smaller than the pad size. It reduces incidences of solder bridging.
Heat Dissipation Pad Design of the PCB Stencil
The exposed pads’ soldering at the chip’s bottom to the PCB’s thermal pads can lead to air holes. It results as the gas located inside the large-size pads and thermal vias overflow outwards during the soldering process. It means, therefore, that whenever the solder paste spot or area proves too enormous, incidences of defects like solder balls and sputtering occur. But it is impossible to get rid of the pores means refocusing and limiting the pore size. It would help if you also took the necessary precaution by considering the aspects needed when designing the thermal pad of the stencil.
Always develop numerous small apertures in the thermal pad area instead of a single large aperture or opening. A common value bandied around includes 50% to around 80% of solder paste treatment or coverage. Additionally, the thickness of the solder joint needs to fall at 50μm, going by the industry practice and outcomes. It helps enhance the board-level reliability. An incredible way of realizing this thickness entails ensuring at least 50% solder thickness at the under-fill thermal vias. However, for the through-hole, the rate of coverage needs to range above 75%.
Importance of a PCB Stencil in the Assembly Process of Printed Circuit Boards
The conventional form of the assembly process of printed circuit boards entailed manual placing of electronic components before soldering. But the ever-increasing complexity of electronic circuit boards, besides the need for miniaturization, has necessitated a machine-operated component placing and solder paste application aided by SMT stencils. It is no longer viable to place and solder numerous small components on smaller circuit boards.
In the assembly process of printed circuit boards, solder paste has to go in the designated spots. Such spots denote the areas electronic components have to get soldered within the conductive path of the board. While manual soldering would otherwise make sense for simple circuit designs involving a few components, complex ones need a more elaborate approach. It is necessary to ensure fewer inconsistencies from human mistakes besides proper alignment and functionality of the PCB.
PCB stencils make repetitive solder impressions and act as templates to guide the solder paste application on the PCB. It gets placed and aligned correctly on the PCB. A runner then moves across the PCB stencil screen while squeezing the correct proportions of the solder paste through the circuit board holes.
Benefits
- Solder paste stencils make it possible to work faster, more accurately, besides making the work more straightforward.
- PCB stencils also reduce the amount of labor and time in assembling printed circuit boards which translates to a faster time-to-market scenario of products.
- It also guarantees a tremendously fine pitch precision when it comes to printing.
- Printed circuit board stencils also assist in the reduction of errors and defects when printing, including ultra-fine pitch board patterns.
- Stencils are instrumental in the uniform application of solder across all the surface-mount device pads.
- It also ensures high pad positional precision.
The Step-Down and Step-Up Stencil Aspect
SMT production demands sophisticated approaches, especially in complex and challenging circuits with components like ceramic BGAs, small components, or RF shields. Other mass-produced techs like the Internet of Things (IoT) require such advanced levels of complexity. However, it also has the overriding importance of cutting down on cost and saving time. Components that need different solder paste volumes get placed closer to avert instances of voltage surges or ESD.
It is a type of assembly that needs a different set of approaches beyond the aperture dimensional adjustment. The need to connect various sized components and cleaner while ensuring optimal transfer efficiency implies deploying stencils possessing varied foil thicknesses. The approach gets referred to as a step-down and step-up stencil. It allows for the adjustment of different quantities of the solder paste on the same circuit board. It also includes the clean application of solder paste around elevated spots or areas.
The Step-down and step-up stencil approach only has a ten-year history since its inception. However, the technology has become an effective method of integrating different or mixed parts/components requiring varying paste requirements. Step stencils prove valuable in streamlining the assembly time besides lowering the assembly costs. It arises because step areas or spots can get replaced, which allows for reusing of the stencils.
Application of Step-Down and Step-Up Stencil
Step-up areas apply in instances where a large volume besides the height of the solder deposit proves necessary. On the other hand, step-down areas limit the height and volume of the solder. Step-less areas or steps utilize ramped areas to accommodate big or large components, especially those needing more paste deposits. Step-relief cavities cater to board irregularities like bar codes or raised areas. Angled steps, however, reduce the damage that arises from the squeegee.
Components with small footprints like 0201 have to vary their solder paste volumes to avert incidences of short-circuiting. It may not become possible to vary the width and length of the tiny fixed area, though varying the height can prove the ideal solution.
Besides accommodating a mix of components with low and high paste requirements, it has other application areas. It includes realizing co-planarity when it comes to components with different heights. It also encompasses the printing of PCBs with separate surface elevations or stickers. Step-down and step-up stencils should have a standard 0.5mm minimum distance between the component pad and the step’s edge.
The crucial factors influencing the step stencil’s efficiency include the squeegee angle, speed, pressure, direction, and material. Additionally, the distance between the step’s edge to the nearest opening or aperture also influences the efficiency of the step stencils. However, it is prudent to ensure the stencil design optimizes the solder deposits for every electronic component. Too much or too little solder paste has adverse effects on the functionality of the stencil and ultimate impact on the printed circuit board assembly.
Frequently Asked Questions (FAQ) about PCB Stencil
What does a PCB Stencil Entail?
It is a metal sheet with apertures or openings allowing solder paste printing on a printed circuit board. PCB stencils come in diverse types based on the manufacturing process and variety of apertures. It is instrumental in allowing for a faster, more straightforward, and more accurate assembly of PCBs.
What do PCB Stencils Do?
Printed circuit board stencils prove instrumental in applying solder deposits on PCBs using a specific screen printing approach. Surface-mounted devices or SMD components get placed atop these spots before their subsequent soldering to fix them in place. Soldering occurs through a vapor phase or reflow process.
What is the purpose of a PCB Solder Stencil?
An SMT stencil primarily functions to transfer the required solder paste to the electronic bare circuit board. The stainless steel gets cut using a laser beam to create apertures for every SMD or surface-mounted device on the circuit board.
What Makes a PCB Stencil Framework?
Stencils represent a bare type of stainless steel sheet that can either come without or without a frame. A framework becomes essential for the automated assembly of PCBs where you use an automatic stencil machine. On the other hand, hand soldering works perfectly with the unframed stencil. A framed stencil comes bigger and heavier compared to the unframed stencil.
Final Thoughts
A PCB stencil is instrumental in the fabrication and assembly process of printed circuit boards. Every PCB design engineer or enthusiast needs to understand the different types, their corresponding importance, and their application in their PCB development process. Additionally, finding the right partner for your PCB assembly needs helps get a suitable PCB solder paste stencil for your circuit board assembly essentials.
