Archive for April, 2016

All about SMT Stencils

Written by Rush PCB Inc on . Posted in PCB, PCB Assembly and component

SMT

What are Stencils?

Surface-mount technology (SMT) is a method of producing PCBs where the components are mounted or placed directly onto the surface of the board.

The stencil is designed to allow a smooth transfer of material (solder paste) onto the bare PCB. It is crucial in ensuring that the material is placed with precision. It is also vital in ensuring that deposits are formed with the proper shape and size. The stencil is critical to ensure that the assembly process can be operated with a high-yield output.

The stencil provides a tool for accurate solder paste deposition, allowing the process to be repeated multiple times. The solder paste is printed through the holes in the stencil, forming deposits that hold the components in place. The stencil provides the accuracy needed to ensure the solder is printed in precisely the right place on the PCB.

The way in which the stencil is designed can be varied based on thickness, the size of the aperture, and the shape of the aperture.

How are stencils formed?

There are various technologies for forming stencils

  • Laser
  • Electroformed
  • Chemically etched plastic
  • Hybrid – a combination of chemically etched and laser-cut.

In chemically etched stencils, the stencil is etched from both sides using two positive images. The etching process leaves aperture walls that taper to an hour-glass shape in the centre of the aperture. In laser-cut stencils, a stainless steel foil is cut by laser, creating an opening for each component that will be included in the final PCB. For both processes, aperture walls are electro-polished to ensure a smooth finish. Following the cutting process, the stencil is aligned over the board. The solder paste is deposited over the apertures. After the solder is laid, the foil is removed.

Ensuring the correct amount of solder paste

It is vital to ensure that the amount of solder being laid down meets design specifications.  If not enough solder is used, there is a risk of inadequate solder joints. In contrast, much solder can result in balling, bridging and tomb-stoning. Both options can disrupt the electrical functionality.

Types of Stencils

There are four types of stencils:  prototypes, framed, frameless and hand/ rework stencils. Each has a different function.

  • Prototype stencils are customised laser cut stencils made for various PCB needs. They are also used in Gerber files. They are generally designed for manual printing.
  • Framed stencils (also known as glue-in stencils) are laser-cut stencils mounted permanently on a stencil frame. They are designed for high speed printing on PCB, and for production runs.
  • Frameless stencils do not require permanent fixing to a frame. They are mainly used for screening printing on PCBs, and are a useful option for short-run production and PCB prototypes.
  • Hand/ rework stencils are used to print individual components onto a PCB. In general, they are used in rework situations, but can also be used in developing prototypes. The stencil includes a flag that allows the stencil to be hand-held.

Mistakes that are Often Made in PCB Design

Written by Rush PCB Inc on . Posted in PCB Design

pcb design

There are many steps to finalizing the design of a PCB. Engineers who are aware of the risks in design and can plan for them will have a more productive output. These are some of the common ways of avoiding errors in designing PCBs.

Plan!

Make sure that you’re using the right tools. While there are many EDA packages available, you will need to be aware of the strengths and limitations of the software. For example, some packages have issues such as footprint mismatches. Ensuring the design is correct at the planning stage will avoid challenging issues in later development. EDA packages may include component footprint libraries. It will be worthwhile checking that the detail in the libraries is correct for each component.

Engage with other team members

A team that works together, understanding the desired outcome will help ensure an effective PCB design. Each team member will have a different perspective, and may be able to identify possible design issues, early in the process. In any field of endeavor, most people will miss errors in their own work. That’s why it is important to have someone else review and critique the design. Experience can be as valuable as theory. An experienced technician can help identify issues. Designers and engineers need to have an ongoing and effective communication process in place.

Making sure the prototype meets specifications

Ensuring that the prototype meets all functional requirements will help minimize the risk of mistakes being present when the PCB is in production. Make sure you have comprehensive prototype testing protocols in place. Develop a test plan, and identify objectives of the testing process. It is a better process to identify issues, create new prototypes, and re-test than it is to short-cut the prototype stage and risk downstream errors. Include testing of high-risk elements.

Substitutions

Board layouts need to be correct for the proposed design, and meet the potential requirements of substitute components. If a substitute component has a slight variation in characteristics, is may be sufficient to affect the performance of the whole design.  Some components will be more tolerant of substitution parts, while others will need to operate within a highly specific range. An error that occurs from a substituted component oscillating and behaving erratically can be challenging when you’re in the troubleshooting stage. Check the datasheets for components to help minimize the risks.

Having detailed documentation and silk-screening

Once the PCB is made, silk-screens will be the main way others will interact with the PCB design. They need to be as thorough as possible. For example, issues can arise when detail such as polarity isn’t clearly identified. Components such as diodes, electrolytic or tantalum capacitors should be clearly marked. Similarly, the first pin should be clearly marked wherever possible, and orientation should be marked for IC packages and components.

Backing Up

While it is an obvious thing to do, it is amazing how often hours of work are wasted through a failure to back up.

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