Know the SMT Stencil Facts

What is SMT Stencil

Introduction

What is SMT stencil? Why do you need to know about it?

If you are a PCB professional or a PCB hobbyist, you need to know about this topic.

SMT refers to Surface Mount Technology, which is a technique used in mounting PCB components.

In this article, we cover the types of SMT stencils, how the process is done, as well as some alternatives for it.

We hope you read on. It would enrich your knowledge about circuit board component mounting.

What is SMT Stencil?

It is an item used to facilitate the placement of Surface Mount Devices (SMD) on a circuit board.

Here is a simple explanation:

If you are making a circuit board, you need to apply solder paste on it.
Solder paste makes the components stick
As we mentioned, SMT stenciling is only for SMD devices
The number of components mounted differs from board to board
Stenciling makes it easier to apply the paste in the SMT process by eliminating the manual process
There is a reduction of errors in the correct placement of the paste using the stencil method

The SMT stencil is used in the automated application of solder paste on a circuit board It can be used for manual application, but it is not the norm.

Why is SMT Important for PCBs

Why is SMT Important for PCBs?

The simple answers are:

Cost-effective if you have large PCB runs
Reduces errors by manual application of solder paste
Produces uniform results
You can always modify after the stencil process is done

Additionally, the SMT stencil is central to the practice of the SMD process.

In modern-day electronics, the SMT process has largely replaced the older PTH or through-hole technology, which used to be the standard practice.

If you are curious to know the difference between PTH and SMT, you can read our blog here.

In addition, if you are eager to learn the difference between SMD, you can read our article here.

Types of SMT Stencil

What you need to know about the types of SMT stencils depends on what purpose you have.

There are different ways of categorizing SMT stencils, just as there are many ways of categorizing PCBs.

But for the most part, most people want to know the types of SMT stencil based on usage and application. Here they are:

Stencils for Prototyping – the most personalized option, mainly for use in manual solder paste application
Stencils for long-term use, which are framed – these can be cost-effective because the goal is to make each finished product uniform. Frames attach to the PCB so that the solder paste deposit comes off neatly
Stencils for short runs, which are frameless

If you are looking for a PCB/SMT stencil service provider, though, you may want to know what type of stencils they use based on the materials the stencils are actually made of. Here they are:

Stencils made of stainless steel – this is the standard stencil material, preferred by 80% of customers
Stencils made of nickel – cheaper but less durable than stainless steel ones; used for shorter runs
Stencils made of Kapton, which is a flexible component

So what type of SMT stencil do you prefer? If you need assistance, you can always contact us, PCBTok. We can help you pick the best option for your PCB project.

Advantages of SMT Stencil

In this section, we give information about the advantages of using SMT stencils.

You can attach mainstream SMT components with it. This includes >0.5mm pitch ≥ 0.4 mm pitch BGA, 1.0×0.5mm, and 1.60×0.8mm chips.

You can attach new generation chips with it (usually using newer stencil products in the market). New chips that can be attached include the > 0.5 mm lead-free types, the super I/O types, and the wirewound 0201 or 0603 coil inductors

Even if the board is densely populated, you can effectively use the stenciling method, as it is accurate and adjustable.

Perhaps the biggest advantage of using the stencil in SMT is speed. Here is a video on fast SMT PCB production using stencils to illustrate:

Think about it, using the pre-made pattern (i.e., the stencil) helps make a production line. Instead of individually applying the solder paste, whole batches can be completed.

Another added benefit is uniformity. PCBs are ordered by the thousands, and stenciled-on solder paste on PCBs saves a lot of time.

Making a stencil may take some time at first, but it is well worth the time investment in the long run.

Disadvantages of SMT Stencil

Generally speaking, SMT stencil-finished PCBs are of high quality. But why do some people avoid the use of the stencil method? Here is a list of reasons, which we can call disadvantages:

Significant investment in time – if you want to have PCB stencil done, it would take some time. You need to contact a stencil manufacturer, send your Gerber files, then wait for the finished product. It’s only after the stencil is finished that your PCB can be produced
Cost can be prohibitive – A stencil can be completed with DIY laser cutting, or you can buy a PCB stencil machine. Both are viable methods, but the latter can be expensive. The best course of action is to get in touch with a PCB manufacturer who can produce both the stencil and the circuit boards in one complete package.
Risk of smears – after the stencil is lifted off, some solder paste might be accidentally left behind. We call this “solder smear”. Although modern machines are less likely to leave undesired remnants, there is still a small risk remaining.

Think about how many PCBs you want to make. If you want mass-produced PCBs, you obviously cannot use manual stenciling. But if you are creating a prototype, Prototyping stencil for a board with SMT components is advisable. At the end of the day, weigh your pros and cons. You get to choose what works for you best.

Explain the SMT Stencil Process

We’re discussing here the PCB or SMT stencil process that is done by professional manufacturers. If you want to know about DIY laser cutting of PCB stencils, kindly consult other sources. Here is the process of SMT stenciling, simply explained:

The SMT stencil follows the PCB design given by the PCB design to the PCB manufacturer.
PCB company uses their stencil machine—the machine laser cuts the pattern on a stainless steel plate
The holes on the stainless steel plate correspond to the places where the SMD components should be.
The plate is aligned on the bare PCB board, then the board goes through a machine. The machine can be a wave solder paste printing machine, also called an automatic SMT solder paste printing machine
This machine puts in the “glue” or solder paste. Then, the board that has glue is passed on to another machine called a PCB pick-and-place machine
The components are accurately surface mounted on the PCB
The PCB then goes to the wave or reflow oven for the next step

With these steps, the SMT process is almost complete. If you want to avail of SMT PCBs, you can always contact a competent manufacturer like us. We’re here to help.

Finding a Good SMT Stencil Provider

If you are not going to do DIY stencils, then find a reliable service provider for them. Here are the necessary qualifications you need to look for:

The knowledgeable employees of the organization continue to receive the expertise and education required to enable them to produce high-quality goods.
They possess a strong foundation of understanding regarding SMT or SMD processes.
They have quality standard accreditations, such as ISO-9001, ISO-14001, and the highly regarded UL standards, which help assess overall performance.

We are PCBTok. We have provided complete technical support for PCBs for over ten years, so we guarantee that you will be happy with our stencil products. You will be satisfied throughout your transaction with us because we always provide the help you need. Our service is fast and comes with 24/7 online support.

Designing Your SMT Stencil

The first thing you should consider is the PCB design. PCB designs with vias and PCB designs with significantly sized copper pads need special consideration.

The second thing to consider in designing is the aperture specs. Aperture means “hole” and stencil sizes are a bit smaller than the exact aperture. If you don’t pay attention to the size, you can have an error.

The third thing you need to consider is the size of your stencil (stencil width). There are ways to measure what stencil thickness you need in relation to the aperture.

The fourth thing you need to consider is what material was used to construct the PCB stencil. Normally, stainless steel is used. So in this case, you can use PCBs with normal pitch, not fine pitch. If a PCB is with fine pitch design, there are adjustments that need to be made in terms of adjusting the height of the stencil as it is placed above the PCB to be stenciled.

Conclusion

The topic of SMT stencil was discussed in this blog entry. Basically, this method is effective in creating PCBs that are classified as HDI and miniaturized.

SMD stenciled PCBs are usually mass manufactured, as producing one stencil for short PCB runs is cost-prohibitive.

There are alternatives, however, like buying your own PCB stencil machine. If you are a serious PCB hobbyist, this can be a good decision.

For the most part, leave the stenciling process to SMT and SMD PCB experts—giant PCB manufacturers like us.

SMT Stencil