Stencil printing

Key takeaways

Stencil printing belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Stencil printing to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Stencil printing from memory before moving on to harder problems.

Reference excerpt

Stencil printing is the process of depositing solder paste on the printed circuit boards (PCBs) to establish electrical connections. It is immediately followed by the component placement stage. The equipment and materials used in this stage are a stencil, solder paste, and a printer. The stencil printing function is achieved through a single material namely solder paste which consists of solder metal and flux. Paste also acts as an adhesive during component placement and solder reflow. The tackiness of the paste enables the components to stay in place. A good solder joint is one where the solder paste has melted well and flowed and wetted the lead or terminal on the component and the pad on the board. In order to achieve this kind of a solder joint, the component needs to be in the right place, the right volume of solder paste needs to be applied, the paste needs to wet well on the board and component, and there needs to be a residue that is either safe to leave on the board or one that can easily be cleaned. The solder volume is a function of the stencil, the printing process and equipment, solder powder, and rheology or the physical properties of the paste. Good solder wetting is a function of the flux.

Inputs Inputs to the process can be classified as design input, material input and process parameter input. The output of the process is a printed wiring board that meets the process specification limits. These specifications usually are consistent solder paste volume and height, and printed solder paste aligned on the PWB pads. This determines the process yield. In electronic design automation, the solder paste mask and thus the stencil is typically defined in a layer named tCream/bCream aka CRC/CRS, PMC/PMS, TPS/BPS, or TSP/BSP (EAGLE), F.Paste/B.Paste (KiCad), PasteTop/PasteBot (TARGET), SPT/SPB (OrCAD), PT.PHO/PB.PHO (PADS), PASTE-VS/PASTE-RS (WEdirekt), GTP/GBP (Gerber and many others). Some (less common) EDA software does not treat the solder paste mask as a regular part of a PCB's layer stack, in which case the paste mask must be derived from the solder stop mask. For improved accuracy, stencils traditionally were often mounted in proprietary aluminum frames of various kinds. Today, the usage of quick mount systems is more common at least for low volume batches, mounting the stencil pneumatically or mechanically. For this the stencil needs additional perforations for alignment following one of several mount system standards including QuattroFlex, ZelFlex, ESSEMTEC, PAGGEN, Metz, DEK VectorGuard, Mechatronic Systems and others.

Printing process The process begins with loading the board into the printer. The internal vision system aligns the stencil to the board, after which the squeegee prints the solder paste. The stencil and board are then separated and unloaded. The bottom of the stencil is wiped about every ten prints to remove excess solder paste remaining on the stencil. A typical printing operation has a speed of around 15 to 45 seconds per board. Print head speed is typically 1 to 8 inches per second. The printing process must be carefully controlled. Misalignment of motion from the reference results in several defects, hence the board must be secured correctly before the process begins. A snugger and vacuum holders are used to secure the X and Y axes of the board. Vacuum holders must be carefully used, as they may affect the pin-in-paste printing process if not secured properly. The longest process is the printing operation, followed by the separation process. Post print inspection is crucial and is usually performed with special 2D vision systems on the printer or separate 3D systems.

Printed wiring boards

Design Vision systems in the stencil printing machines use global fiducial marks for aligning the PWB. Without these fiducials the printer would not print the solder paste in exact alignment with the pads. The PWB should have close dimensional tolerances so that it mates to the stencil. This is necessary to achieve the required alignment of solder blocks on the pads.

Masking The required accuracy in alignment can also be achieved by controlling the flow of solder on the PWB during reflow soldering. For this purpose, the space between the pads is often coated with a solder mask. The solder mask materials have no affinity to the molten solder and hence, no positive bonding is formed between them as the solder solidifies. This process is often referred to as solder masking. The mask must be aligned correctly. The mask protects the PWB against oxidation, and prevents unintended solder bridges from forming between closely spaced solder pads. Also the height of the solder mask should be lower than the pad height to avoid gasketing problems. If the height of the solder mask is greater than that of the pad, then some of the solder paste would settle in the empty space between the mask and the pad. This is what is referred to as gasketing. It is a seal that fills the space between two surfaces to prevent leakages. Gasketing is a problem as the excess solder paste around the pad may be more than a nuisance factor for circuits having very small line spacing.

Finishing The pads on the PWB are made of copper and are susceptible to oxidization. Surface oxidization on the copper will inhibit the ability of the solder to form a reliable joint. To avoid this unwanted effect, all exposed copper is protected with a surface finish.

Aperture fill and release The core of a well printed PWB lies in the fill and release of solder paste into the aperture. When the stencil is in contact with the PWB, solder paste is applied over the top surface of the stencil using a squeegee. This causes the aperture to fill with solder paste. The PWB is then lowered from the stencil. The amount of solder paste which is released from the stencil apertures and transferred to the PWB pads, determines whether or not the print is good. Ideally, all volumes of solder paste should be equal to the volume of the corresponding stencil aperture. In reality however, this is never the case. Hence, a print is considered to be good if a certain fraction of the paste is released. One way of quantifying print performance is to calculate the transfer efficiency. This is mathematically stated as:

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Stencil printing

Start with the simplest possible case. Write down what Stencil printing claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Stencil printing before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Stencil printing ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Stencil printing

In research: Stencil printing appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Stencil printing in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Stencil printing is common in secondary-school and first-year university syllabi. It links to neighbouring topics Printed circuit board manufacturing, Printing processes, so understanding it makes those chapters shorter.
In everyday life: Look for Stencil printing outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Stencil printing in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Stencil printing means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Stencil printing out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Stencil printing in simple terms?

Stencil printing is the process of depositing solder paste on the printed circuit boards (PCBs) to establish electrical connections. It is immediately followed by the component placement stage.

Why does Stencil printing matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Stencil printing?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Stencil printing.