Look at a printed circuit board (PCB) with all components and wiring placed on it (figure 1). It just looks like a jigsaw puzzle which requires more time for engineering to assess.
On the other, figure 2 shows another version of the design where component are identified along with essential points marked on the circuit board.
It’s clear that 2nd one looks more sophisticated regarding its visual appeal and ease of use. That is the point where the art of PCB silkscreen comes into the play.
Now Follow me to read this guide to know more about the silkscreen.
1、What is PCB Silkscreen?
You can imagine that roads and streets on a map do not make any sense if you cannot see text and legends identifying where every connection leads to. The role that is played by landmarks, legends, and other markings on a map are same as silkscreen. PCB silkscreen provides text-based information of different circuit point, components, and additional circuitry information as per requirement.
Figure 3: Yellow PCB-Silkscreen
You should understand that PCB silkscreen is just one process of overall PCB manufacturing.
With the technological advancement, not only the text but beautification images can also be the part of silkscreen artwork design. However, the pitfall is that you may overlook the fundamental purpose of design and may make the information unreadable easily.
Therefore, You must keep the primary goal of silkscreen in mind while creating the artwork.
Now that you have developed a basic understanding about PCB silkscreen let’s take a look at the significance of PCB silkscreen in overall PCB manufacturing process.
2、Significance of PCB Silkscreen
First, let us remind you of the primary use of PCB. Also known as Print Wiring Board (PWB) or Engraved Wiring Board, PCB is used to provide mechanical support to connect the various electronic parts in a circuit board electronically.
It is done using conductive pathways, signal traces or tracks imprinted from copper sheets laminated onto a non-conductive substrate. Components and conductive paths are spread all across the board without identification.
This creates a need for you to place customized information to help the end user/customers.
• Print company logos like the name of the company that made the product,
• Warning signs like High Voltage,
• Version numbers like V2, V3, etc.
• Manufacturer identifiers,
• Testing points,
• Part numbers,
• Components orientation,
• Processing types etc.
In a broader perspective, you can say that the role of PCB Silkscreening extends to but not limited to
• Circuitry identification such as testing points, ground point, and component interconnect, etc.
• Troubleshooting of circuits in case of a burnt part or undesired result produced.
• Parts identification in case of replacement required or design evaluation needs to be done.
• Safety precautions by putting in warning signs like high voltage, grounding, etc.
• Branding and publicity of manufacturer. It can also serve as protection against illegal copying of circuit design.
• Product evolution such as component replacement testing or rating verifications.
It may be noted that silkscreen information is dependent on the board size of PCB you choose. In case of limited space available, you should prioritize the critical information. This becomes more critical if you have chosen a flexible PCB design for your project.
PCB silkscreen process impacts the overall quality of PCB. You can learn more about it once you understand the process of PCB silkscreen as described in the next chapter.
3、Process of PCB Silkscreen
PCB silkscreen is a sub-process in the board tree of solder masking along with other ones as baking, UV exposure and developing. However, due to its overall impact on the product's grading and its reliability, you can consider it as the most sophisticated ones among the four in its class.
Now PCB silkscreen is a part of the overall PCB manufacturing assembly line.
Overall silkscreen process consists of at most 32 steps which can be categorized into six broad processing areas. These processes may vary in actual scenarios depending on the material of PCB elements, quality requirements and cost.
Figure 4: Automatic Character Ink Printer
3.1 PCB Silkscreen Design using EAGLE
This is the phase where artwork you designed to be printed on PCB is readied. You can do it very easily and artistically with Computer Aided Design software EAGLE.
Silkscreen section, as part of PCB design package, lets you pleasingly design your silkscreen layer along with circuitry and other design elements.
Rectangle shape of yellow color represents the silkscreen outline. You can look for it above grey package dimension lines. Once the silkscreen menu is selected, it is easier to move ahead with your design.
From parts placement points to text writing can be done through different menu options available. You can refer to EAGLE Silkscreen tutorial for more details.
3.2 Preparation of PCB board
Once the CAD blueprint is finalized, it's time to move towards hardware implementation. You need to perform some steps to finish the product with high quality.
The first step involves in this process tree is the selection of the type of board to be used for PCB manufacturing.
The material selected is then picked-up for cleaning followed by tape trolling and board handling. PCB board then gets ready for the printing.
At this point, the PCB Designs should be ready and printed (black) on a transparent paper.
The designs are placed on the dried silkscreen. In this case, the silkscreen PCB Design should be placed on the prepared silk fabric the way you would like the writings or the markings to appear on the PCB.
It is important to make sure that the PCB Design paper does not move from where you have positioned it. This can be done by fixing the designs firmly on the surface using a TRANSPARENT tape.
When this is done, the whole setup (silkscreen and the attached designs) is turned upside down such that the designs face downwards. The top side is then covered with a dark cardboard.
The UV light is then shown from below for about 15 minutes, after when the PCB silkscreen artwork is fully developed.
To expose the developed silk screen template, pressurized water is used to wash away the emulsion parts that were not exposed to the UV (not cured).
3.3 Silkscreen Printing
In these steps, actual silkscreen printing on the board occurs. The board is fixed on the machine for printing. In the next 4 steps, the screen moves upward, downward, left and right for direction printings. The design you made on EAGLE is now imprinted on PCB.
To print the silkscreen on the PCB, the ready PCB (already etched) is placed below the silkscreen fabric. The PCB should exactly align with the silkscreen artwork on top.
Again, after positioning the PCB directly below the silkscreen and aligning well, the PCB should be fixed firmly in place to ensure that it does not move during the application process. This can be done using stoppers at the sides of the PCB.
The silkscreen UV curable ink, usually white, is applied evenly on top of the artwork (silkscreen) using a squeegee. From here, the PCB is dried for about 10 minutes using a drying oven. The writings or markings are well imprinted on the PCB surface.
3.4 Handling Procedures
Although printing has been completed at this point but to retain the quality of PCB board, proper handling mechanism is vital. That is why the highest number of steps are gauged in handling procedures.
You can see that around 12 different stages are done as shown in the figure below to make sure that the quality of PCB is not affected due to silkscreen printing and that information you imprinted on the board meets the overall criteria.
3.5 Inspection Procedure
Inspection procedure is conducted in the middle of handling steps and at the end of processing steps. The process is to check the quality of the PCB panels made visually.
This procedure ensures the information to be properly readable by the human eye on board. You need to make sure that there are no misprints or missing info.
3.6 Treatment of Silkscreen
In the final step, treatment of silkscreen print is done, and inspection for product readiness is made. Once quality assurance is passed, you have PCB with silkscreen processes completed.
Figure 5: Process of PCB Silkscreen
An emulsion is mixed with the developer in a ratio of about 10:1. This is preferably done in a dark room. This mixture is applied evenly on the surface of the silk fabric using a wide squeegee or a similar tool.
It is advisable to apply around 2 layers on each side to make a total of 4 layers. This layer must be uniform and there should be no spaces or bubbles. Leave it to dry up or accelerate the process using a drier.
One of the depending factors on which some steps in PCB silkscreen process tree are Method of PCB silkscreen that you choose. Methods of PCB silkscreen for your detailed understanding are described in the next section.
4、Methods of PCB Silkscreen
Choice of PCB silkscreen method is dependent on many factors including time, cost, the precision required, board spacing available, the material of PCB used. There is a fair trade-off in these parameters under each method described below to help you in PCB design choices.
4.1 Manual Screen Printing
Manual screen printing is the oldest and most cost-effective PCB Silkscreen method. Epoxy ink is used to carry out this process. The ink is pushed onto the laminate and dried to get the impression on board.
Setup requirements for manual screen printing are much easier. Its cost efficiency for setup also makes it a preferred choice. Also, the application of manual screen printing still makes it preferred the option for many tasks.
As a downside, this method requires more board spacing and impression drying. That is due to the larger text size that is supported by this method. Pad clearance is another challenge which is on the higher end.
You should opt for manual screen printing if you want to cut the cost and board spacing is not an issue.
4.2 Liquid Photo Imaging
This process utilizes the similar process that was base of PCB manufacturing in the past. The difference is that white color replaces green color. The same procedure as the solder mask is used for silkscreening.
Laminate is first coated through the epoxy. This epoxy is liquid and has the property of photo imaging. The sketch of silkscreen is transferred to plates through exposure of UV light.
The results that are obtained from liquid photo imaging have better precision than manual silkscreen process. Also, the required spacing is reduced in this method utilizing the board space more efficiently.
This process requires more ink than the other two processes. Also, the time required to set up along with application time is higher.
You can go for liquid photo imaging to save space on board along with fineness for your project.
4.3 Direct Legend Printing
This process has the same mechanism as used by paper printers. High-end inkjet printers with the capability of ink transfer on PCB are used for the process.
Another ease to use this process is that CAD application data is directly printed on the circuit boards. The ink used in Direct Legend printing has also been upgraded to acrylic to save the impression drying time.
Being more digitalized process among all, this process has the highest accuracy and best time efficiency. It can have a very low text height capacity but lower than 0.02 inches is not used as it may not remain readable by the human eye.
As per the downside, this process cannot be used for boards with the silver material survey. Also, there is a limitation of acrylic ink for some applications.
You can gain benefits of time and board spacing by choosing DLP.
Choosing the right method of PCB silkscreen helps you to save overall cost. Details about other factors affecting silkscreen cost are described in next section.
5、Understanding The Cost of PCB Silkscreen
PCB Silkscreen cost is placed under the head of nonrecurring tooling during the PCB design process due to fixed cost characteristics. Silkscreen process posts around 12-15% contribution of the overall manufacturing cost of PCB.
Text design refers to character width, character height, character thickness, character spacing and character placement variation. Artwork cost is usually not separately charged.
Cost of ink to be used is dependent on the method of silkscreen being used. Acrylic ink is used for the DLP method whereas other methods utilize epoxy ink.
Ink cost is dependent on specific properties such as substrate compatibility, adhesion properties, curing procedure and immunity to acid/alkali treatment.
The color of ink is also a decisive cost factor. White color ink cost less than other color inks. Around 5% of overall PCB cost can be reduced with cost optimization for PCB silkscreen regarding material choice.
Method of silkscreen being used also has an impact on overall costing. Liquid Photo Imaging (LPI) and Direct Legend Printing (DLP) both cost more than manual screen printing as the capital cost of setup is high.
Another vital factor to consider in silkscreen costing is either you want to print artwork on both sides, or you want it on one side only. In the case of both sides, the cost jumps up.
You can also consider the DIY option for PCB silkscreen or opt for a manufacturer. This factor also controls the budget as per your desire. Details about DIY PCB silkscreen are given in next chapter.
6、DIY PCB silkscreen
PCB silkscreen manual process is more natural to set up at home or at your business place. This process is part of overall PCB Manufacturing which can be seen from below video.
Homemade PCB by the method of screen printing
There are a few pre-requisites to implement PCB silkscreen at home. You need to know the use of a CAD tool for PCB design and should be able to design the color, font, and placements of text.
Referred to as artwork, this is a critical part of the silkscreen process. A creative approach can be taken to make PCB charming, but it must be readable as well.
Secondly, the most crucial aspect of learning is the transfer of ink to the laminate. That is a delicate process which has a direct impact on the finished product. Steps involved in manual silkscreen require proper attention for the fine finished product.
First, polyester screens are stretched on an aluminum frame. The coating is done on the screens in the second step. If screen and panels are not placed in the synchronized manner, otherwise it can ruin the whole board.
Once the coating is done, it should be adequately dried for use. These all steps should be kept in mind while developing DIY PCB Silkscreen.
Still, there are some factors which make you prefer a manufacturer for your work. If you want to choose a manufacturer rather than DIY, check out our next chapter for guidelines.
7、How to choose PCB silkscreen manufacturer
It is better to select a PCB silkscreen manufacturer if you are looking for time-saving and high quality of work. Manufacturer have strong learning curves developed over time which makes them the better choice for PCB silkscreen.
Still, you should be aware of pitfalls to avoid if you want to choose a vendor for your PCB design.
First, an important aspect is the preparation of the artwork. You should be closely working with the vendor to complete CAD or manual PCB design before going into production.
This is the point when you can decide the font size, clearance spacing, color, single or double size printing and other important silkscreen design considerations. Remember that these points have the direct impact on the overall cost, so you need to choose it carefully.
Once ready, you should double check the design for errors and readability. The die design may be used over and over once developed but remaking it in case of any precision issues will cost time and money.
Your designer can always help you in making these decisions.
You can evaluate the manufacturer capabilities through communication with him regarding different aspects. A better manufacturer has a strong command over the subject and will be willing to guide you throughout the process.
Choosing a right vendor such as WELLPCB makes your work hassle-free.
PCB silkscreen is not only a delicate process which can impact the overall quality of your PCB, but it can also add value through classy artwork. Cost trade-off is an essential consideration for you in finalizing designs.
In-depth understanding of the whole process along with the pros and cons of each method and step makes it easier whether you want to DIY or chose a manufacturer for the procedure.
WellPCB has a team of professionals having extensive experience in PCB manufacturing process and assembly jobs. You can save cost and time by choosing WellPCB for manufacturing of your next electronic project.
The services extend from standard to customized options as per your needs. We provide additional color options of white, yellow and black. Also, you can save the silkscreen cost of purchasing PCB from us.
You can contact our technical support team for further information and your queries regarding PCB silkscreen. You can reach through email at [email protected]et or visit our website www.wellpcb.com for more details.