Creating Your PCB Solder Mask in Altium Designer
You’ve just finished the electrical layout of your board and now you are working through the final steps to prepare for manufacturing. What steps do you need to take to get your board ready for production? Aside from generating deliverables like Gerber files, a bill of materials, drill files, silkscreen, and assembly instructions, you’ll need to define the solder mask on the surface layers of your PCB. This important production planning step is much easier when you use the right PCB design software with a full suite of design tools in a single program.
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Once your layout is finished, you might be tempted to think that the last step in the design process has been completed and you can start planning for manufacturing and assembly. While this is technically true, there is on last design step that is critical for proper fabrication and assembly of your board. This step requires defining solder mask throughout your board. The goal is to provide sufficient protection to traces and ensure that solder only bonds to specific areas of exposed copper in your board.
There are other benefits to using solder mask that include preventing corrosion, bridging, and tombstoning during wave soldering. With the right design software, you’ll be able to define the important areas of your board that require solder mask and verify your coverage using native 3D design tools. Altium Designer is the only PCB design package that includes these features and many other tools that are built to help you move to manufacturing successfully.
In short, solder mask (also called solder resist) is an epoxy or photoimageable polymer layer that protects copper on the surface layer of the PCB. This layer covers exposed traces on the surface of the board, preventing oxidation and providing an extra protective barrier against mechanical damage.
Solder mask provides another benefit to your board; it helps prevent unintended connections from forming during assembly when components are soldered onto pads and holes. A solder bridge forms when two pads on a board or two leads on an IC package are unintentionally soldered together, which typically happens due to the use of too much solder paste during assembly. Defining the right solder mask opening size and thickness helps ensure that solder remains on a conductive pad during wave soldering. This also helps prevent tombstoning of components on the surface layer.
How the Solder Mask Process Affects Your PCB
Solder mask was created primarily to facilitate wave soldering during in mass assembly. Proper use of solder mask is critical for ensuring proper wave soldering and unintended solder bridges during assembly of rigid circuit boards. As modern boards require higher trace density routing and component placement, PCB solder mask is something of a requirement rather than an option in most boards.
With flex ribbons in rigid-flex PCBs, or with fully flexible PCBs, you’ll need to decide whether to use flexible polyimide coverlay or flexible solder mask to protect critical traces on polyimide substrate. The answer to this important question really depends on whether you intend to solder components directly to the flex ribbon and the level of flexibility required on the finished PCB.
- Should you use an epoxy-based, lacquer-based, or photoimageable polymer-based solder mask in your board? The answer depends on industry standards, required solder mask thickness, and the level of surface complexity.Learn about choosing the right solder mask for your next PCB.
- In addition to solder mask, there are other options for preventing corrosion on exposed conductors in your PCB.Learn more about methods for preventing corrosion in exposed PCB traces.
- With flex and rigid-flex circuits, you’ll need to decide whether to use a flexible solder mask or polyimide coverlay to protect traces on a flex ribbon.Learn more about flexible solder mask with Tara Dunn.
3D view of a PCB with blue solder solder in Altium Designer
The process of applying solder mask to the surface layer of a PCB includes several steps. First, the surface layer must be cleaned and degreased, and the copper surface is roughend mechanically or chemically. A solder mask layer is then applied as an epoxy-based coating or a polymer coating. More complex boards with finer features will use a liquid photoimageable polymer that is hardened using a photolithography mask. If you are working with a solid or liquid photoimageable polymer, your manufacturer will need to take your design files and construct a photolithography mask that contains the exposed conductors.
The final step after applying and exposing a solder mask involves removing excess material and curing the mask with a UV lamp or in a tunnel over. This process fully hardens the solder resist so that it will adhere to the board and roughened copper conductors in your PCB. Each component pad will then be left exposed through the solder mask opening and the board will be ready for PCB assembly.
Standards on Solder Mask Design and Manufacturing
Just like all other aspects of printed circuit design, there are industry standards that define design and manufacturing practices for solder mask. The IPC-SM-840D standards define solder mask requirements for a printed circuit board intended for use in certain industries or applications.
Silkscreen printing is another important aspect of preparing for assembly and will be placed after the solder mask has been placed and cured. Clearance and printing requirements on silkscreen on a the surface layer of a circuit board are specified in the IPC standards. A lot number and a unique manufacturer’s mark will need to be placed on the board using silkscreen in order to provide traceability in the event of a recall. Make sure to allocate space on your circuit board for this information and clearly specify this in your design.
- Resist exposure is critical for ensuring your solder mask is placed accurately and exposes conductors with the proper clearances.
- Your solder mask design and silkscreen will appear in your Gerber files for your manufacturer. The right design software will allow you to easily generate these files for your manufacturer.
- Silkscreen should contain a number of important pieces of information for your fabricator and assembler.Learn more about silkscreen in a printed circuit board.
Preparing Gerber files and assembly drawings in Altium Designer
The design tools in Altium Designer are built on a single rules-driven design engine that allows you to define solder mask clearances around pads and tolerances throughout your board. You'll usually want a two mil border around each feature to compensate for tolerances in the solder mask. You'll need to define clearances on other features in your board like pads and vias to ensure the solder mask can prevent solder bridges from forming.
When preparing your printed circuit board for manufacturing, the solder mask should be defined in its own layer in your Gerber files. The solder mask color is just one of many important points that will determine the type of solder mask that can be used on your board. It is best to check with your manufacturer regarding their capabilities and available materials when selecting a solder mask color, thickness, and other manufacturing tolerances while designing your board.
Preparing for Manufacturing in a Unified Design Environment
Once you’ve laid out your board and defined your solder mask openings, pad clearances, and thickness, it’s time to prepare your deliverables for your manufacturer. These deliverables include Gerber files, bills of materials, sourcing information, drill files, and other assembly instructions. These tasks are much easier when you work with design software that integrates these tools into a unified design environment.
Working in an integrated environment means that your design, verification, data management, and production planning tools have access to the same set of design data within a single program. You won’t have to export design data between programs, and each tool will be accessible within a single interface. Instead of resigning yourself to an outdated workflow, you design and prepare for manufacturing with maximum productivity when you use Altium Designer.
- Native 3D design tools in Altium Designer allow you to accurately place components, verify clearances, and examine your solder mask without using a separate program.Learn more about native 3D design in Altium Designer.
- Silkscreen is one important aspect of design that will need to be placed on top of your solder mask.See how you can define silkscreen to solder mask clearance in Altium Designer.
- The goal in any manufacturing process is to maximize yield. With panelization tools in Altium Designer, you can maximize your output and customize your panels for manufacturing.Learn more about panelization in Altium Designer.
Solder mask is one of those all-important design steps that designers must complete prior to sending a board off for manufacturing. Your design tools should simplify the process prior to moving on to manufacturing. Altium Designer does more than just offer tools for solder mask design. The rigid-flex PCB design tools, PCB editor, schematic editor, documentation features, and circuit board analysis tools are integrated in a single environment.
Integrated design features are built to work together and have set the standard for productivity in the PCB design industry. If you need helpful design tips and resources, Altium gives you access to webinars and podcasts, the AltiumLive forum, and a thorough knowledge base with design examples. You’ll be able to design printed circuit boards for any application.
When you work with Altium Designer, you won’t have to sacrifice productivity by switching between multiple programs just to complete essential design tasks. Instead of using outdated command line-based design software or multiple programs to design your solder mask, try working with the industries best design features in a unified environment. The integrated design tools in Altium Designer are built to work together and help you maximize your productivity.