Understanding How the Solder Mask Process Affects Your Manufactured PCB

One of the greatest honors that can be bestowed upon any person is the ability to serve their country through military service. The fact that we need a military is a necessary evil, to be sure. Yet, putting yourself at risk for others is probably the greatest sacrifice one can make. It is quite comforting to know that family, friends and even people that you will never meet feel safe and protected because of you. In my experience, the only negative part about serving was having to wear those “ugly” fatigues, as that pale green color was certainly an eyesore. However, it represented protection that all of us could rely upon and although it has been years since I left the service, pale green still represents protection. Now that I’m deeply entrenched in the PCB world, it also represents the solder mask, which not only protects your board but is essential for PCB assembly.

The solder mask is your board’s primary protection against corrosion and oxidation. It impacts PCB quality by guarding against performance degradation and the shortening of the device’s normal operational lifetime that may occur without solder mask protection. Even more importantly, solder mask provides a barrier between soldered joints and other conductive elements of the board during PCB assembly, thereby helping prevent solder bridges from forming. Although green may not be the most appealing option, there are properties that make it a good choice for your solder mask and, by far, the most commonly used color. By exploring the solder mask process and understanding how it affects the board, you can make the best solder mask choice for your design.

The Solder Mask Process

The solder mask process is an important PCB manufacturing step that is performed during fabrication. Probably, the most important factor of the solder mask process is how it is applied. Common methods of application include:

• Screen Print
  • Single or Double sided coating
  • Recommended for thin cores/flexible substrates
  • Can be used for via plugging
  • Prone to air entrapment
  • ≅ 17 m²/kg typical coverage¹
• Curtain Coat
  • Single sided coating only
  • Not recommended for thin cores/flexible substrates
  • Cannot be used for via plugging
  • Prone to air entrapment
  • ≅ 12 m²/kg typical coverage¹
• High Pressure Low Volume (HPLV) Air Spray
  • Single or Double sided coating
  • Difficult to apply for thin cores/flexible substrates
  • Cannot be used for via plugging
  • Not Prone to air entrapment
  • ≅ 4.5 - 14 m²/kg typical coverage¹
• Electrostatic Spray
  • Single or Double sided coating
  • Can be applied for thin cores/flexible substrates
  • Cannot be used for via plugging
  • Not Prone to air entrapment
  • ≅ 16 m²/kg typical coverage¹

It should be noted that if your design requires via tenting, you should not use a liquid application. There are several considerations for solder mask processes, which may be imaged directly or indirectly, as shown in the table below:

SOLDER MASK PROCESSES
Direct Imaging		Indirect Imaging
Manual		Laser Direct Image (LDI)
Screen Print		Liquid Photoimageable Solder Mask (LPISM)
Ink Jet		Dry Film

 

As shown, the methods of solder mask application available depend on your PCB design and specific needs.

How the Solder Mask Process Affects your PCB

Now that we have delved into the solder mask process, let’s examine how this process affects your PCB. As previously stated, the solder mask impacts PCB assembly and board quality. The most notable impacts of solder mask are listed below:

• Prevention of solder bridges
  • Provides a solder dam between solder joints and other conductive areas of the board where solder bridges may form, potentially leading to short circuits or board damage.
• Reduction of solder paste consumption
  • Allows less solder paste to be used during solder process.
• Additional insulation of the components from PCB
  • Serves as an insulator between board components.
• Protection of the PCB
  • Guards against contaminants that may be introduced from handling the board.
  • Prevents oxidation that may shorten the operational lifetime of the device.
• Increasing the breakdown voltage
  • May increase the breakdown voltage of the board’s dielectric material, providing additional protection against arcs or corona typically associated with high voltage (above 1 kV) boards.
• Prevention of metal whiskers
  • Help prevent the growth of tin whiskers mostly associated with lead-free solder or tin-plating for electronic components.

Obviously, the solder mask process has important implications for your board’s manufacturability and operation once deployed. Nevertheless, there are situations where it may not be possible or desirable to have solder mask, like small components and fine pitches on ball grid arrays (BGAs), for instance. Before deciding to forego solder mask and pursue design alternatives, you should check with your CM to ensure there are no adverse effects on your board’s performance or reliability.

Solder Mask Considerations

Although, there is no clear consensus on why green is the predominant color used during the solder mask process, there are attributes that support its usage. First, green solder mask can be used to create the smallest solder mask dams (0.1 mm). Red, blue and yellow can produce dams of 0.12 mm. Black and white, which may provide the greatest contrast for your silkscreen, produce 0.15 mm solder mask dams. Supporting the usage of green may be the fact that green light has a wavelength of around 550 nm, which makes green the color most visible to the human eye in daylight.

However, color is but one of several considerations that influence the type of solder mask that must, or even can, be used. The primary consideration is the type of system in which the board will be deployed. PCBs destined for usage in certain industries or for certain applications must adhere to standards set forth in IPC-SM-840D.

To ensure that the solder mask process applied to your board meets all regulations, as well as your own requirements, you should partner with your contract manufacturer (CM) early in the design and development process. Engaging your CM will ensure that PCB assembly considerations, as well as the quality of your boards, will be well-integrated into the design and development process.

Tempo‘s Custom PCB Manufacturing Service Performs entire turnkey process in as fast as 3 days. Emphasizes DFM to eliminate time-consuming back-and-forth design corrections. Performs BOM verifications to reduce possibility of component obsolescence issues. Sources components from the most reputable suppliers in the industry to reduce procurement time. Performs multiple automated inspections during assembly to ensure PCB quality for prototyping. Provides support throughout the PCB manufacturing process, beginning with design. Smooth transition from prototyping to production.

At Tempo Automation, we help you get started on the best path by furnishing information for your DFM and enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on the solder mask process and how it affects the manufacturing of your boards, contact us.

 

 

 

 

 

 

 

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¹ This is the board coverage or thickness of the solder mask based on the weight used.