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Solder Mask and Its Design Tips
Solder mask, also called solder resist or solder-stop mask/coating, is a thin layer covering copper traces without necessity of soldering on a printed circuit board (PCB) on both top side and bottom side to help ensuring PCB reliability and high performance. Resin is usually selected as the leading material of solder mask because it works excellent in terms of humidity resistance, insulation, solder resistance and high temperature resistance plus aesthetics.
It's believed that most PCBs are regarded to be in green color that is actually the color of solder mask green oil. However, solder mask can be displayed in different colors apart from commonest green color including white, blue, black, red, yellow etc. Different colors are applied based on different demands. For example, some RDs tend to pick up red solder mask for prototypes in the stage of NPI (New Production Introduction in order to make them distinct from mass-produced boards). Black solder mask is selected just to be compatible with the color of end product enclosure when those circuit boards need to be partially or fully exposed.
Even two sides of the same piece of board possibly contain solder mask with different colors. Take Arduino Uno board foe example:
Functions of Solder Mask
Solder mask becomes increasingly popular and significant for circuit boards as density of boards rockets and SMT (Surface Mount Technology) starts to become the leading selection due to market demand of volume and efficiency.
As its name indicates, solder mask, or solder resist aims to stop soldering bridges from taking place on covered area. Reflow soldering plays a key role in SMT assembly as it makes electronic components totally and accurately mounted on circuit boards through solder paste. If solder mask isn't applied, copper traces tend to be connected with solder paste so that short cuts will be possibly caused. As a result, the reliability and performance of assembled circuit boards will be comprised.
Apart from its main responsibility, solder mask is also able to prevent copper traces from oxidation, corrosion and dirt.
Solder Mask Manufacturing Process
Some people think that it is not a cutting-edge technology to manufacture solder mask and lots of engineers are able to DIY at home. It's never too late to be aware that it's a complete myth. Solder mask DIY only works for simply-designed boards and it's a bit difficult to ensure the reliability of products unless it is formally applied in the final project.
When it comes to professional PCB fabricators, solder mask manufacturing is never that easy. On the one hand, stringent regulations such as ISO9001, UL or RoHS etc. have to be conformed to. On the other hand, solder mask manufacturing consists of a couple of stages each of which calls for high accuracy deriving from mature technologies, rich manufacturing experience and up-to-date equipment.
The ordinary procedure of solder mask manufacturing goes on in accordance with the depiction in the following figure.
• Board Cleaning. This step aims to clean board surface so that tarnish or dirt can be eliminated with surface maintaining dry.
• Solder Mask Ink Coating. The clean board is then loaded into a vertical coater for solder mask ink coating. Thickness of coating is determined by some elements such as reliability requirement of circuit boards, fields that PCBs serve and board thickness. What's worse, board surface isn't so smooth as is imagined. Solder mask ink thickness differs when being located in different sections of the board like on the traces, on the substrate or on the copper foil. Experiential PCB manufacturers usually stipulate specific coating thickness due to considerations of equipment capability and their manufacturing experience.
• Pre-hardening. Far from total hardening, pre-hardening aims to make coating relatively solid on board so that unwanted coating can be easily eliminated from board in the developing stage.
• Imaging and Hardening. In this stage, a transparent film with some circuit images is mounted on board and then goes through UV exposure. This process makes solder mask covered by transparent section of film become hardened while section film covered with circuit images maintained pre-hardening. As a result, correct alignment has to be ensured when hardening is carried out to stop exposure of non-designated copper foil from generating short cuts or further affecting final performance of circuit board.
• Developing. After imaging and hardening, circuit board is then put into developer to clean off unwanted solder mask so that designated copper foil can be correctly exposed.
• Final Hardening and Cleaning. In order to optimize functions of solder mask, final hardening should be implemented to make available solder mask ink completely mounted on the board surface. Then, boards covered with solder mask have to be cleaned prior to further processing such as surface finish, assembly etc.
Design Tips of Solder Mask
As a matter of fact, solder mask is optional no matter what type of PCB design software you prefer to use. Solder mask can be easily designed through completing filling some parameters. Some software even can provide an automated solder mask.
Prior to real design, it's of much necessity to contact with contracted PCB manufacturer to be correctly aware of their capability in terms of solder mask thickness and minimized spacing between copper pads neither of which is cure-all for every piece of board.
Circuit board will fail due to silly issues concerning solder mask such as insufficient solder mask opening, excessive openings, mismatching between number of openings and that of copper pads in circuit plane. Those issues possibly derive from carelessness or design file modification but do take much time for confirmation. And some even arouse catastrophes. Therefore, your design files are dramatically worth your careful examinations.