How to Use PCB Inks
Printed circuit manufacturers use various types of PCB inks on the boards they manufacture. These mainly include liquid inks for the solder resist or solder mask layer and for the surface printing or silkscreen layer. Eminent PCB manufacturers like Rush PCB Inc. select liquid PCB inks carefully, after considering various factors for their performance. These factors include several technical parameters that in combination, bring out the quality and environmentally friendly nature of the PCB ink.
Application of PCB Inks
PCB manufacturers apply the solder mask layer for a variety of reasons. The primary reason is to prevent the copper traces from meeting air. Various impurities and chemicals present in air immediately surrounding the PCB can corrode the copper traces. Covering the traces with a solder mask layer prevents such corrosion, thereby improving the life of the board.
Another important function of the solder mask layer is to prevent solder bridging. When present between two adjacent solder pads, solder mask prevents molten solder from crossing over to the nearby pad when the board is undergoing soldering.
Manufacturers usually apply liquid photo-imageable solder mask by screening an epoxy liquid through a pattern on to the board. After defining the pattern, the board must undergo a thermal curing process to dry and fix the ink on the PCB surface.
Assemblers require plenty of information for properly mounting components on the PCB. Manufacturers print such information on the surface of the PCB over the solder mask. Information such as component orientation, component numbering, positioning of mechanical features help assemblers to position components without errors.
Manufacturers apply liquid epoxy through a pattern on to the board. Color of the surface printing liquid is usually in contrast to that of the solder mask, providing good readability. Some manufacturers may use a printer to dispense the lettering for the surface printing.
Important Technical Parameters of PCB Inks
Ink quality is a combination of several technical parameters of the PCB ink. These include multiple properties like:
- Dynamic Viscosity
- Chemical Resistance
- Physical Resistance
- Insulating Properties
- Safety and Environment Protection
For the liquid PCB ink, its dynamic viscosity is its resistance to deformation at a given rate. The common concept of dynamic viscosity is the thickness of the liquid. One can also imagine dynamic viscosity as individual layers of the fluid resisting the movement of adjacent layers because of friction.
Highly viscous PCB inks can be difficult to apply through the screen-printing process—the thick liquid might not pass easily though the screen openings—resulting in patchy printing. On the other hand, low viscosity liquids might easily smear during lifting the screen, resulting in the PCB ink covering areas it should not.
Plasticity of a liquid defines its capability of being able to deform easily without rupturing. It is a measure of the resistance of the liquid to free flow. PCB inks with low plasticity will result in patchy printing after passing through the screen as the ink will rupture. On the other hand, a highly plastic ink will easily deform after printing, smearing the result.
Thixotropy is the reversible variation of viscosity over time. The PCB ink generally loses its viscosity immediately after the operator has forced it through the screen, but regains its viscosity after some time. A good thixotropic PCB ink must quickly lose its viscosity during the printing process, but it must quickly regain it after the printing is over.
While printing, the operator forces the PCB ink through the screen openings. This causes the ink to spread under the influence of the external force. The external force makes the ink more fluid by reducing its viscosity. Therefore, to some extent, fluidity of the ink is the reciprocal of its viscosity. Also, fluidity of the ink depends on its plasticity and thixotropy. Inks with high fluidity are easy to work with, as they spread easily after printing. Inks that have low fluidity cause netting or sticking, resulting in patchy printing.
PCB inks must exhibit both viscous and elastic properties during the printing process. The ink will initially resist the deformation imposed by the printing process, especially when passing through the screen. Once the printing force ceases, the PCB ink must return to its original viscous nature as quickly as possible.
The PCB ink must remain in wet state after the printing process ends. It must not dry out before the curing process.
The pigment and solid materials making up the PCB ink must be small enough to allow the ink to pass through the screen mesh without hindrance. Rush PCB Inc. prefers keeping the fineness to lower than one-third of the mesh openings.
When an operator picks up some liquid PCB ink in an ink shovel, filaments of ink may form between the shovel tip and the original container. The filaments must break away easily to allow clean printing. If the liquid ink exhibits stringiness, that is, the filaments remain as long strings, it is detrimental to the printing process, as the strings can adhere to the printing mesh and make it impossible to obtain a clean print.
Depending on the application, PCB inks must adhere to strict standards for resistance to acids, alkalis, salts, and various solvents.
After curing, the PCB ink must be able to withstand external forces such as thermal shock, scratches, and peeling off.
As manufacturers use PCB inks as solder mask between adjacent pads and to cover bare copper tracks, PCB inks must exhibit good electrical insulating properties. Once cured, PCB inks must not form pin-holes that can be detrimental to insulation and aid arcing.
Safety and Environment Protection
PCB inks must be odorless and non-toxic. As human hands may encounter the PCB ink during assembly and mounting, the PCB ink must also be safe. Discarded PCBs must not have PCB inks that can pollute the environment in any way.
Although the above list includes almost all parameters that Rush PCB Inc. considers while selecting PCB inks for an application, the two most important parameters for any PCB ink are its viscosity and thixotropy. While viscosity affects the overall printing quality of a PCB ink, its thixotropy affects the printing accuracy.