Following from our first post on Solder Masks, we wanted to release today’s post in celebration of #TechTuesday and reveal to you a little behind the scenes information about the processes used for Solder Mask application.
Solder Mask is applied to PCBs for a number of different reasons, some of which are as follows:
- Enable mass soldering techniques
- Prevent solder shorts under components
- Prevent corrosion to underlying circuitry
- Plating resist for surface finishes
- Prevent growth of metal whiskers
- Insulate substrate from debris and environment
- Assist with component placement
Solder Mask Selection
A number of factors need to be taken into consideration when selecting an appropriate Solder Mask. If Solder Mask is selected post design phase then the feature sizes must be examined to ensure that the Solder Mask type and application process is suitable for the technology of the PCB submitted.
In scenarios where a particular Solder mask type is required it is important to examine the technical data sheets and required application process and discuss this with the selected PCB manufacturer prior to the design phase. In the interest of time, cost and efficiency it is prudent to understand any restrictions at the outset to reduce the possibility of design failures at a later stage.
Factors that you may wish to consider include:
- Feature Sizes
- Registration Tolerances of PCB Manufacturer
- Required Electrical Performance (impedance control)
The Application Process
Solder Mask is applied to the PCB panel post etching. The panels are first scrubbed, brushed and cleaned. It is extremely important to ensure that Solder Mask is applied in a clean and dust free environment. There are a number of different methods for applying Solder Mask including screen printing, curtain coating, HPLV Air Spray and Electrostatic Spray. The most common method in the UK is screen printing either automated or by hand.
Here at Ragworm HQ we use a single sided manual screen print methodology which we believe provides us with both flexibility and high visual integrity. Manual hand printing has a lower set up time and cost involved when looking at prototype and small batch productions and is therefore perfect for the services we offer. Our other UK sites offer automated double sided printing to ease the time burden of high volume manufacture.
Following the coating process the panels are placed into racks and put into an oven which hardens the resist just enough to allow it to be printed. This process is commonly referred to as tack-drying. At this stage our operator will inspect the panel carefully checking each panel for an even coating.
The next process is the imaging of the coated panels. For this we use a UV printer. The operator mounts the photo tool films on the machine and then places the panel onto the registration pins. The operator will run through a number of checks to ensure that the film and the copper layer are precisely aligned. Any poor registration at this point could result in the job needing to be scrapped and started again costing both time and money. The UV lamps in the machine harden the ink where the film is clear; the required locations for Solder Mask on the finished board.
The imaged panels are next moved into the developer which strips off the unhardened and unwanted resist. Later the required resist will be further hardened or “cured” to provide a robust and permanent coat. For this we use the same process as previously explained for the ‘tack-drying’ process. Solder Mask is cured at 150 degrees for a period of 60 minutes. But first the operator checks the alignment of the Solder Mask on the panel and makes sure that there are no traces of ink on the pads or through the holes. Even slight traces will compromise the solderability of the finished PCB.
As we can see – Solder Mask has a far greater function that just providing the design/colour to the PCB! It is important that as designers and engineers we understand all of the elements involved in order to provide the best results!