Solder masks, which are also called solder resist or solder stop masks can be described as a lacquer-like thin polymer layer, which is usually placed or applied to a printed circuit board’s copper traces. These offer protection against any oxidation, as well as prevent the occurrence or the formation of solder bridges between solder pads that are closely spaced. Furthermore, solder bridges can be described as an electrical connection that is unintended and formed in-between two conductors through a small solder blob. PCBs make use of solder masks in order to prevent any of this from occurring.
Solder mask isn’t usually utilized for assemblies that are hand soldered. However, it is very important for boards that are mass-produced, which are automatically soldered making use of solder bath or reflow techniques. Immediately this is applied, the openings have to be made inside the solder mask where the components get soldered. This is achieved by making use of photolithography. Traditionally, solder mask is green in color, but now, they come in many colors.
In general, there are different pad designs. The soldermask defined pad and the copper defined pad. For the purpose of this article, we will be introducing the benefits as well as the pitfalls of the soldermask defined pad.
What are the Types of Solder Masks?
Solder masks comes in two major types
- LPI (Liquid Photo-Imageable Solder Mask)
- DFSM (Dry Film Photo-Imageable Solder Mask)
The LPI solder mask could be silkscreened onto the printed circuit board, which is exposed onto the pattern and then developed so as to offer some window openings for those parts that need to be soldered onto copper pads. The DFSM is usually vacuum-laminated onto the printed circuit board, which is then developed and exposed. The two processes above typically passed through a type of thermal cure after the defining of the pattern. This happens, even though LPI solder masks also come in the UV or ultraviolet cure.
Back then, the application of solder masks were done in the dry film sheets, however, with the invention of the LPI solder masks as well as different techniques, the DFSM is rarely utilized today. Furthermore, they are very reliable, and it could be printed accurately.
Also, it can make a much better contact when brought to the board’s surface as well as the copper, which they are supposed to offer protection to. In addition, the LPI solder masks are usually epoxy-based materials. This is why they are extremely durable and almost impossible to take out immediately after curing them.
Ways to Apply the LPI Solder Mask Onto the Printed Boards
The LPI solder mask can be applied onto a circuit board with the steps outlined below.
First, the panel for PCB production is cleaned thoroughly. This is to make sure that no dust particles found its way underneath your solder mask.
Next, these panels would be covered completely on one or the two sides using your LPI solder mask
These coated panels would be placed inside the oven to help in tack-drying the solder masks, which would be more than enough to complete the processing.
The film is usually made up of the Gerber files of your solder mask, where one takes each of the board’s sides. Wherever you want the soldermask to stick onto the board, this film would have to be clear. Also, wherever this mask should be taken off would be black on your film.
These boards which are tack-dried would be placed in the UV developed, where this film would be aligned in a precise manner over the circuit board. In addition, the film’s blacked-out locations would prevent ultraviolet light from curing masks wherever it isn’t wanted.
What are the Soldering Methods for Exposed Pad Packages?
The proper solder pad PCB designs are very important for soldering components onto a board in a proper and efficient way. Soldering methods come in two main types for exposed pad packages. These are non solder mask defined pads and the solder mask defined pads. Each of these solder mask defined pads comes with its own benefits and pitfalls. Let’s consider them
Solder Mask Defined Pads
The SMD pads help in defining the pad areas where the solder balls could be soldered. The method helps in the reduction of any possibility that the lifted pad on PCB during the desoldering or soldering process. However, there is a disadvantage. The method reduces the quantity of copper area available for connecting the solder ball, as well as reduce whatever space is present between the adjacent pads. It limits the traces thickness between the pads. This could be affect the utilization of the boas
Benefits of the Solder mask Defined Pads
- The pads improves the pads’ strength effectively
- Also, it helps in improving the BGA strength as well as its reliability
- Serves as a great and reliable option for any portal electronic product
Disadvantages of Solder Mask Defined Pads
The solderability becomes worse: Because the solder mask would be affected by high temperatures present in a reflow oven, which would affect the solder paste’s tin area.
Bad pad positioning: The soldermask’s tolerance is larger compared to the copper. Therefore, this might affect the pads’ size as well as its relative position.
Due to the increase in the copper area, the area for trace is reduced. This makes the design of the trace become very difficult.
Non-Solder Mask Defined Pads
The Non-solder mask defined pads make use of copper in defining the pad area where you’ll be soldering a solder bump. Using this method would provide a greater surface area for solder ball connections as well as offer more clearance in contrast to the solder mask defined pads. This allows trace widths that are wider as well as greater flexibility when making use of the vias.
However, non solder mask defined pads usually experience a lifted pad on PCB easily during desoldering and soldering. By this, this method aids more effective solder connections, as well as permit the solder joints to the package pads.
Comparing SMD and NSMD: Which is better as a Solder Pad PCB Style?
For SMD pads, the aperture of the solder mask is usually outlined to be much smaller compared to the pad’s diameter which it encompasses. The approach helps in limiting the copper pad’s size which the component get soldered onto. This provides two main advantages.
First, the overlay of the solder mask assists in the securing of the pads on the printed circuit boards against any mechanical or thermal stresses. Secondly, the second mask opening usually creates a guide for every BGA ball when soldering.
The non solder mask defined pads, on the other hand, have a different structure. With these pads, the design of the solder mask is done so as not to have any interaction with copper pads. This forms a gap in-between the pad’s edge as well as the solder mask.
When you allow a pad size that is smaller, the NSMD pads help in ensuring easy trace routing. This is a feature that is almost necessary when you are working with fine-pitch, high-density BGA chips. Moreover, the non solder defined pads usually experience delamination easily from the printed circuit board when under thermal and mechanical stresses. Working with standard handling and manufacturing practices should help in preventing this from happening.
Manufacturing Process of the Solder Mask for Your Solder Pad PCB
There’s this general misconception that it is easy to make a solder mask PCB and you may handle the entire procedure yourself. This isn’t really true.
DIY solder masks only function on printed circuit boards with simple designs. Besides, you must know of the present trends, make use of the right manufacturing processes and equipment in developing solder masks that are reliable.
Here are the steps that you must work with during the production of solder masks
Cleaning the board
Before you begin, make sure that you have a clean board and it is completely dust-free. Besides this board has to be dry
Ink coat your solder mask
When you are done cleaning, the board must be placed vertically. There is a need to device the solder mask’s thickness by considering the electronic components, the thickness of the board, and the reliability demand.
Pre-hardening
Pre-hardening is necessary to ensure a solid ink coating on the circuit board as well as take out any excessive coating
Imaging and Hardening
Here requires you to cover the board using transparent films having circuit images. After this, go ahead and subject this board to ultraviolet exposure.
Developing
Next, is to place the board in the developer. This helps in cleaning excess solder mask. Furthermore, this clearly exposes parts where you wish to implant these copper foils.
Hardening and then Cleaning
Harden the circuit board to ensure that its ink coating is hardened on the surface of the board. After this, clean this circuit board to get rid of dust particles out of the surface.
Conclusion
The significance of solder pad PCB cannot be overemphasized. The SMD pads help in defining the pad areas where the solder balls could be soldered. The method helps in the reduction of any possibility of lifted pad on PCB during the desoldering or soldering process.
