Options include the traditional screen printable epoxy inks, liquid photoimageable solder mask (LPSM) inks and dry film photoimageable solder mask (DFSM).

Solder Mask Options

Precision have worked with all of them and can therefore discuss the technical and cost advantages and disadvantages of all three without bias.

Originally, the main requirement for a solder mask was its ability to prevent solder from bridging components to conductors. With the advent of surface mounting and the introduction of fine pitch components, the tolerance requirements for the solder mask have tightened considerably, in many cases below the clearance requirements for screen printing.

Clearance Requirements

The performance requirements for solder masks have also increased and today’s solder masks can be expected to:

• prevent solder bridging and wicking under components.
• provide environmental protection to circuitry.
• prevent metal migration.
• aid the assembly of SMDs.
• be compatible with SMD adhesives & conformal coatings.
• tent via holes.
• provide an insulation coating.
• be aesthetically pleasing.
• help to achieve high first pass yields during application.
• help to achieve high first pass yields in assembly process.
• be easy to clean to meet ionic contamination tests.
• meet UL & IPC strict criteria.

The selection criteria is not always easy, with the designer, the fabricator and the assembler all having different needs. Some of the design considerations include mask thickness, track & spacing, via tenting and dielectric requirements. Issues which concern the manufacturer are cost of labour & materials and expected first-pass yields during application. The assembler’s needs revolve around the ability of the mask to reduce reworking and its compatibility with the loading, soldering & cleaning processes.

A close co-operation between the designer, manufacturer and assembler will help to define the most cost-effective solution, depending on the packaging density, the soldering & testing processes and the performance level.

All three different types of solder mask materials are processed differently and whilst they may share some advantages they differ in others and certainly have distinct disadvantages.

SCREEN PRINTABLE (EPOXY)
The screenprint process has been used for centuries and is widely used in many diverse industries, albeit generally on flat surfaces, where few difficulties are encountered. However, when one introduces the uneven surface of a PCB, many problems can be experienced. These problems have many times ruined what was otherwise a perfect board.

THE PROCESS
The following is a simplified flowchart of the screen printing process:

THE ADVANTAGES

Cost
Of the three solder mask finishes, screen printable ink is undoubtedly the lowest cost process. Generally, however, this material is most suited to the lower technology end of the market.

THE DISADVANTAGES

Number of Variables to Control
The total list of variables associated with the control of screen printing are numerous resulting in an increased likelihood of problems. Due to the number of these variables, set up time is significant and can vary a great deal. Highly skilled operators are employed in the screen printing department, each adding their own idiosyncrasies to the standard method of set up.

Inconsistent conductor coverage
Almost all PCBs designed have tracks which run in every conceivable direction possible. Conductors which run in the same direction of print are the easiest to cover. Any deviation from this can lead to coverage problems, especially when perpendicular to print.

Isolated tracks which invariably plate thicker in the pattern plating process, are another reason why screen-printers can struggle to achieve required quality levels.

Various methods can be tried to combat this problem; i.e. printing at angles, softer squeegee, increase mesh size, double printing, increase printing pressure (which further stretches the mask causing misregistration), but these only add to the already long list of variables which haunt screen-printers.

Limitations on higher technology PCBs
In our experience the repeatability of screen printing is around +/-0.2mm, therefore the required annulus around the pad has to be at least that much. For the ever increasing percentage of PCBs requiring gaps of 0.2mm or below, screen printing is not an option. It may be possible on very small boards, but is certainly not feasible on panels.

LIQUID PHOTOIMAGEABLE (LPSM)
Since their introduction in recent years, photoimageable inks have proved to be a reliable system in meeting the requirements of printing precision. Similar image transfer tolerances can be achieved to those experienced in the process for the reproduction of conductive patterns.

THE PROCESS

The following is a simplified flowchart of the liquid photoimaging process:

There are three methods of applying the photoimageable ink to the boards:

1. screen printing
2. electro-static coating
3. curtain coating

The large investment cost of $500K+ for curtain coating and electro-static systems, has proved too large a barrier for local manufacturers.

THE ADVANTAGES

Suitable for higher technology PCBs
The ability to reproduce size for size images enable this process to be more suited for high technology boards.

This ability coupled with the finished thickness of the mask makes it the preferred finish for fine pitch SMD (below 1.27mm), by enabling a good coverage of the solder paste when screen printed.

Conductor Coverage
Conductors should be completely covered although thinning out can be expected on line edges.

Ease of Control
The photoimageable process is easier to control and variation experienced is reduced with many of the key parameters easily controlled and monitored.

THE DISADVANTAGES

Unable to tent via holes
Tenting or blocking of via holes is not possible with wet inks.

High cost investment for equipment
Initial investment for equipment is high especially for electrostatic and curtain coating.

Uneven finished surface topography
The finished mask follows the contour of the circuitry making the application of legend ink more difficult. Greater controls are required to ensure legibility is not impaired.

DRY PHOTOIMAGEABLE (DFSM)
Dry film solder mask is supplied as a photo-sensitive film. This type of mask has now been available for a good ten years or so and is widely used in the industry around the world.

THE PROCESS
The following is a simplified flowchart of the dry photoimageable process:

THE ADVANTAGES

Suitable for higher technology PCBs
The ability to reproduce size for size images enable this process to be ideally suited for high technology boards.

Conductor Coverage
Irrespective of design of PCB and copper thickness, the dry film solder mask completely covers the conductive pattern, with minimal thinning on line edges.

Ease of Control
The supply of resist coated dry film along with the relatively few process steps makes this process easy to control and highly predictable.

Ability to tent holes
Many dense circuit designs call for via holes to be tented with solder mask.

Tenting is used for various reasons:

1. prevention of flux entrapment and solder wicking under SMDs.
2. reduction in solder consumption.
3. minimisation of component insertion errors.
4. replacement of spacers under critical components.
5. clarity of unbroken outlines and text on the legend artwork.

THE DISADVANTAGES

Higher Cost
Dry film is the most expensive of all the solder mask types discussed.

Colour Range
Unlike the wet printable and photoimage-able inks, dry films are only available in green, albeit in different shades.

Vacuum Rings
Rings can form around large holes & slots.

Thickness
The 75mm thick wall can prevent even deposition of the solder paste onto the pads on fine pitch SMDs.

END USER REQUIREMENTS
It is important that the end user understands the soldermask he is specifying in order to maximise the desired benefits.

In many cases, there are now requirements for PCBs with mixed technology comprising of Surface Mount Devices (SMD) as well as leaded components for through hole mounting. To allow for this mixture, both wave soldering and reflow soldering will be used.

Whilst one solder mask type may have some limited advantage depending on the assembly techniques used, it is generally the case that each can perform satisfactorily during assembly provided that the placement, gluing and soldering operations are set up accordingly; eg dry film solder mask provides a thicker coating than wet ink, therefore application and control of gluing and solder paste would require to be set in accordance with the thickness variation in mind.

The following is a simplified flowchart designed to help facilitate the decision making process for determining the best solder mask type for your needs.

CHARACTERISTIC	SCREEN PRINTABLE	LIQUID PHOTO- IMAGEABLE	LIQUID PHOTO- IMAGEABLE
Cost of materials Cost of equipment Suitability for fine line PCBs Suitability for standard SMDs Suitability for fine pitch SMDs Conductor coverage Ease of control Tenting Capability Legend Application Definition Definition of Mask Suitability for high volume automation Handling/Scratch resistance	1 1 3 3 3 3 3 3 3 3 3 2	2 3 1 1 1 2 2 3 2 1 1 3	3 2 1 1 2 1 1 1 1 1 2 1

SUMMARY
The following table attempts to summarise and rate all the key issues.      (Note: 1=best and 3=worst)