Topic

Application methods and their influence on solder mask processing

The course team are very grateful to Shaun Tibbals of Electra Polymers & Chemicals Ltd for permission to convert material originally prepared for publication in CircuiTree magazine. Shaun may be contacted at shaun.tibbals@electrapolymers.com, and we would encourage you to visit the company web site at http://www.electrapolymers.com/.

Introduction

Advances in Printed Circuit Board (PCB) technology place ever increasing demands on solder masks in terms of their processing capability and final properties.

This paper describes the application methods currently available for Liquid Photoimageable Solder Mask (LPISM) and discusses how these different methods influence solder mask processing and capability at the three main stages of coating, drying and imaging.

The paper concludes with the use of an evaluation matrix based on LPISM application Key Success Factors (KSF) derived from current and future demands.

LPISM application methods

There are currently four main categories of photoimageable solder mask application:

Screen print

LPISM is applied to the PCB with a squeegee blade through a tensioned mesh. Mesh count and print settings (angle, speed, pressure) can be varied to control ink deposit within a relatively narrow range (10–50µm) with a single coat.

Application can be anywhere from manual to fully-automatic by way of horizontal and vertical techniques, thereby allowing for single and double-sided coating (Figure 1).

Screen-printing technology is well established and understood within the industry with a wide variety of equipment suppliers.

Figure 1: Double-sided and single-sided coating

Double-sided and single-sided coating

Curtain coat

LPISM is applied as the PCB passes through a ‘curtain’ of low viscosity (<10 Poise) ink which falls through a narrow (0.3–1.0 mm) slot (‘nip-gap’) in a holding ‘head’.

Application is generally automatic and because of the low viscosity of the LPISM, all panels have to be maintained in the horizontal position until complete or partial evaporation of the solvent has taken place. The very nature of the coating methodology restricts the coating process to a single-sided operation.

Coating speeds are relatively high (80–100 m·min−1) and in principle, the coating thickness is infinitely adjustable provided sufficient ink flows through the nip-gap to maintain a stable curtain. As with screen-printing, curtain-coating is well established and understood within the PCB industry.

Electrostatic spray

LPISM is applied from a serrated turbine bell rotating at 25–50,000 rpm. This rotating bell, aided by compressed air, atomises the ink and deposits it on the PCB. In addition, the LPISM is given a negative charge and the PCB is earthed. Therefore, based on the fundamental principle that unlike charges attract; the LPISM is attracted to the PCB.

Application can be semi or fully automatic and both horizontal (single-sided) and vertical (double-sided) units are available. Compared to the other application methods, equipment tends to be higher cost and there are limited installations worldwide.

Air spray

LPISM is applied using conventional, well established spray gun technology where the ink is mixed with decompressing air resulting in atomisation. In most cases the ink is heated prior to spraying in order to drop the viscosity and compensate for the cooling effect of the expanding air. Systems lend themselves to double-sided coating and are available in semi-automatic and fully-automatic formats.

LPISM spray technology is based around two methodologies;

  1. Horizontal systems where coating is carried out by a single-gun traversing across the surface of the board.
  2. Vertical systems where panels are coated as they pass a stationary multiple gun array.

Air-spray is the newest of the four application methods, with the first machines being introduced in 1994.

Influence of application methods on LPISM processing

Coating: cosmetics and uniformity

Providing parts are clean and undamaged, both screen-printing and curtain-coating systems generally apply a uniform coating to the board surface.

Multiple gun spray systems are generally more difficult to set up and have a tendency to form stripes across the PCB. Striping is caused by overlap or interference between adjacent guns and as such is not experienced with single-gun designs.

Table 1 shows the results of a thickness uniformity test carried out across the surface of ten panels (18" x 24") sprayed with a single-gun system.

Table 1 : Cured thickness (mm) across 18"24" panel
29.5 29.0 31.0 30.2 29.7
30.2 29.7 31.2 30.7 29.7
30.2 32.0 31.0 30.2 30.2
31.0 31.2 30.7 30.0 30.0
Mean: 30.4 µm; Std.Dev: 0.7 µm;
UCL: 32.6 µm; LCL: 28.2 µm

Electrostatic spray systems are prone to non-uniform coatings because the electrostatic effect preferentially attracts the LPISM to the copper areas. This poses two problems:

The copper coating characteristics will vary depending on circuitry layout, leading to certain part numbers requiring special voltage settings.

Areas of bare laminate coat with a thinner deposit, because they are ‘robbed’ of LPISM by larger areas of copper.

The above problems can be overcome by reducing the voltage potential or increasing the LPISM conductivity by use of alternative diluents. However, these simply reduce the electrostatic effect and in turn lower the transfer efficiency.

When spraying it is important that the LPISM has good atomisation characteristics. If difficult to atomise, the solder mask will form a mottled surface or require excessive atomisation pressure and/or dilution, leading to lower transfer efficiency and inferior track coverage.

Coating: track encapsulation and coverage

Table 2: Track encapsulation and coverage characteristics
Screen print Curtain coat Electrostatic spray Air spray
Good coverage of tracks <100–125µm with a double print stroke. Above this track-height it becomes necessary to apply two coats. Tracks above 80–90µm tend to suffer from poor edge coverage. This can be compensated for with higher coating weights but generally requires a double coat. Most tracks cover well unless robbed of solder mask by a larger copper area. Tracks in excess of 400µm can be successfully coated with a single coat.
Viscosity and thixotropy generally prevent slumping from track edges after coating. To maximise encapsulation at 80–90 m·min–1, viscosity and thixotropy must be significantly reduced. This leads to slumping from track edges after coating. Although solder masks are sprayed at a low viscosity (similar to CC), thixotropic properties and ‘in-flight’ solvent loss prevent slumping from track edges.
Spray solder masks’ rheological properties are optimised to prevent slumping from track-edges.
Skips can generally be overcome with slower print speeds and/or multiple print strokes. Once track/gaps fall to certain levels (typically 100–150µm depending on track height) full encapsulation becomes impractical. Skips tend not to be a problem unless viscosity is too high or the solder mask has poor atomising characteristics.
Both spray technologies allow encapsulation below 100 m m track/gap configurations.

Optimised rheological characteristics are key to achieving the best possible performance but the nature of some application methods imposes considerable formulation boundaries. This often results in a trade-off between encapsulation and coverage.

Curtain coat solder masks must be long flowing at low shear stresses to ensure that the materials flow between tracks during and immediately after coating. Apart from affecting edge coverage this also imposes restrictions regarding very matt formulations. This is because the overall level and oil absorption properties of the fillers used in matt solder masks tend to decrease material flow, particularly at low shear stresses.

Screen print processes involve much higher shear stresses and slower coating speeds which enable the use of more thixotropic materials.

The spray processes permits very high levels of thixotropy resulting in excellent track coverage.

Note that there are substantial differences in rheological characteristics between materials that have been optimised for a different application methods: under typical conditions, the viscosity of the three materials spans two decades. That is, spray process materials are two orders of magnitude higher in viscosity than those for curtain coating, whilst screen print materials are intermediate in their resistance to flow.

These differences lead to a difference in consumption and not just a difference in capability. Table 3 shows typical coverage figures for the different application methods. The values are based on achieving a minimum of 10µm over 65–70µm tracks.

Table 3: Typical solder mask consumption
Application method Dry thickness (µm) Coverage (m2·kg−1)
Screen print 25–30 17
Curtain coat 40–45 12
Electrostatic spray 35–40 14
Air spray 30–35 16

Drying

The objective of the drying stage is to remove the solvent from the coating with minimal activation of the curing reaction. Excessive activation of the cross-linking process will cause reduced hold-time capability and ultimately solder mask residues from poor developing.

The main differences between the various application methods that affect the drying process are:

a) Single or double-sided coating

b) Solder mask thickness

c) Solvent level

d) Solder mask in/around holes

e) Air bubble inclusion

Table 4 : Factors and characteristics affecting LPISM drying
  Screen–print Curtain coat Electrostatic spray Air spray
1- or 2-sided

All the application methods discussed, with the exception of curtain coating and horizontal electrostatic spray, allow both sides to be coated and dried in a single stage (double-sided coating). This avoids the trade-off between drying the second side coated sufficiently to prevent artwork marking or increased undercut and not over-drying the first side coated.

Solder mask thickness

Approx. 25–50µmm over most of the PCB.

Thickness between tracks can easily reach 60–80 m m depending on track height/gap.

Approx. 80–90µm over most of the PCB.

Coating speed means that thickness between tracks is lower than screen printed LPISM.

Approx. 50–75µm over most of the PCB.

Thickness between tracks is minimised. Certain copper areas are prone to thicker ink deposits caused by electrostatic effect.

Approx. 50–65µm over most of PCB.

Thickness between tracks minimised.

Solvent level

Approx. 25% (w/w) solvent on PCB.

Equates to ≈16 g·mm−2 needing evaporation

Approx. 42% (w/w) solvent on PCB.

Equates to ≈46 g·mm−2 needing evaporation

Approx. 30% (w/w) solvent on PCB.

Equates to ≈28 g·mm−2 needing evaporation

Approx. 35% (w/w) solvent on PCB.

Equates to ≈28 g·mm−2 needing evaporation

Ink in/around holes

Significant LPISM deposition in holes which must be dried to prevent artwork damage. Tends to lead to increased drying times depending on level of fill.

Large tooling holes and slots tend to have heavy LPISM deposition. Under certain conditions thick ‘tear–drops’ may form on the underside of the PCB.

Generally both spray methods leave the holes almost free of LPISM eliminating the need for extended drying times.

However, a thin deposit of LPISM does coat the barrel of the holes which is quick to dry and can be a source of residues.

Air bubble inclusion

Mixing and shearing action during printing induces air into the LPISM.

Viscosity and thixotropy of SP materials slow the release of these bubbles.

Slow solvents and reduced thixotropy can be introduced to aid the release from between high tracks but this results in a trade–off with edge coverage.

Pumping and coating action induces air into the LPISM.

The low viscosity and thixotropy of CC materials allows quick release from between high tracks. Temperature ramp up must be controlled to prevent the LPISM surface ‘skinning–over’ and trapping bubbles in the coating.

Both electrostatic–spray and air–spray applications apply LPISM via a fine mist.
As a result, neither method is prone to air entrapment.

The low risk of air inclusion allows the use of faster temperature ramp–up during drying leading to reduced cycle–times.

Imaging

The main differences between the various application methods which affect the imaging process are:

Solder mask thickness

Table 3 and Table 4 identified the differences in thickness typically deposited by the alternative application methods. Ink deposit around tracks and pads is key to imaging capability.

The greater the solder mask thickness, the more difficult it is for the ultraviolet light to penetrate to the base of the coating. leading to a cure differential between the top of the LPISM and the base and potentially excessive undercut.

Figure 2: Effect of LPISM thickness on light penetration

 

Greater thickness can result in insufficient polymerisation at the base of the coating manifesting itself as poor resolution capability.

To compensate for a higher LPISM thickness it is necessary to increase the exposure energy. This reduces productivity, impairs artwork stability and, unless the light source is highly collimated, causes image growth at the LPISM surface.

Figure 4: Diagrammatic view of undercut & image growth

Diagrammatic view of undercut & image growth

 

Solder mask in holes

As pointed out in Table 4, differing amounts of LPISM are deposited in the holes by the four application methods, requiring different developer dwell times to ensure they are washed clean.

The increased undercut caused by longer developer dwell times adversely affects resolution capability and although this can be compensated for by increased exposure energy it leads to reduced productivity.

Despite elaborate methods to prevent the alignment of holes on successive prints or scraping excessive LPISM from the back-side of the mesh, screen-printing tends to deposit large amounts of ink into both component and via holes. Unlike curtain-coating which tends to deposit ink in larger tooling holes or slots, these smaller component or via holes are difficult to wash clean due to reduced impingement by the developing solution.

Table 5 demonstrates this difference and highlights the gain in line speed delivered by the spray applications.

Table 5 : Typical LPISM developing speeds
Typical developing speed
for 2m chamber
Screen print 1.3–2.0 m·min−1
Curtain coat 2.0–2.5 m·min−1
Spray 3.5–4.0 m·min−1

These differences in line speed and capability become even more apparent when using very matt LPISM formulations which, because of their overall solubility, are prone to being stubborn to remove from via holes.

Air bubble inclusion

Process control can yield minimal undercut but if bubbles are present in the base of coating they can increase the risk of solder-dam adhesion loss during developing or hot-air-solder-level. The low risk of air-inclusion offered by the spray systems offers a distinct capability advantage when imaging solder-dams of 100µm and below.

Figure 4: Diagram displaying air inclusion at base of solder-dam

Diagram displaying air inclusion at base of solder-dam

 

Table 6 shows the results of an experiment to investigate the practical differences in imaging of LPISM when applied by screen-print and air-spray. The tests were carried out using a purpose designed board with the following requirements:

Table 6 : LPISM imaging characteristics – screen print and air spray
  Green matt High resolution Dark green matt High resolution
Screen print Air spray
Developing speed:
clean 0.2mm holes
90s dwell 90s dwell 30s dwell 30s dwell
Note: Holes were plugged after printing
Exposure energy:
hold 50µm dams
822 mJcm−2 583 mJcm−2 270 mJcm−2 134 mJcm−2

From the above results it is evident that benefits attributed to reduced ink deposit in holes and between pads allow significant improvements in capability and, in turn, productivity.

Transfer-efficiency refers to the percentage of material leaving the sump that is actually deposited on the PCB.

Increasing demands on LPISM

Reduced feature size

Reduced hole diameter

Aggressive surface finish chemistry

No-clean and organic flux assembly

Reduced cost

These demands can be used to compile a range of key success factors for a LPISM and its application.

LPISM application Key Success Factors

Table 7: LPISM application key success factors
Analysis of demand LPISM – Key Success Factor
Good cosmetics
  • Uniform coating attributes
  • Good track coverage
No residues after developing
  • Wide drying window
  • Adequate hold-time after drying
No solder mask in holes after developing
  • Restricted ink deposit in holes during coating
  • Good solubility of ink in developing process
No broken solder-dams
  • Good through exposure characteristics
  • Minimal micro-bubbles against tracks and pads
  • Fast developing speed (reduced dwell times)
No resist breakdown
  • Good track coverage (optimum – min. 10µm)
  • Adequate final cure
  • High chemical resistance
No assembly related defects
  • Anti-solderball capability
Output
  • Maximum line speed at all process stages
  • Minimal need for operator intervention
Cost
  • Minimal material consumption
  • Equipment cost

Summary

By comparing the characteristics and capabilities discussed in the earlier sections with the key success factors identified in Table 7 it is possible to rate the overall suitability of each application method for today’s market place.

An example of this evaluation is detailed in Table 8. The results of this table are based on a pragmatic view of the LPISM process. It should be pointed out that depending on the PCB fabricator’s market position, business strategy and financial situation it is possible to weight certain KSF factors higher than others.

Table 8: Evaluation matrix of application methods against key success factors
Key Success Factor SP CC ES AS
Uniform coating attributes 8 8 5 8
Good track coverage 7 4 7 9
Wide drying window 7 4 9 9
Adequate hold time after drying 7 4 9 9
Restricted ink deposit in holes during coating 2 6 9 9
Good through exposure 3 6 9 9
Minimal micro-bubbles against tracks/pads 3 5 9 9
Fast developing speeds (<dwell times) 2 5 9 9
High chemical resistance 8 6 9 9
Anti-solder ball capability 5 3 8 8
Max. line speed at all process stages 3 6 9 9
Minimal need for operator intervention 6 9 4 6
Minimal material consumption 9 4 6 7
Equipment cost 8 6 2 7
Overall rating 78 76 104 117
Key: 1 = Poor; 10 = Excellent

Conclusion

Although a single application method cannot satisfy all of the demands placed on a liquid photoimageable solder mask and its processing; careful selection of the appropriate coating technology can open the process window, improve quality and reduce costs.

Based on advances in technological demand, it is felt that spray application of solder mask offers distinct process and end-user advantages through the nature in which the materials are deposited on the PCB.

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