Aqueous and Solvent Processing of Solder Mask Materials

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Aqueous and Solvent Processing of Solder Mask Materials

A.Ekman, B.Sailer, A.von Krannichfeldt

Abstract

Different polymer systems are used for solvent and alkaline developable systems for solder masks. Full epoxy polymers as well as acrylated epoxy / epoxy resin hybrid based solder mask materials have been used in the PWB manufacturing for many years. There are two types of development processes used in the market which influences the environmental impact of waste and on the environment.

There are rather big regional differences of using solvent or aqueous developing processes. The European market is dominated, by market share, by solvent developing process material types including full in-house recycling systems. Properties and performance of these formulated insulation materials are well-known and well established at OEM level.

This paper outlines the differences between the two product groups in respect of chemistry, processes, performance and final properties.

1 Liquid Photo Imageable

Solder Mask (LPISM)

1.1 Background

LPISM was first introduced in the market end of the 1970's by using a "photosensitive FR-4 epoxy resin". This material was fulfilling the needs of a photo processing method for solder mask when circuitry was increasing in density and registration in production became an issue. Based on full epoxy resin this product is still setting the standards for market requirements. At that time curtain coating, a new application method, was introduced to the printed circuit industry for high volume production. Until then, selective screen printing using thermal or UV curing had dominated the solder mask application method. Today many other different application methods are used like screen print, electrostatic spray, airless spray and vertical roller coating.

At present, full epoxy polymers are used for dielectrics, as build-up or as plugging material thanks to its similar performance and capabilities to FR4 types of substrates.

For solder masks there was a switch from full epoxy based systems to new polymer binders which are pure acrylated systems for aqueous-alkaline development or acrylated/epoxy hybrid systems for solvent development.

What are the differences between theses systems in respect of chemistry, processes, performance and final properties as well as environmental impact?

1.2 Polymer Chemistry

In order to understand the advantages and disadvantages of a solvent developable system compared to an aqueous developable system, it is important to understand the difference in chemistry.

A solvent system has:

- a polymer with acrylic or methacrylic groups attached to the binder and additionally epoxy groups attached to the same binder polymer

- a hardener which can react with these epoxy groups: e.g. an amine based hardener or an anhydride hardener.

- the first case is a so called "epoxy-amine",

the second case is an "epoxy-anhydride" curing system.

- The "epoxy-amine" system provides better moisture and chemical resistance, therefore is the preferred system for solvent developable solder masks.

- in the exposure step, the acrylic groups react with the photoinitiator to form a film which is insoluble in organic solvents. The unexposed parts remain soluble and are developed off in solvent.

- in the thermal curing step, the epoxy groups of the binder polymer will crosslink (=cure) with the amine hardener providing a cured and moisture resistant film.

An aqueous system has:

- a polymer with acrylic or methacrylic groups attached to the binder and additionally carboxylic groups attached to the same binder polymer. These carboxylic groups render the polymer hydrophylic and provide the solubility in aqueous alkaline solution.

- the hardener is a multifunctional epoxy molecule: I

- n the exposure step, the acrylic groups react with the photoinitiator to form a film which is insoluble in aqueous alkaline systems .The unexposed parts remain soluble and are developed off in alkaline water solution.

- In the thermal curing step, the carboxylic groups of the binder polymer will crosslink (=cure) with epoxy groups of the hardener providing a cured, but still slightly moisture sensitive (hydrophilic) film.

- the reason for this is the fact, that not all carboxylic groups will react with the epoxy, leaving a small percentage of the hydrophilic groups un-reacted which is shown in the higher water pick-up and lower MIR values of an aqueous developable solder mask compared to organic developable solder mask system.

2. The Manufacturing Process

2.1 Coating and Drying of Substrate

2.1.1 Coating Characteristics

There is no major difference when applying a "solvent" or "aqueous" developable solder mask. Both contain a carrier solvent system of the same type which is removed to > 85% from the applied wet film at low temperature (75-85*C) drying, in order to achieve a tack-free panel, which can be handled in the subsequent processes.

The solvent content varies depending on the application method: for spray and curtain coating it is in the range of 40-50%, for screen printing 20-25%. For each application method the ideal solvent type is chosen to optimize the evaporation rate. However, material manufacturers tend to use the most environmental friendly solvents.

2.1.2 Drying Characteristics

The main difference between aqueous and solvent developable systems is the drying window. Due to the different developing mechanisms the solvent developable systems are much more forgiving to overdrying. Whereas for alkaline systems drying time and temperature must be precisely followed, if this is not done the product cannot be developed properly anymore.

Thanks to a more robust product chemistry the dried film of solvent systems is harder compared to the one of aqueous systems which shows a thermoplastic behaviour. Thus, solvent systems are less sensitive for handling defects especially at elevated temperatures. Also hold times, e.g. the time between drying and developing, are not critical. Solvent systems can be easily developed after 1-2 weeks using the same processing parameters.

Picture 1 Environmental general overview

The environmental impact does not depend on the type of developing process, i.e. if it is a solvent or aqueous-alkaline development, but on the application method used:

The amount of solvent evaporated from the coating line could be treated and reduced by applying different kind of solvent treatment techniques. Proven in the industry is for example catalytic incineration, biofilters and bioscrubbers and absorption methods. The solvent treatment is influenced by the actual situation and volumes at the manufacturer as well as legislation and other requirements (see picture 1).

2.2 Exposure / Imaging

Conventional aqueous and solvent solder mask systems need the same type of imaging equipment with Fe-doped mercury lamps. Today's solvent systems, i.e. the new generation of halogen-free solder masks, have the same performance in photospeed and reactivity (e.g. <300 mJ) as aqueous developable systems.

2.3 Developing

From an environmental point of view the developing step is the key step in the whole process in which solvent and aqueous systems reveal their main differences.

2.3.1 Solvent Developing (closed loop)

The solvents used for this process are typically g-butyrolactone (GBL) or ethyldiglycol (EDG). Both are high boiling solvents with low vapor pressure (low evaporation) and high flashpoint (not highly flammable).

Solvent

GBL

C4H6O2

EDG

C6H14O3

Cas Nr.

96-48-0

111-90-0

Flash point (*C)

104

94

Ignition Temp. (*C)

445

220

Vapor pressure(10-20C)

0.2-0.3 mbar

< 0.1 mbar

Threshold health values

( MAK)

No, please check regional

legislation

No, please check regional legislation

Fully biodegradable

(OECD)

Yes / Good

Yes / Good

COD (mg/l) BOD5 (mg/l)

1674 1160

1910 955

The solvents applied for the development are different from the ones used as carrier in the ink. They can be recycled by distillation for continuous use.

In order to prevent the solvent drag out into the rinse water an air-knife is blowing off the solvent from the panels, thus returning the solvent into the last solvent zone. The small residual solvent quantity (drag out) remaining on the panel is going into the rinse water. The rinse water has normally a very low COD of <600 mg.

Fresh or recycled solvent is fed into the last solvent zone in the developer. The solvent cascades into the 2nd and 1st zones. The used developer is then pumped from the 1st zone to the distillation unit and distilled. Then the solvent and water absorbed from humidity in the air is distilled, separated and returned to the last and clean third zone of the developer. The final waste which contains the developed solder mask polymer is collected and can be incinerated at high temperature in a controlled way (see pictures 2, 3 and 4).

Picture 2 Close<?xml:namespace prefix = st1 />Loop Solvent Developing Process

Picture 3 Differences in waste water

Picture 4 Biodegradability of g-Butyrolactone

2.3.2 Solvent Developing Mechanism

The developing mechanism is a softening and dissolving process of the polymer (see picture 5).

Picture 5 SolventDeveloping Mechanism

2.3.3 Aqueous Developing (direct disposal)

The developer waste from the 1st zone has a high COD value of 3000-7000 mg/l and should be waste-treated accordingly. The final COD value might depend on waste treatment method, but after neutralization precipitation methods, the sludge comes down to 500-2000 mg/l levels, which is higher than in the solvent developing process (see picture 6).

Picture 6 Aqueous Developing process

Instead of feeding solvent into the last developer zone new sodium or potassium carbonate (0.8-1.2%) alkaline solutions are used.

2.3.4 Aqueous Developing Mechanism

The aqueous developing mechanism is different to the solvent as the carboxylic group on the polymer chain reacts with Na or K ions forming a Na or K salt which allows dispersion of the polymer into the developer solution. The feed and bleed of new Na2CO3 developer solution (4-6 l/m2 panel area) can be controlled be conductivity or pH as sodium-bicarbonate NaHCO3 is formed which is inactive in the developing process but reduces the pH. pH could then be used as process control in range 10.8-11.2 to ensure active and good developability (see picture 7).

Picure 7 Aqueous Developing Mechanism

2.3.5 Consumables

The consumable materials and volumes are considerably higher in the aqueous process, since higher volumes of developer solutions as well as rinse water are used which should be considered in the total process cost calculation.

2.4 Thermal curing

The last process step is the thermal curing of the solder mask polymer. The curing chemistry can consist of different types depending on the hardener system used. However, there is normally no difference in processing time or temperatures. Typically 50-70 min at 150 *C are used in production.

3 Final performance

The solder mask has to fulfill the requirements of the final application.

Some of these basic requirements are:

A. Post solder mask plating processes

- Selective immersion Sn, ENIG or Ag

- Selective immersion High P ENIG (SIT)

- Galvanic Au

- HASL (tin/lead or lead free process)

- OSP

Some of these processes are more or less aggressive to the solder mask, where SIT and immersion Sn has special requirements of resistance and performance.

B. Electrical & other requirements

- Solderability & repair at high temperature

- Flammability (UL 94 V-O)

- Halogen-free (JPCA standard)

- High dielectric strength

- Low ionic contamination

- Moisture insulation resistance

- E-corrosion resistance

- Thermal cycling resistance

- High temperature stability

- Specific chemicals / environment for final

application

- Laser bar code compatibility

International standards, such as IPC SM 840B, outline basic requirements for solder masks and UL 94 for flammability, thus fulfilling most of the standard electronic requirements.

For high-end applications, the specific OEMs may have additional requirements.

4. High Temp. Requirements

New automotive designs require higher temperature stability up to 150 *C for 1500-2000 h, as the mounted boards are closer to the motor.

The individual polymers used for solvent or alkaline

developed solder masks perform differently at these elevated high temperatures. A halogen free solvent solder mask was tested together with the alkaline equivalent.

Test criterias:

A. Insulation resistance after thermal cycling test

-40*C to + 140*C, 30 min hold time at temperatures with 10 sec transfer time, 1038 cycles (=1038h)

B. Insulation resistance after temp. storage test 2000 h at 140*C or at 150*C

Test results

A. Insulation resistance after thermal cycling test.

Conditions: 85 *C / 85% relative humidity

Alkaline developed Solder Mask

Solvent developed solder mask

The insulation resistance level under humidity, before and after testing, was one decade lower for the alkaline product. After thermal cycling the resistance recuperates with both solder masks.

B. Insulation resistance after temp. storage test.

Conditions: 85 *C / 85% relative humidity

Alkaline developed solder mask

Solvent developed solder mask

The solvent product had superior performance to the alkaline system showing a higher temperature stability of up to 2000h at 150 *C.

The alkaline product had a temperature stability of up to 1500 h at 140 *C, since the mask was peeling or had significant adhesion loss. The insulation resistance level under humidity, before and after testing, was one decade lower for the alkaline product.

5. Conclusion

There is no basic difference between solvent or alkaline products concerning the solvents in the products, which are evaporated in the drying process. Solvent developable products are generally more forgiving and less sensitive in the complete production process.

The solvent developable process is environmental friendly as it allows full recycling of the solvent developer media. The solvents used are fast biodegradable in water and in the environment.

The hydrophobic polymer chemistry used for the solvent product formulations give an outstanding performance, such as:

- Resistance to all known subsequent post solder mask processes

- High dielectric strength ( 150-170 kV/mm)

- High Insulation resistance and e-corrosion at high temperatures & humidity

- Excellent thermal cycling resistance

- Excellent chemical resistance to aggressive environment (chemicals, gases, etc)

- High temperature stability > 2000h at 150*C, meeting high temperature application requirements (e.g. of the automotive industry)

For more information contact:

Huntsman Advanced Materials,<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Klybeckstrasse 200, CH-4057 <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />Basel, Switzerland.

www.probimer.com

Watch their Product Demo Video at Productronica 2005