Introduction of Solder Mask

Process Purpose and Solder Mask Features

Principle of solder masking

A.Solder masking: leave the vias and pads for soldering on the PCB, while masking all other circuits and copper surfaces to prevent short-circuits caused by wave soldering, as well as reducing solder consumption.

B.Protecting board: this prevents the invasion of moisture and electrolytes and protects wires from oxidation and affecting the electrical quality. It also prevents external mechanical damage to maintain good insulation for the board.

C.Insulation: As boards become thinner and the width of wires becomes smaller, insulation between conductors has become an issue of more concern, and the insulation property of the solder mask becomes more important.

Lead-free surface treatment and solder mask development history

2-1. Foreword

The high-temperature property and corrosiveness of the lead-free process bring about many problems in the PCB process. Therefore, the solder mask must be improved comprehensively to meet the requirements of assembly and manufacture nowadays. The future of solder mask is described below. The options of solder masks include traditional epoxy resin IR baking, UV hardening, liquid photo imageable solder mask, and dry film solder mask. Currently, liquid solder mask is most common.

2-2. Solder mask property requirements

2-2-1. Acid resistance

The traditional sulfuric acid resistance test is not the only way to ensure acid resistance. Now, to reduce the incomplete use of tin due to cold soldering, contamination, etc., fluxes with “better cleaning capability” (Kester 985, αRF 800t3) are used for this purpose. However, the compositions of these fluxes such as bromic acid and fluoric acid may substantially invade the polymer bonds and destroy them, leading to “lumpy” wrinkles on the solder mask. This often leads to damage compensation for “unknown” reasons. Hence, the acid resistance capability of the solder mask must be improved.

2-2-2. Heat resistance

The lead-free process leads to a 20-30 °C temperature rise. Even when using old-fashioned reflow ovens (6 steps) to produce the most advanced products there is no awareness of the risks (e.g. hole cracking, solder mask color change, text color change). Hence, in consideration of the heat resistance, the Td and Tg of the substrate must meet higher standards, and the heat resistance of the solder mask must be improved. The solder mask is deemed qualified only if it is resistant in three tests at 288°C with each test lasting for 10 seconds.

2-2-3. Heat and color change resistance

Heat and color change resistance is not only a requirement that must be met, as color change resistance is a requirement set by the customer. For example, the LED white solder mask may not be yellowed. Hence, solder mask must be resistant to color change. However, more costs are needed to achieve this goal.

2-2-4. Retention capability

Like the unpredictable weather, solder mask changes easily and problems emerge in moist and changing weather conditions as well as when temperatures change significantly. The retention capability of the solder mask must meet the dry film standard 48H to ensure that the solder mask will not have “inner ringlike shadows” resulting from humidity, time for sensitization of films, and other factors after the solder mask has been put aside for a “longer” period of time after printing.

2-2-5. Solder dam

As ICs are designed smaller and denser, “opening a skylight” is no longer a good idea to avoid solder dam. When the line spacing/width is 4/3 or 3/3, a solder dam of 1 mil is usually accepted. Hence, the only rule is how to select solder mask with better light transmittance. Therefore, the final “winner” is the manufacturer that can control a solder dam of 1 mil under “normal” operation conditions without increasing the exposure, reducing the thickness of the solder mask, or lowering the developing temperature.

2-2-6. Immersion resistance

Solder mask must be resistant to cyanide in the ENIG process, thiourea in the immersion tin process, nitric acid in the immersion silver process, and high temperature in the lead-free HASL process. It must also have the ability to resist cooking in chemicals. Empty bubbles are often found at the end of the ENIG cooking process (nickel bath and gold bath). Though the condition of the vias takes some responsibility, wouldn’t it be better if the solder mask could “make more contributions”? The features of the solder mask are described in the hope that they can be taken into account when assessing the solder mask to overcome the challenges to solder mask manufacturers in the era of lead-free surface treatment.

2-2-7. Hole plugging capability

“Vias” are the most important issue for the solder masking staff. Previously, via printing was used to speed up the process. In the lead-free era, however, the problems of heat resistance, contractility, chemicals resistance and so on have emerged one after another. The operators in the field often say “The baked board after solder masking is OK in the 3M glue tensile force test. But why do empty bubbles, loosening and many other problems emerge in the back-end process?”.  Ha!  They don’t know that the weakness of solder masking is discovered in the subsequent lead-free HASL (heat resistance, contractility), OSP, and ENIG (chemicals resistance) processes. Hence, after introduction of the hole plugging ink, the “vias and then printing” mode becomes an important issue this day.

As for hole plugging ink, however, selection of the brand is very important, because the amount of the solvent, the size of the molecules, the number of grindings, the type and percentage of fillers may affect the heat resistance, chemicals resistance, contraction and expansion. Users must pay attention to these. Otherwise, there could be risk of negligence in considerations.

This refers to hole plugging ink. After holes are plugged, the ink must be based and subjected to the impact of SMT heat. If the contractility of the hole plugging ink is not good, overflow of solder mask and other defects on the surface may occur.

solder 1b
Ink overflow after solder mask development
solder 2
Ink overflow after HASL

2-3. Major compositions and functions of solder mask

Currently, most of the solder mask is liquid photo imageable solder mask mainly made of UV-hardened and heat-hardened resin in combination with photoinitiator, thermal hardener, filler, pigment, and other aids. The physical property of the hardened material differs depending on the raw material, blending proportion and operating conditions. The hardened products have an IPN (Interpenetrating Polymer network) structure.

Major components are described below:

2-3-1. Synthetic resin

This is photosensitive base epoxy resin or urethane resin. Since diluted alkaline water solution is needed for development, a carboxyl group (COOH) or hydroxyl group (OH) must be introduced. This resin is a prepolymer and framed with bisphenol A epoxy resin, phenolic epoxy resin, O-Methyl phenolic epoxy resin, and urethane resin.

2-3-2. Photoinitiator, photosensitizer, thermal hardener

The photoinitiator, photosensitizer and UV hardener share the same solder mask. However, since the UV light must pass through a layer of glass and a PET film for exposure, an applying additional UV sensitizer must be considered. Thermal hardener contains aromatic amine and acid anhydride. The photoinitiator contains benzophenone, benzoin ether, acetopheonone, and Lewis acid and base. The photosensitizer contains amine.

2-3-3. Filler

The more filler, the drier, and the stress is reduced after UV and heat hardening with better heat resistance and thermal cycling. However, excessive filler may reduce the resolution. Hence, the proportion and type of the filler is very important in the formula. Filler is usually comprised of SiO2+BaSO4+TiSO4…

The selection of the thermal hardener, pigment and filler is similar to traditional solder mask. As for binding agent and photoinitiator, the following must be taken into account for the selection and design of the photosensitive monomer:

a.High photosensitivity (surface hardening)
b.Solvable in weak base solution (1% Na2CO3) prior to exposure
c.Resistant to the erosion of the weak base solution after exposure (development resistance)
d.Resistant to strong base solution (10% NaOH) after thermal hardening

The thixotropy of the solder mask is another important factor. The thixotropic agent, also named anti-sagging agent, can flow more easily in the movement or swinging of the solder mask, persists in its shear resistance when put aside and does not move easily. This makes solder mask roll easily during printing and keeps it fixed and immovable after printing to avoid the sagging of solder mask from the board or accumulation of ink when it is put aside. In practice, it is not really good with higher or lower thixotropy. It should be appropriate for the application. During screening printing, the solder mask rolls and squeezes under the push power of the scraper. The viscosity of the solder mask is reduced in favor of penetration. When the solder mask is printed to the PCB substrate, the slow restoration of the viscosity facilitates the slow flow of the solder mask. When the solder mask reaches a state of balance , the edge of the printed pattern attains satisfactory straightness.

2-3-4. Pigment

The color of the ink is not directly related to its functionality. However, in consideration of the appearance and the preference of the operator, control of color becomes an important indicator. The purpose of the color is for easy visual or optical inspection. Blue and green are the most commonly used colors. Both are good complementary colors in contrast to the dark red or pink of the copper surface. There are organic and inorganic pigments. The former is non-mineral material with bright colors covering the full spectrum. However, the masking capability of organic pigment is not good. Inorganic pigment is mineral material with good masking capability and strong light and aging resistance. This is the most commonly used pigment in PCB solder mask. The pigment is adjusted upon request of the customer. The major components of the pigment are the same except for the proportion. Hence, the solder mask of different brands has slight differences in hues.

2-3-5 Leveling agent, etc., and other aids

The aids added to the solder mask are used to improve the physical properties of the solder mask, make it more suitable for printing, and improve the printing effect. Aids: Defoaming agent, dispersant, thinner, homogenizing agent, retention agent, color separation inhibiting agent, settlement inhibiting agent, plasticizer, coupling agent, UV absorber, activating agent, thickener, etc.

2-3-6 Solvent

The major function of the organic solvent is to dissolve resin, pigment and aid, adjust the viscosity of the solder mask, and control its drying speed to improve the dissolution, penetration and adhesion to the printed material. Using thinner randomly may lead to gelation (agglomeration), stripping and uneven color of the solder mask and reduction of its chemical resistance, thermal shock resistance, and adhesion. Using “anti-white water” as a versatile solvent and pouring it in solder mask randomly is incorrect.

 

2-4. Hole plugging ink and coating ink

2-4-1. Coating ink

Main agent: hardener mix ratio 85: 15, 86:14, 75:25, 80:20
Solder mask viscosity:  190 ±30 PS.
Solid content:  70–75 %.

2-4-2 Hole plugging ink

Main agent: hardener mix ratio  70:30, 75:25.
Solder mask viscosity:  220 ± 30 PS —400 PS
Solid content:  80–90 %.

The above table shows that the viscosity, solid content and hardener mix proportion of the hole plugging ink are higher than the coating ink. To ensure the operability and coating capability of the ink, the manufacturer may adjust the hardener mix ratio, viscosity, rheological property, and so on of the ink according to its application.

Process Outline

2-1. Pre-treatment:

The purpose of the pre-treatment is to treat the board surface to ensure appropriate roughness and gloss. Foreign matter can also be removed from the holes and on the surface in this process to provide good processing conditions and to improve the bonding capability of the copper surface and solder mask.

solder 3

2-2. Printing:

Simply put, the purpose of printing is to coat a layer of solder mask evenly on the surface of the PCB. It is divided into flat printing and hole plugging printing.

solder 4
solder 5

2-3. Pre-baking:

Purpose:

The purpose of pre-baking is to remove part of the solvent from the solder mask to make it nonadhesive (to the film) and maintain hardness without being damaged easily (the solder mask is still a monomer after the pre-baking).

Pre-baking parameters:

Pre-baking parameters vary depending on the manufacturers, their equipment and the solder mask pre-baking parameters provided. The temperature is adjusted appropriately according to the environment in which the oven is located and the climate in order to avoid inner ringlike shadows.

solder 6

2-4. Exposure:

The solder mask is directly exposed to 7 kW & 8 kW UV light. The photoinitiator is decomposed into free radicals. They attack optically active resin to perform radical polymerization (i.e., the monomer exposed to the UV light becomes polymer, while the monomer not exposed to the UV light remains in the monomeric state). The polymer solder mask does not dissolve in weak base solution Na2CO3, but in NaOH (strong base solution).

solder 7
solder 8

2-5. Development:

A.Function:
Use 0.95–1.05% NaCO3 solution to dissolve the part not exposed to UV light (not polymerized) to expose the required patterns.

B.Developing process:
Development (Na2CO3) * 2 → pressurized wash * 2 → wash * 4 → city water wash → dry → baking (40-60 °C).

C.Five development control element:
Concentration, temperature, pH value, speed, pressure

2-6. Post-baking

Purpose:

The purpose of the post-baking is to make solder mask harden permanently with premium quality by putting it through a final burning and polymerization process (also known as secondary polymerization).

A.When baking on the hole plugging plate, the temperature must be increased in phases or there could be abnormalities such as explosions and bubbles. There are usually 3 temperature rising phases: low-temperature phase, medium temperature phase, and high temperature phase. The post-baking parameters differ depending on the equipment.
Under normal circumstances: 
Low temperature phase: 30–60 min
Medium temperature phase: 20–40 min
High temperature phase: 40–60 min

B.Like the pre-baking process, the entire air-extracting apparatus must be cleaned when cleaning the post-baking aluminum duct to avoid fire.

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