Solder mask application is a core process in PCB manufacturing that determines yield and reliability, and pre-treatment is the cornerstone of its success.
Its purpose is by no means merely to “clean the copper surface,” but rather to create a clean, highly wettable, appropriately roughened interface that bonds tightly with the ink.
Improper pre-treatment can result in issues ranging from insufficient ink adhesion—leading to ink peeling and bubbling—to more severe problems such as unclear development and plating bleed-through causing short circuits.
This article will systematically review the main types of solder mask pretreatment currently in use, their process differences, and their design objectives.
Types and Principles of Solder Mask Pretreatment
Solder mask pretreatment can be divided into three main categories based on the roughening mechanism:
Mechanical
Non-woven brush grinding (pumice powder/alumina), sandblasting: These methods physically cut the copper surface using abrasives or brush wheels to create a plow-like texture.
1. Non-woven Brush Grinding
Nylon non-woven brush wheels contain silicon carbide or aluminum oxide abrasives, used either with pumice (volcanic ash) slurry or with water alone.
The board is ground by the oscillating motion of upper and lower brush wheels.
The mesh size of the non-woven fabric determines the roughness.
There are two methods: “wet with pumice” and “dry/wet without pumice.”
The former provides strong cutting action and a uniform matte finish, while the latter relies solely on the brush wheel abrasives and results in lower material removal.
The non-directional abrasion marks created by the oscillating brush wheels form extensive physical anchor points, making this method particularly suitable for thick copper power boards that require extremely high bond strength.
The pumice powder prevents the brush wheels from clogging and helps create uniform, dense grooves.
2. Sandblasting
Silicon carbide or aluminum oxide micropowder is propelled at high speed onto the copper surface using compressed air.
The sandblasting pressure and grit size determine the depth and morphology of the surface roughening.
There are dry and wet methods; the wet method produces less dust.
3. Ceramic Brushes/Pure Nylon Brushes
These are used only for light roughening or oxide layer removal.
They have extremely low material removal rates and are rarely used alone for pre-solder mask treatment.
Chemical Formulas
Sulfuric acid-hydrogen peroxide micro-etching, organic acid super-roughening: Copper is selectively dissolved through chemical etching to form microscopic irregularities or an organometallic conversion layer.
1. Conventional Micro-Etching (H₂SO₄/H₂O₂ System)
Typically involves sulfuric acid, hydrogen peroxide, copper ions, and controlled temperature and processing time.
Primarily etches copper grain boundaries to form micron-scale grooves, with a surface roughness (Ra) of approximately 0.2–0.35 μm.
2. Super-Roughening (Organic Acid/Hydrogen Peroxide System—Current Mainstream Trend)
Primarily utilizes organic acids, hydrogen peroxide, and nitrogen-containing heterocyclic additives to generate a uniform honeycomb-like or fine-needle-like copper surface, while simultaneously forming an extremely thin organometallic conversion layer with a Ra of 0.3–0.5 μm, appearing dark brown to grayish-black.
› Uniform roughening with no directionality; absolutely no damage to fine circuits (<2 mil line width).
› The organic film generates polar groups (such as -OH and -NH-), which form hydrogen bonds or chemical bonds with the hydroxyl and carboxyl groups in the ink resin, resulting in bonding strength far exceeding that of pure mechanical anchoring.
› The honeycomb structure accommodates ink, allowing for more uniform release of interfacial stress during thermal shock and providing excellent anti-blistering performance.
› This embodies the intended dual mechanism of “physical anchoring + chemical bonding.”
› Chemical super-roughening has become the preferred pretreatment for solder mask applications on HDI, fine-line, and high-frequency boards.
Mechanical + Chemical Combination
Brushing + Micro-etching in-line: The process forms macroscopic grooves mechanically first, then chemical modification creates microscopic roughness and removes copper powder.
Process flow: Feed → Brushing (non-woven brush + pumice powder) → High-pressure water rinse → Chemical micro-etching/super-roughening → Water rinse → Drying.
› Brushing creates deep physical grooves to remove severe oxidation and resin residue.
› Subsequent chemical treatment removes copper powder and embedded abrasives generated by brushing, while creating a micro-roughness that serves to “modify surface morphology and enhance adhesion.”
› Suitable for products with “combined requirements”—such as thick copper, large copper areas, and extremely stringent bonding strength demands—while also accommodating fine circuit patterns.
› A drawback is that the brushing process may damage the top corners of circuit lines, making it unsuitable for fine-pitch boards.
How to Choose a Pretreatment Method
Decisions should not be based on intuition or experience alone, but must take into account multiple factors, including product characteristics, ink systems, reliability requirements, equipment capabilities, and cost.
1. Selection Based on Product Type
Generally, for adhesion requirements, we recommend using mechanical brushing combined with micro-etching or pure chemical micro-etching to balance cost and quality.
Mechanical brushing is highly effective at removing the oxide layer from large copper surfaces.
For fragile circuits requiring uniform surface roughness, we recommend pure chemical super-roughening to eliminate mechanical stress damage.
This method ensures uniform roughening without damaging the line shoulders while meeting the adhesion requirements for fine lines.
For extremely smooth copper foil requiring low surface roughness to minimize skin effect losses but presenting challenges for ink adhesion, we recommend low-etch-rate super-roughening or proprietary high-adhesion chemical treatment.
This compensates for the lack of physical anchor points through chemical bonding, with minimal increase in surface roughness, thereby balancing electrical performance and reliability.
Large copper surfaces with high thermal capacity and significant internal stress require a combination of mechanical brushing and chemical super-roughening.
Deep mechanical grooves provide strong adhesion, while chemical modification removes residual flux and copper powder, ensuring thermal cycle resistance.
Surfaces with resin or foreign matter spots must not damage via holes.
Pure chemical super-roughening should be recommended to avoid stripping resin from the via holes during brushing.
Chemical roughening uniformly treats via edges, ensuring seamless ink coverage.
2. Equipment and Cost Considerations
Solder mask pretreatment has evolved from a simple “roughening” process into an interface engineering discipline.
Mechanical methods primarily contribute to macro-scale anchoring, while chemical methods focus on micro-scale morphology and interfacial bonding.
After understanding these fundamental differences, engineers must precisely match each specific product.
Only by treating the pretreatment process as an integral part of the entire solder mask system can we truly ensure a smooth transition from R&D to high-yield mass production.
