In PCB manufacturing, solder mask material is one of the core elements to ensure the long-term reliability of circuit boards. It not only prevents solder bridging short circuits, but also plays an important role in protecting copper layer oxidation and improving insulation performance. The current mainstream solder mask technologies include liquid phase imaging solder mask (LPI) and dry film solder mask (DFSM), both of which have their own advantages and disadvantages in terms of process adaptability, performance, and cost control. This article will comprehensively analyze the characteristics of these two types of solder mask materials from the perspectives of technical principles, application scenarios, and future trends, providing scientific basis for enterprise selection.
Table of Contents
1、 Liquid state photo imaging solder mask (LPI): a balance between high precision and cost-effectiveness
Technical principles
LPI is a liquid solder mask material cured through photolithography technology, and its process includes steps such as cleaning, coating, pre drying, exposure, development, and post curing. Its core advantage lies in its high-resolution pattern forming ability, such as achieving uniform thickness coverage (0.8-1.2 μ m) through screen coating or spray coating technology, and supporting fine window design below 5 μ m.
Performance advantages
1. Wide adaptability: LPI has better compatibility with PCBs with uneven surfaces, such as multi-layer boards or flexible boards, and can effectively fill small gaps to avoid the generation of bubbles.
2. Strong thermal stability: LPI materials can remain stable at high temperatures (such as 260 ℃ for lead-free soldering), with dielectric loss (Df) as low as 0.005, making them suitable for high-frequency communication equipment.
3. Cost controllable: Compared to dry film, LPI has lower raw material costs and can achieve large-scale production through automated spraying equipment, reducing labor costs.
limitation
-Thickness uniformity challenge: Complex circuit areas may experience local thinning (0.3 μ m) or thickening (1.5 μ m), which requires optimization through dynamic compensation techniques.
-High environmental requirements: Some LPIs contain organic solvents and require a closed-loop system to treat waste gas and wastewater. The initial equipment investment is relatively high.
2、 Dry film solder mask (DFSM): an ideal choice for high-density design
Technical principles
Dry film solder mask adopts vacuum lamination process, attaching photosensitive film to the surface of PCB, and forming patterns through exposure and development. Its biggest feature is that it does not require liquid coating and is suitable for high-density interconnect (HDI) and miniaturization design. For example, the latest SUNFORT launched by Asahi Kasei ™ TA series dry film can achieve micro lines with a line width of 1.0 μ m, suitable for laser direct writing (LDI) and step exposure equipment.
Performance advantages
1. Ultra high precision: The resolution of the dry film can reach a pitch of 4 μ m, especially suitable for scenarios such as BGA packaging and 5G base stations that require strict line density.
2. Process stability: There is no liquid flow problem in the dry film, and the welding coverage inside the hole is more uniform, avoiding the risk of exposed copper on the solder pad caused by “solder mask cutting”.
3. Resistance to mechanical stress: The cured dry film has high hardness and excellent scratch resistance, making it suitable for vibration environments such as automotive electronics.
limitation
-High cost: The unit price of dry film materials is about 1.5-2 times that of LPI, and specialized vacuum laminating equipment is required, which may put investment pressure on small and medium-sized enterprises.
-Flexible board has weak adaptability: the dry film is prone to poor adhesion on curved or irregular surfaces, and needs to be optimized by combining rigid and flexible bonding processes.
3、 Comparison of Application Scenarios between LPI and Dry Film
| Dimension | LPI | Dry Film |
| Applicable board types | multi-layer board, flexible board, consumer electronics | HDI board, semiconductor packaging, high-frequency communication |
| Process complexity | Medium (requires spray/screen printing equipment) | High (requires vacuum lamination and precision exposure) |
| Cost effectiveness | Low to medium (suitable for small and medium-sized production) | High (suitable for high-end and large-scale demand) |
| Environmental friendliness | Solvent emissions to be treated | No solvent pollution, compliant with RoHS standards |
Typical application cases
-LPI: The white goods control board adopts matte LPI solder mask to reduce the generation of solder balls; The automotive dashboard PCB uses high temperature resistant LPI (Tg ≥ 170 ℃) to ensure long-term stability.
-Dry film: The AI server’s Interposer uses dry film technology to achieve 4 μ m pitch wiring; Folding screen mobile phone flexible PCB protects the integrity of high-frequency signals through dry film.
4、 Future Trends: Technological Innovation and Green Manufacturing
1. Material performance upgrade: LPI is developing towards halogen-free and low dielectric loss, such as the M8 level LPI developed by Shengyi Technology, with a dielectric constant (Dk) as low as 3.2; Dry film is modified with nano fillers to enhance its thermal expansion resistance (CTE ≤ 30ppm/℃).
2. Intelligent production: The AI driven LPI spraying system can monitor coating thickness in real time, increasing yield to 99.95%; The dry film process combined with laser direct writing technology reduces the use of film and energy consumption.
3. Green transformation: Accelerated research and development of water-based LPI ink and recyclable dry film substrates to promote PCB manufacturing towards carbon neutrality goals.
Conclusion
The competition between LPI and dry film solder mask is essentially a game of precision and cost. For consumer electronics and small to medium batch orders, LPI has the advantage of high cost-effectiveness and process flexibility; However, high-end communication, semiconductor packaging and other fields rely more on the ultra-high density and reliability of dry film. Enterprises need to choose a suitable solder mask solution based on product positioning, technical reserves, and environmental requirements. With the advancement of materials science and intelligent manufacturing, solder mask technology will continue to break through performance boundaries, empowering the electronics industry to develop towards higher integration and sustainability.
