Component Technology and Miniaturization
Component technology and assembly have historically been driving new material developments to meet requirements for solder mask materials and application processes. Th
e requirements for smaller component pitches with advanced functionalities have rapidly increased in the past few years. There is a continuous request for miniaturization of electronic designs, especially in consumer handheld electronics, where the component pitches are reduced to the minimum. The solder mask material has to be able to form smaller features on the surface combined with high registration and imaging accuracy on large production panels.
Fine pitch designs from 0.4 mm are in volume production while 0.3 mm has been tested and will be introduced in the next few years. The PCB producer needs to be able to process large production panels with fine pitch designs in both outer layer and solder mask HDI production processes.
So far this has been possible with standard contact printing equipment using optical registration technology. For the next-generation components this will be extremely challenging and it will be difficult to achieve acceptable production yields. Depending on pad size designs, solder mask clearances are down to 20 µm, with small features of 50 µm between pads (see Figure 1).
LDI Imaging Technology
LDI imaging technology was introduced in PCB manufacturing some years ago. PCB producers started to introduce the LDI imaging process to improve the registration accuracy in the production of inner and outer layers. There are a number of compatible resists available for LDI primary imaging where dry films are the most common materials in use for inner and outer layer imaging.
Digitalization, through elimination of artwork, will help producers to complete the whole PCB process, with high imaging accuracy and yield. The solder mask process has to follow at the end of the PCB production cycle without creation of a single artwork.
LDI equipment manufacturers are using single wavelength light sources of 355 or 405 nm. Recent developments increased the effective output energies, shortening the exposure time, but mechanical and registration cycle time has also been reduced for a faster total cycle time and higher productivity. Other market drivers are fast prototyping to eliminate artwork and reduce total lead times through production (see Figure 2).
Can Standard Solder Mask Material Be Used for LDI?
Not in an optimized way. The main reason is the absorption during imaging is different from LDI. The polymer/photo system of the solder mask material has to be able to absorb at a specific single wavelength of 355 or 405 nm, while a traditional standard exposure is emitting energy at a wider UV spectra (see Figure 3, Fe-doped lamp [blue] and Ga-doped lamp [orange]). The photo package can then absorb at various wavelengths, which is at an optimum for each photo initiator component.
The standard solder mask material will not absorb enough UV energy at LDI wave lengths to perform acceptably with fast and complete photo reaction. For high performance, a high cross linking density is needed to complete the layer thickness. If the cross linking density is too low, then undercut and low resolution will result, which in turn will not form small solder mask features such as 50 µm solder dams, on the surface. With decreased undercut, the risk of penetration or entrapment of chemicals in post-solder mask processes is reduced (see Figure 4).
LDI Imaging at 355 nm
An LDI solder mask applied in various thicknesses results in a solder dam thickness of 30 to 50 µm. To achieve a 50 µm (2 mil) dam in 30 µm film thickness, a thickness commonly applied for HDI outer layers, t
he exposure energy needs to be 50 to 60 mJ but requires 80 to 90 mJ at approximately a 40 µm thickness.
The exposure values for this process will be strongly influenced by the developing process, which has to be optimized for the aspect ratio of the panels, minimum hole diameters, and buried vias to be cleaned out from solder mask material. This also means that the wet solder mask application process will influence the exposure values and how much solder mask material is present in the plated holes or vias, which need to be cleaned out. The optimal application process is electrostatic spray or spray application in general as less volume of material is present in the plated holes (see Figure 5).
LDI Solder Mask
The excellent imaging properties of an LDI solder mask are demonstrated in Figure 6 to 8. The 50 µm (2 mil) dam in 30 µm thickness is processed on a UV 355 LDI (50 mJ exposure energy). The dam fe
ature shows excellent shape and sidewall with a very low undercut. The demonstrated LDI products are formulated from IP-protected high-performance polymer binders, and they are modern state-of-the-art solder mask formulations that are halogen-free, RoHS-conforming, and UL-listed. LDI solder mask ranges are used for all conventional solder mask application technologies like screen print, spray, or curtain coating.
Split Imaging Process
PCB producers, in some cases, do not need to use LDI imaging for all panels they produce. The
required registration accuracy could be handled in a standard contact exposure process while some panels do require LDI imaging due to tight tolerances. An LDI solder mask could be applied and processed on the same wet application line. After coating and drying, the processing of panels could be either imaged in LDI or in common used standard exposure using good vacuum.
PCB producers can benefit from high production flexibility where the difficult designs could be done with LDI imaging and conventional designs with standard exposure, using only one product applied in one wet solder mask application line.
Printed Circuit Boards
e requirements for smaller component pitches with advanced functionalities have rapidly increased in the past few years. There is a continuous request for miniaturization of electronic designs, especially in consumer handheld electronics, where the component pitches are reduced to the minimum. The solder mask material has to be able to form smaller features on the surface combined with high registration and imaging accuracy on large production panels. Fine pitch designs from 0.4 mm are in volume production while 0.3 mm has been tested and will be introduced in the next few years. The PCB producer needs to be able to process large production panels with fine pitch designs in both outer layer and solder mask HDI production processes.
So far this has been possible with standard contact printing equipment using optical registration technology. For the next-generation components this will be extremely challenging and it will be difficult to achieve acceptable production yields. Depending on pad size designs, solder mask clearances are down to 20 µm, with small features of 50 µm between pads (see Figure 1).LDI Imaging Technology
LDI imaging technology was introduced in PCB manufacturing some years ago. PCB producers started to introduce the LDI imaging process to improve the registration accuracy in the production of inner and outer layers. There are a number of compatible resists available for LDI primary imaging where dry films are the most common materials in use for inner and outer layer imaging.
Digitalization, through elimination of artwork, will help producers to complete the whole PCB process, with high imaging accuracy and yield. The solder mask process has to follow at the end of the PCB production cycle without creation of a single artwork. 
LDI equipment manufacturers are using single wavelength light sources of 355 or 405 nm. Recent developments increased the effective output energies, shortening the exposure time, but mechanical and registration cycle time has also been reduced for a faster total cycle time and higher productivity. Other market drivers are fast prototyping to eliminate artwork and reduce total lead times through production (see Figure 2).Can Standard Solder Mask Material Be Used for LDI?
Not in an optimized way. The main reason is the absorption during imaging is different from LDI. The polymer/photo system of the solder mask material has to be able to absorb at a specific single wavelength of 355 or 405 nm, while a traditional standard exposure is emitting energy at a wider UV spectra (see Figure 3, Fe-doped lamp [blue] and Ga-doped lamp [orange]). The photo package can then absorb at various wavelengths, which is at an optimum for each photo initiator component.
The standard solder mask material will not absorb enough UV energy at LDI wave lengths to perform acceptably with fast and complete photo reaction. For high performance, a high cross linking density is needed to complete the layer thickness. If the cross linking density is too low, then undercut and low resolution will result, which in turn will not form small solder mask features such as 50 µm solder dams, on the surface. With decreased undercut, the risk of penetration or entrapment of chemicals in post-solder mask processes is reduced (see Figure 4).LDI Imaging at 355 nm
An LDI solder mask applied in various thicknesses results in a solder dam thickness of 30 to 50 µm. To achieve a 50 µm (2 mil) dam in 30 µm film thickness, a thickness commonly applied for HDI outer layers, t
he exposure energy needs to be 50 to 60 mJ but requires 80 to 90 mJ at approximately a 40 µm thickness.The exposure values for this process will be strongly influenced by the developing process, which has to be optimized for the aspect ratio of the panels, minimum hole diameters, and buried vias to be cleaned out from solder mask material. This also means that the wet solder mask application process will influence the exposure values and how much solder mask material is present in the plated holes or vias, which need to be cleaned out. The optimal application process is electrostatic spray or spray application in general as less volume of material is present in the plated holes (see Figure 5).
LDI Solder Mask
The excellent imaging properties of an LDI solder mask are demonstrated in Figure 6 to 8. The 50 µm (2 mil) dam in 30 µm thickness is processed on a UV 355 LDI (50 mJ exposure energy). The dam fe
ature shows excellent shape and sidewall with a very low undercut. The demonstrated LDI products are formulated from IP-protected high-performance polymer binders, and they are modern state-of-the-art solder mask formulations that are halogen-free, RoHS-conforming, and UL-listed. LDI solder mask ranges are used for all conventional solder mask application technologies like screen print, spray, or curtain coating.Split Imaging Process
PCB producers, in some cases, do not need to use LDI imaging for all panels they produce. The
required registration accuracy could be handled in a standard contact exposure process while some panels do require LDI imaging due to tight tolerances. An LDI solder mask could be applied and processed on the same wet application line. After coating and drying, the processing of panels could be either imaged in LDI or in common used standard exposure using good vacuum. PCB producers can benefit from high production flexibility where the difficult designs could be done with LDI imaging and conventional designs with standard exposure, using only one product applied in one wet solder mask application line.
Printed Circuit Boards
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