CircuiTree - December 2008 - (Page 22) Is Your Solder Mask Process Ready for the Fine Pitches? 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). There is a continuous request for miniaturization of electronic designs, especially in consumer handheld electronics, where the component pitches are reduced to the minimum. 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, the 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 Figure 3 Typical Emission Spectra of Standard Fe/Ga Dote Lamps Used for Photo Polymers Figure 4 Functionality of Photo Polymerization Figure 5 Exposure Energy Versus Resolution Capability at 30 μm/ 40 μm/60 μm Solder Mask Thickness Figure 6 50 μm Dam Exposed at 50 mJ 22 December 2008 • circuitree.com Figure 7 50 μm Dam Exposed at 50 mJ Figure 8 Small 50 μm Dots After Developing 24 http://www.circuitree.com
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