14 Fundamentals of Advanced Materials …
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if needed [13]. Another layer of dielectric material is then applied and the buildup process repeats itself. Copper filled vias have high reliability, high density, and
higher heat dissipation. This process is relatively expensive and often used for high
performance applications such as smartphone applications.
14.3.6 Solder Mask Materials
Advancement of solder mask materials and lithographic process technology has
resulted in the utilization of solder mask being used more than just preventing solder
bridging and short circuit. Primarily, solder mask is used as a protection layer against
oxidation of underneath copper traces. Additionally, it also prevents the chemical
attack of dielectric material during surface finish plating and via deterioration under
assembly and application.
Solder mask consists of two main chemistries; which can be generally grouped
under epoxy and acrylates. In addition to these the formulation has certain additives
such as adhesion promoters for copper, pigments, cure catalysts, and fillers. The
thermally cured epoxies require considerable amount of energy, and curing is typically 30–60 min at 120–150 °C, while UV-cure uses very low energy and last only
several seconds [9]. Since UV cure is sensitive to surface contamination, the surface
cleanliness is very important for UV cured acrylate solder masks.. Application of
solder mask have also evolved over the years and depends on the PWB design and
copper features underneath. In-expensive options of application are screen printing,
spray coating, curtain coating roll coating methodology. However, it is challenging to
apply similar printing technologies for ultra-thin packages e.g. coreless packages due
to panel warpage concerns or thickness control in high routing packages. To overcome these, solder mask are now being manufactured as dry film with additional
additives such as surface levelers, adhesion promoters which can subsequently be
applied on large PWB panels using advanced vacuum lamination tool and provide
superior thickness control.
Similarly, pattern registration techniques for solder mask have also evolved over
multiple generations. The various lithographic techniques have been discussed in
Table 14.2. Although, the tool capability is generally higher (as listed here), the
actual feature size in production today is much larger; this is due to material limitation and end-customer requirement. Initially, due to application of screen printing
the resolution offered was low and overall accurate registration is difficult. Hence,
they typically cannot meet feature size requirements for the high density substrate
applications. Higher resolution photoimageable solder mask development opened
the door for finer pitch and features [14, 15].
Photoimageable solder masks can be either pure epoxy resin systems or epoxyacrylate mixtures. These two photoimageable solder mask materials can be in both
liquid form and dry film form. Many application techniques are available for liquid
photoimageable solder mask (LPSM) materials. These techniques include screen
printing, spray coating, and curtain coating. In the context of dry film solder mask
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if needed [13]. Another layer of dielectric material is then applied and the buildup process repeats itself. Copper filled vias have high reliability, high density, and
higher heat dissipation. This process is relatively expensive and often used for high
performance applications such as smartphone applications.
14.3.6 Solder Mask Materials
Advancement of solder mask materials and lithographic process technology has
resulted in the utilization of solder mask being used more than just preventing solder
bridging and short circuit. Primarily, solder mask is used as a protection layer against
oxidation of underneath copper traces. Additionally, it also prevents the chemical
attack of dielectric material during surface finish plating and via deterioration under
assembly and application.
Solder mask consists of two main chemistries; which can be generally grouped
under epoxy and acrylates. In addition to these the formulation has certain additives
such as adhesion promoters for copper, pigments, cure catalysts, and fillers. The
thermally cured epoxies require considerable amount of energy, and curing is typically 30–60 min at 120–150 °C, while UV-cure uses very low energy and last only
several seconds [9]. Since UV cure is sensitive to surface contamination, the surface
cleanliness is very important for UV cured acrylate solder masks.. Application of
solder mask have also evolved over the years and depends on the PWB design and
copper features underneath. In-expensive options of application are screen printing,
spray coating, curtain coating roll coating methodology. However, it is challenging to
apply similar printing technologies for ultra-thin packages e.g. coreless packages due
to panel warpage concerns or thickness control in high routing packages. To overcome these, solder mask are now being manufactured as dry film with additional
additives such as surface levelers, adhesion promoters which can subsequently be
applied on large PWB panels using advanced vacuum lamination tool and provide
superior thickness control.
Similarly, pattern registration techniques for solder mask have also evolved over
multiple generations. The various lithographic techniques have been discussed in
Table 14.2. Although, the tool capability is generally higher (as listed here), the
actual feature size in production today is much larger; this is due to material limitation and end-customer requirement. Initially, due to application of screen printing
the resolution offered was low and overall accurate registration is difficult. Hence,
they typically cannot meet feature size requirements for the high density substrate
applications. Higher resolution photoimageable solder mask development opened
the door for finer pitch and features [14, 15].
Photoimageable solder masks can be either pure epoxy resin systems or epoxyacrylate mixtures. These two photoimageable solder mask materials can be in both
liquid form and dry film form. Many application techniques are available for liquid
photoimageable solder mask (LPSM) materials. These techniques include screen
printing, spray coating, and curtain coating. In the context of dry film solder mask