14 Fundamentals of Advanced Materials …
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Polyimide is another major material used for substrate build-up layers. This is
due to its outstanding integrated properties and availability in different formulations. There are three types of polyimides used for dielectric build-up layers: nonphotosensitive polyimide, negative photosensitive polyimide, and positive photosensitive polyimide. Since polyimide is often used in pre-cursor form in both liquid
solution or film form, cure shrinkage must be sufficiently considered in processing
polyimide dielectric material. Multiple methods can be used to generate microvias
in non-photosensitive polyimide layers. Laser drilling is a commonly used technique
of making microvias in fully cured polyimide dielectric layers. The via diameter
is limited by the laser beam spot size. Eximer laser drilling can generate vias with
diameters of around 10 μm. Dry and wet etching, with the help of photoresist, is also
a commonly used method to form microvias in polyimide dielectric layers. However,
etching processes require many steps, and the achieved via hole diameter depends
on the dielectric layer thickness because of etching isotropicity issues. The diameter
of via made by etching methods often ranges from 75 μm to several hundreds of
microns. Photosensitive polyimides have gained lots of attention because they allow
simplified process and have higher resolution compared to eximer laser generated via.
Much more material choices are available for negative photosensitive polyimide than
positive photosensitive polyimide. Negative photosensitive polyimides are primarily
derived from polyamic acid precursors which have either covalently bonded ester or
ionic-bonded photosensitive groups attached to the backbone polymer chain. Positive photosensitive polyimides are derived either from incorporating a photosensitive
group into the backbone as a side branch of the base polyamic acid, or by blending
a photosensitive compound with the polyamic acid precursor. A detailed discussion of the properties and application processes of different types of photosensitive
polyimides can be found in Microelectronics Packaging Handbook edited by Rao
Tummala et al. [8]. Polyimide dielectric build-up layers are utilized in wide varieties
of substrates and PWBs such as thick core, thin core, and coreless substrates, as well
as rigid, flex, and rigid-flex PWBs.
LCP is a thermoplastic polymer whose molecular chains have one or two dimensional spatial orders. LCP gains its attention in substrate applications because of
many of its outstanding properties. LCP is “near” hermetic with low 0.02–0.04%
moisture absorption and its CTE can be custom tuned from 3 ppm/°C to 30 ppm/°C
to match copper CTE of 17 ppm/°C. LCP is halogen free, has a similar dielectric
constant and loss factor to PTFE and is claimed to be capable of enabling 110 GHz
frequency applications [11]. The desirability for aerospace applications is its flexibility and light weight, with its density of 1.4 g/cm
3 , which is 40% lighter than
silicon, 65% lighter than alumina and almost 30% lighter than FR4. LCP is a good
material candidate for embedded substrate applications because it can be made as
thin as 25 μm and as thick as desired. It is radiation hard and biocompatible which
is suitable for medical applications, and it is thermoplastic and easy to process for
lower costs. Coreless multilayer substrates have been built only by LCP dielectric
material and copper. Laser drilling is used to make 50 μm diameter through vias and
buried vias. Some substrate base material suppliers have commercialized LCP dielectric material such as Rogers Ultralam 3850 and 3908, and Nippon Steel Espanex L.
415
Polyimide is another major material used for substrate build-up layers. This is
due to its outstanding integrated properties and availability in different formulations. There are three types of polyimides used for dielectric build-up layers: nonphotosensitive polyimide, negative photosensitive polyimide, and positive photosensitive polyimide. Since polyimide is often used in pre-cursor form in both liquid
solution or film form, cure shrinkage must be sufficiently considered in processing
polyimide dielectric material. Multiple methods can be used to generate microvias
in non-photosensitive polyimide layers. Laser drilling is a commonly used technique
of making microvias in fully cured polyimide dielectric layers. The via diameter
is limited by the laser beam spot size. Eximer laser drilling can generate vias with
diameters of around 10 μm. Dry and wet etching, with the help of photoresist, is also
a commonly used method to form microvias in polyimide dielectric layers. However,
etching processes require many steps, and the achieved via hole diameter depends
on the dielectric layer thickness because of etching isotropicity issues. The diameter
of via made by etching methods often ranges from 75 μm to several hundreds of
microns. Photosensitive polyimides have gained lots of attention because they allow
simplified process and have higher resolution compared to eximer laser generated via.
Much more material choices are available for negative photosensitive polyimide than
positive photosensitive polyimide. Negative photosensitive polyimides are primarily
derived from polyamic acid precursors which have either covalently bonded ester or
ionic-bonded photosensitive groups attached to the backbone polymer chain. Positive photosensitive polyimides are derived either from incorporating a photosensitive
group into the backbone as a side branch of the base polyamic acid, or by blending
a photosensitive compound with the polyamic acid precursor. A detailed discussion of the properties and application processes of different types of photosensitive
polyimides can be found in Microelectronics Packaging Handbook edited by Rao
Tummala et al. [8]. Polyimide dielectric build-up layers are utilized in wide varieties
of substrates and PWBs such as thick core, thin core, and coreless substrates, as well
as rigid, flex, and rigid-flex PWBs.
LCP is a thermoplastic polymer whose molecular chains have one or two dimensional spatial orders. LCP gains its attention in substrate applications because of
many of its outstanding properties. LCP is “near” hermetic with low 0.02–0.04%
moisture absorption and its CTE can be custom tuned from 3 ppm/°C to 30 ppm/°C
to match copper CTE of 17 ppm/°C. LCP is halogen free, has a similar dielectric
constant and loss factor to PTFE and is claimed to be capable of enabling 110 GHz
frequency applications [11]. The desirability for aerospace applications is its flexibility and light weight, with its density of 1.4 g/cm
3 , which is 40% lighter than
silicon, 65% lighter than alumina and almost 30% lighter than FR4. LCP is a good
material candidate for embedded substrate applications because it can be made as
thin as 25 μm and as thick as desired. It is radiation hard and biocompatible which
is suitable for medical applications, and it is thermoplastic and easy to process for
lower costs. Coreless multilayer substrates have been built only by LCP dielectric
material and copper. Laser drilling is used to make 50 μm diameter through vias and
buried vias. Some substrate base material suppliers have commercialized LCP dielectric material such as Rogers Ultralam 3850 and 3908, and Nippon Steel Espanex L.