Patent Description:
Power semiconductor modules typically include a molded plastic body such as a plastic molding compound in which power semiconductor dies are embedded, or a plastic housing which covers the power semiconductor dies. Commonly used package materials are epoxies used in transfer molding devices or as potting material in modules, silicone gel used as potting material in modules, and engineering plastics such as PBT (polybutylene terephthalate), PET (polyethylene terephthalate), polyamide, PPS (polyphenylene sulfide), etc. as housing/envelope materials. Such materials not only have to fulfil predetermined mechanical requirements (e.g. elongation at yield ≥<NUM>%, preferred; electrical RTI ≥<NUM>, mechanical RTI ≥<NUM>), but also environmental (e.g. related standards: RoHS; REACH), fire and smoke (e.g. related standards: UL94V0; EN45545, including HL2 and R22) as well as electrical (e.g. related standards: IEC60664, including CTI><NUM>, <NUM>) and chemical (e.g. low content of solvable ions) requirements.

Mechanical requirements are a consequence of joining the plastic material to metals, substrates and/or semiconductors. The plastic material becomes elongated, bent or compressed during temperature cycling as the CTE (coefficient of thermal expansion) of the different materials does not match, and also the temperature is not homogenous if temperature cycles are generated by power losses in the power semiconductors and a temperature gradient exists between the heat sources and the environment and heatsinks. The plastic material should not crack during such stress. Therefore, the elongation at yield is an important figure. High temperature applied to the plastic also should not change the mechanical and electrical characteristics too much, which is covered by RTI (relative thermal index). RTI indicates the ability of a material to retain a particular property (physical, electrical, etc.).

Electrical and chemical requirements are combined for harsh environmental conditions. Under humid atmosphere and applied voltage bias, the plastic materials should not cause corrosion of terminals, substrates or semiconductors. An <NUM>/<NUM>%RH (relative humidity) test with applied blocking voltage (e.g. <NUM>%, <NUM>% or <NUM>% of blocking voltage) of the semiconductors should for example last for <NUM>, <NUM> or even <NUM> without causing excessive leakage current between terminals, substrate tracks or semiconductor electrodes. The REACH regulation (Registration, Evaluation, Authorisation and Restriction of Chemicals) promulgated by the EU outlines a directive for restricting the use of hazardous substances in electrical and electronic equipment - the so-called RoHS (restriction of hazardous substances) directive (see http://eur-lex. eu/LexUriServ/LexUriServ. do?uri=OJ:L:<NUM>:<NUM>:<NUM>:<NUM>:de:PDF).

To fulfil mechanical, environmental and fire and smoke requirements, additives are typically used. Additives such as flame retardants, fibers, minerals and other chemicals often comprise the electrical characteristics of the package. For example, dielectric strength of the molded plastic body may be weakened over a long lifetime by ions or interfaces introduced by additives. Insulating characteristics may be weakened especially in a humid environment, or in a salt mist or acidic gaseous atmosphere. Also, the package terminals may corrode where contacted by the molded plastic body and corrosive by-products may propagate over the surface of insulating portions. Semiconductor characteristics may be influenced as well by ionic additives. The predetermined electrical requirement CTI (comparative tracking index), which relates to creepage between life parts, typically requires another kind of additive.

Document <CIT> discloses a semiconductor package which meets a plurality of predetermined environmental requirements, the semiconductor package comprising: a semiconductor die embedded in or covered by a molded plastic body, the molded plastic body satisfying only a subset of the plurality of predetermined environmental requirements; a plurality of terminals protruding from the molded plastic body and electrically connected to the semiconductor die; and a coating applied to the molded plastic body, wherein the coating is applied to the molded plastic body at least in a region where the plurality of terminals protrude from the molded plastic body, including between adjacent ones of the terminals, the coating comprising a plastic material and satisfying each predetermined environmental requirement not satisfied by the molded plastic body.

Document <CIT> discloses a die interconnect system comprising a die having a plurality of connection pads, a heat generating element thermally isolated from the die, one or more leads extending from the die to the heat generating element, each lead having a metal core with a core diameter, a dielectric layer surrounding the metal core with a dielectric thickness, and an outer metal layer attached to ground, wherein one or more leads are exposed to ambient conditions and/or are convectively or contact cooled for at least a portion of their length to minimize heat transfer from the heat generating element to the die.

Document <CIT> discloses an integrated circuit device wherein layer insulation is attained by at least one of (i) an insulator layer interposed between two adjacent conductor layers, and (ii) a surface protecting layer fixedly covering the surface of the semiconductor chip, each of the insulator layer and the surface protecting layer being made of a heat-resistant resin obtainable by heating a heat-resistant resin paste. Said paste consists essentially of a first organic liquid, a second organic liquid, a heat-resistant resin which is soluble in an organic liquid mixture consisting of the first organic liquid and the second organic liquid, and fine particles of a heat-resistant resin which is soluble in the first organic liquid but insoluble in the second organic liquid. The first organic liquid, the second organic liquid and the heat-resistant resin are brought into a solution in which the fine particles of a heat-resistant resin are dispersed.

Document <CIT> discloses a semiconductor package which meets a plurality of predetermined electrical, mechanical, chemical and/or environmental requirements, the semiconductor package comprising: a semiconductor die embedded in or covered by a molded plastic body, the molded plastic body comprising a flame-retardant material, and satisfying only a subset of the plurality of predetermined electrical, mechanical, chemical and/or environmental requirements; a plurality of terminals protruding from the molded plastic body and electrically connected to the semiconductor die; the coating comprising a plastic material and satisfying each predetermined electrical, mechanical, chemical and/or environmental requirement not satisfied by the molded plastic body.

As such, there is a need for a power semiconductor module which satisfies a plurality of predetermined electrical, mechanical, chemical and/or environmental requirements but without adversely affecting various characteristics of the package components.

According to an embodiment of a semiconductor package which meets a plurality of predetermined electrical, mechanical, chemical and/or environmental requirements, the semiconductor package comprises a semiconductor die embedded in or covered by a molded plastic body, the molded plastic body comprising a flame-retardant material, and satisfying only a subset of the plurality of predetermined electrical, mechanical, chemical and/or environmental requirements. The semiconductor package further comprises a plurality of terminals protruding from the molded plastic body and electrically connected to the semiconductor die, and a coating applied to the molded plastic body, wherein either the coating coats the molded plastic body completely, or the coating is applied to the molded plastic body only in a region where the plurality of terminals protrude from the molded plastic body, including between adjacent ones of the terminals. The coating comprises a plastic material and satisfies each predetermined electrical, mechanical, chemical and/or environmental requirement not satisfied by the molded plastic body. The molded plastic body achieves a first comparative tracking index value that does not fulfil the comparative tracking index requirements for the semiconductor package, and the coating achieves a second comparative tracking index value that is higher than the first comparative tracking index value of the molded plastic body and that fulfils the comparative tracking index requirements.

According to an embodiment of a method of manufacturing a semiconductor package which meets a plurality of predetermined electrical, mechanical, chemical and/or environmental requirements, the method comprises: applying a coating to the molded plastic body, wherein either the coating is applied to coat the molded plastic body completely, or the coating is applied to the molded plastic body only in a region where the plurality of terminals protrude from the molded plastic body, including between adjacent ones of the terminals, wherein the molded plastic body comprises a flame-retardant material; and embedding or covering a semiconductor die by the molded plastic body. The terminals protrude from the molded plastic body and are electrically connected to the semiconductor die. The molded plastic body satisfies only a subset of the plurality of predetermined electrical, mechanical, chemical and/or environmental requirements. The coating comprises a plastic material and satisfies each predetermined electrical, mechanical, chemical and/or environmental requirement not satisfied by the molded plastic body. The molded plastic body achieves a first comparative tracking index value that does not fulfil the comparative tracking index requirements for the semiconductor package, and the coating achieves a second comparative tracking index value that is higher than the first comparative tracking index value of the molded plastic body and that fulfils the comparative tracking index requirements.

The features of the various illustrated embodiments can be combined unless they exclude each other. Embodiments are depicted in the drawings and are detailed in the description which follows.

Embodiments described herein provide a power semiconductor package which meets a plurality of predetermined electrical, mechanical, chemical and/or environmental requirements. The semiconductor package includes a molded plastic body that satisfies only a subset of the plurality of predetermined electrical, mechanical, chemical and/or environmental requirements. The semiconductor package further includes a coating applied to at least part of the molded plastic body and optionally to part of the package terminals, the coating satisfying each predetermined electrical, mechanical, chemical and/or environmental requirement not satisfied by the molded plastic body. With such a package construction, certain requirements are fulfilled just by the coating while other requirements are fulfilled just by the plastic material. By separating the package requirements between the coating and the bulk plastic material, some conventional additives can be omitted while still fulfilling the overall requirements of the package.

<FIG> illustrate different embodiments of a semiconductor package which meets a plurality of predetermined electrical, mechanical, chemical and/or environmental requirements, and which includes one or more semiconductor dies <NUM> embedded in a molded plastic body <NUM>. A plurality of terminals <NUM> protrude from the molded plastic body <NUM> and are electrically connected to the semiconductor die <NUM> e.g. as leads of a leadframe or via bond wires, metal clips, metal ribbons, etc. Electrical connections between the package terminals <NUM> and each semiconductor die <NUM> included in the package are not shown for ease of illustration in <FIG>.

One semiconductor die <NUM> is shown for ease of illustration, however, the package may include more than one semiconductor die. Each semiconductor die <NUM> housed within the package may include one or more active devices such as transistors and/or diodes, one or more passive devices such as capacitors and/or inductors, or a combination of passive and active devices. The type of device(s) implemented by each semiconductor die <NUM> is unimportant, and any type of semiconductor die(s) can be included in the package.

The molded plastic body <NUM> is out of view in <FIG>, and illustrated with a dashed box. The semiconductor die <NUM> is out of view in <FIG>, and illustrated with a dashed square in <FIG>. In each embodiment, the molded plastic body <NUM> satisfies only a subset of the plurality of predetermined electrical, mechanical, chemical and/or environmental requirements.

According to the claimed invention one predetermined electrical requirement for the package is the electrical breakdown (tracking) properties of the package. CTI (comparative tracking index) is used to measure the electrical breakdown (tracking) properties of insulating materials. One applicable standard for CTI may be IEC60664, which defines CTI><NUM> and CTI> <NUM>. Another electrical requirement may be creepage, which refers to the leakage path along the surface of an electrically insulating material interposed between two conductors. Still other electrical requirements may be electrical RTI ≥<NUM>, and/or the ability of the module to tolerate an <NUM>/<NUM>%RH test with applied blocking voltage (e.g. <NUM>%, <NUM>% or <NUM>% of blocking voltage) of the semiconductors for <NUM>, <NUM> or even <NUM> without causing excessive leakage current between terminals, substrate tracks or semiconductor electrodes.

One predetermined mechanical requirement for the package may be thermal resistance at the interface between the molded plastic body <NUM> and another body such as a heatsink. Another predetermined mechanical requirement may be robustness to mechanical stress e.g. through compression, vibration and/or shock. Still other mechanical requirements may be elongation at yield ≥<NUM>%, preferred, and/or mechanical RTI ≥<NUM>.

One predetermined chemical requirement for the package may be corrosion or other chemical action. Certain components of the power semiconductor package may be susceptible to corrosion or other chemical action, e.g. such as the package terminals <NUM> and semiconductor components <NUM> housed within the package. Materials sensitive to corrosion or other chemical action, electrochemical migration and dendritic growth include aluminum, copper and silver.

One predetermined environmental requirement for the package may be the degree to which smoke is vented and released from the package. Another predetermined environmental requirement for the package may be the likelihood of the package catching fire or combusting during a failure event or under extreme operating conditions. For example, the package may have to satisfy one or more environmental requirements specified in the following environmental standards: RoHS; RoHS directive defined in REACH; UL94V0; and/or EN45545, including HL2 and/or R22.

The above list of predetermined electrical, mechanical, chemical and environmental requirements is provided for merely illustrative purposes, and is not intended to be limiting. The semiconductor package may be required to satisfy fewer, additional or different electrical, mechanical, chemical and/or environmental requirements. In each case, the molded plastic body <NUM> which encases the semiconductor die <NUM> satisfies only a subset of the predetermined electrical, mechanical, chemical and/or environmental requirements which the package must satisfy.

To ensure that the semiconductor package satisfies all predetermined electrical, mechanical, chemical and/or environmental requirements assigned to the semiconductor package, the package further includes a coating <NUM> applied to at least part of the molded plastic body <NUM> and optionally to part of the package terminals <NUM>. The coating <NUM> satisfies each predetermined electrical, mechanical, chemical and/or environmental requirement assigned to the semiconductor package not satisfied by the molded plastic body <NUM>.

According to the embodiment illustrated in <FIG>, the coating <NUM> is applied only to the molded plastic body <NUM>.

According to the embodiment illustrated in <FIG>, the coating <NUM> is applied only to the package terminals <NUM> in a region where the terminals <NUM> protrude from the molded plastic body <NUM>.

According to the embodiment illustrated in <FIG>, the coating <NUM> is applied only to the molded plastic body <NUM> in a region where the package terminals <NUM> protrude from the molded plastic body <NUM>, including between adjacent ones of the terminals <NUM>.

In each embodiment, the semiconductor die <NUM> is attached to a substrate (not shown) such as a die paddle of a leadframe, circuit board, ceramic laminate, etc. and the package terminals <NUM> are electrically connected to contact pads on the die <NUM> e.g. by wire bonds, metal clips, metal ribbons, or by direct attach in the case of a die paddle, etc. The semiconductor die <NUM> and substrate are then embedded in the molded plastic body <NUM> via a molding process such as injection molding, transfer molding, film assisted molding (FAM), etc. The coating <NUM> is then applied in certain regions e.g. as shown in <FIG>, and provides a corrosion barrier between the package terminals <NUM> and the molded plastic body <NUM>.

<FIG>, and <FIG> illustrate further embodiments of a semiconductor package which meets a plurality of predetermined electrical, mechanical, chemical and/or environmental requirements, and which includes one or more semiconductor dies <NUM> covered by a molded plastic body <NUM>. According to these embodiments, the molded plastic body <NUM> forms a housing or lid that is attached to a substrate <NUM>. Each semiconductor die <NUM> included in the package is attached to the substrate <NUM> which can be, e.g., a die paddle of a leadframe, circuit board, ceramic laminate, a ceramic substrate such as a DCB (direct copper bonded), AMB (active metal brazed) or DAB (direct aluminum bonded) substrate, substrates assembled on a base plate, etc. Terminals <NUM> of the package are attached to the substrate <NUM> and protrude through the molded plastic body <NUM>. The package terminals <NUM> are electrically connected to contact pads on each semiconductor die <NUM> e.g. by wire bonds, metal clips, metal ribbons, flexible boards, chip embedding technology such as eWLB (embedded wafer level ball grid array), etc. Electrical connections between the package terminals <NUM> and each semiconductor die <NUM> included in the package are not shown for ease of illustration in <FIG>. Each semiconductor die <NUM> housed within the package may include one or more active devices, the type of which is unimportant as previously explained herein in connection with <FIG>.

According to the embodiment illustrated in <FIG>, a coating <NUM> is selectively applied to desired regions of the molded plastic body <NUM> to ensure that each predetermined electrical, mechanical, chemical and/or environmental requirement assigned to the semiconductor package and not satisfied by the molded plastic body <NUM> is satisfied by the coating <NUM>. Selective coating can be realized by applying a mask to regions of the molded plastic body <NUM> not to be covered by the coating <NUM>. The mask is removed e.g. using a solvent after the coating is applied. In another example, a blanket coating process is followed by a selective etching process which removes the coating <NUM> from regions of the molded plastic body <NUM>.

<FIG> illustrates an embodiment of a semiconductor package which is similar to the embodiment illustrated in <FIG>. Different, however, the sidewalls 208a, 208b of the molded plastic body <NUM> are also covered by the coating <NUM>.

According to an example illustrated in <FIG> and not falling under the scope of the claims, the package terminals <NUM> are inserted into the molded plastic body <NUM> after the coating <NUM> is applied to the terminals <NUM>. In the case of injection molding, the coating <NUM> is applied to the package terminals <NUM> and the coated terminals <NUM> are then injection molded into the plastic body <NUM>. In the case of transfer molding, the coating <NUM> is applied to the package terminals <NUM>, the coated terminals <NUM> are attached to the substrate <NUM>, and afterwards the module is transfer molded. In either case, the coating <NUM> withstands the temperature of the molding process.

According to the embodiment illustrated in <FIG>, the package terminals <NUM> are molded into the plastic body <NUM>. The coating <NUM> is then applied to the entire package with the terminals <NUM>. The coating <NUM> is removed from external contact surfaces 206a of the package terminals <NUM>, to ensure proper connection to another body e.g. such as a circuit board. A mask can be applied to the external contact surfaces 206a of the package terminals <NUM>, or the coating <NUM> can be removed by an etching process.

<FIG> illustrates an example of a semiconductor package not falling under the scope of the claims which is similar to the example illustrated in <FIG>. Different, however, a thicker coating 208a is applied to adjacent package terminals 206p which are at different electric potentials (e.g. source and drain potentials, collector and emitter potentials, or anode and cathode potentials) as compared to other ones of the package terminals (e.g. gate or signal terminals). This way, the thicker coating 208a applied to the power terminals 206p of the package can isolate the full voltage. In one embodiment, the thicker coating 208a applied to adjacent ones of the package terminals 208a at different potentials has a thickness in a range between <NUM> micrometres (µm) and <NUM> millimetre (mm) to isolate the adjacent terminals 206p at different potentials.

Additional semiconductor package embodiments are described next, which apply to both the embodiments illustrated in <FIG> (semiconductor die embedded in molded plastic body encapsulant) and the embodiments illustrated in <FIG>, and <FIG> (semiconductor die covered by molded plastic body housing). In each case, the coating <NUM>/<NUM> can be poly-para-xylylene, a thermoplastic or thermoset polymer, silicone or thixotropic silicone, an epoxy, polyester, etc..

In an example not falling under the scope of the claims but useful for the understanding thereof, the molded plastic body <NUM>/<NUM> of the semiconductor package comprises a flame-retardant material and fulfils predetermined mechanical, fire, smoke and CTI>600V requirements. Corrosion or other chemical action on the package terminals <NUM>/<NUM> which are in physical contact with the molded plastic body <NUM>/<NUM> is prevented or at least minimized by a coating <NUM>/<NUM> applied to at least part of the molded plastic body <NUM>/<NUM> and/or part of the package terminals <NUM>/<NUM> by a CVD (chemical vapor deposition) process. In the CVD process, para-cyclophane is used to deposit a poly-para-xylylene layer having a thickness of at least <NUM>, preferably between <NUM> and <NUM>. The molded plastic body <NUM>/<NUM> is coated completely or only in areas where the package terminals <NUM>/<NUM> contact the body <NUM>/<NUM>, and also between the terminals <NUM>/<NUM>, e.g. along creepage paths. The CVD coating process can be selective in that the package terminals <NUM>/<NUM> can be coated outside contact areas of the terminals <NUM>/<NUM> which make contact to other electrical conductors of the system. As previously described herein, a mask can be applied to the regions of the molded plastic body <NUM>/<NUM> and/or package terminals <NUM>/<NUM> not to be covered by the coating <NUM>/<NUM>. The mask is removed e.g. using a solvent after the coating <NUM>/<NUM> is applied. In another example, a blanket coating process is followed by a selective etching process which removes the coating <NUM>/<NUM> from regions of the molded plastic body <NUM>/<NUM> and/or package terminals <NUM>/<NUM>.

In another example not falling under the scope of the claims, the molded plastic body <NUM>/<NUM> of the semiconductor package comprises a flame-retardant material and fulfils predetermined mechanical, fire, smoke and CTI>600V requirements. Corrosion or other chemical action on the package terminals <NUM>/<NUM> which are in physical contact with the molded plastic body <NUM>/<NUM> is prevented or at least minimized by a coating <NUM>/<NUM> applied to at least part of the molded plastic body <NUM>/<NUM> and/or part of the package terminals <NUM>/<NUM> by an electrostatic powder coating process. Clean thermoplastic or thermoset polymers are deposited during the electrostatic powder coating process. The resulting coating <NUM>/<NUM> can have a thickness of least <NUM>, e.g. > <NUM>. The electrostatic powder coating process can be selected as described above, so that the package terminals <NUM>/<NUM> are coated outside contact areas which make contact to other electrical conductors of the system.

In yet another example not falling under the scope of the claims, the molded plastic body <NUM>/<NUM> of the semiconductor package comprises a flame-retardant material and fulfils predetermined mechanical, fire, smoke and CTI>600V requirements. Leakage current through the molded plastic body <NUM>/<NUM> and between the package terminals <NUM>/<NUM> is prevented by a selective coating, accomplished by an electrostatic powder coating process. In this embodiment, the coating <NUM>/<NUM> is a thermoplastic or thermoset polymer having a deposited thickness in a range between <NUM> micrometres and <NUM> millimetre. Such a thick coating ensures dielectric strength and takes over a significant part, if not all, of the voltage to be isolated between the package terminals <NUM>/<NUM>.

In an embodiment according to the disclosure, the molded plastic body <NUM>/<NUM> of the semiconductor package comprises a flame-retardant material and fulfils predetermined mechanical, fire and smoke requirements but not the CTI requirement for the package. Corrosion or other chemical action on the package terminals <NUM>/<NUM> which are in physical contact with the molded plastic body <NUM>/<NUM> is prevented or at least minimized by a coating <NUM>/<NUM> that comprises a plastic material which does not require additives for flame retardance and fire and smoke requirements, such as PPS (polyphenylene sulfide), LCP (liquid-crystal polymer), silicone, thixotropic silicone, etc. The plastic-based coating <NUM>/<NUM> achieves a high CTI-value e.g. >600V. The coating <NUM>/<NUM> is deposited on selective areas of the molded plastic body <NUM>/<NUM> and optionally on the package terminals <NUM>/<NUM> where high CTI is required, e.g. by an electrostatic powder coating process.

In another embodiment, the coating <NUM>/<NUM> is applied by whirl sintering which allows for thicker layers such as <NUM>, <NUM> or even thicker coatings applied to the molded plastic body <NUM>/<NUM> and/or the package terminals <NUM>/<NUM>. For example, coatings based on epoxy, polyester or thermoplastics can be used in both electrostatic powder coating or whirl sintering processes.

Any of the coatings described above preferably have a low content of aggressive ions, which are known to cause corrosion or change characteristics of semiconductors, directly or indirectly. Some examples of such ions are chlorine, sodium, potassium, fluorine, phosphorous or any other ions and ionic molecules which can be extracted by water or water vapor from the coating material. The maximum content of ions and/or ionic molecules which can be extracted from the coating <NUM>/<NUM> by water or water vapor preferably have a maximum content below 1000ppm or 100ppm by weight, according to an embodiment. The extractable ionic content of the coating material can be detected as follows. The coating material in its final form is reduced to power form for investigation (e.g. particle size <<NUM>). The powder is mixed with deionized water in a clean environment (e.g. clean room <NUM> or better). The sample is stored at a temperature of about <NUM> for about <NUM> minutes. The sample is then filtered. Ion chromatography of water with dissolved ions indicates the content of aggressive ions.

Claim 1:
A semiconductor package which meets a plurality of predetermined electrical, mechanical, chemical and/or environmental requirements, the semiconductor package comprising:
a semiconductor die (<NUM>) embedded in or covered by a molded plastic body (<NUM>), the molded plastic body (<NUM>) comprising a flame-retardant material, and satisfying only a subset of the plurality of predetermined electrical, mechanical, chemical and/or environmental requirements;
a plurality of terminals (<NUM>) protruding from the molded plastic body (<NUM>) and electrically connected to the semiconductor die (<NUM>); and
a coating (<NUM>) applied to the molded plastic body (<NUM>), wherein either the coating (<NUM>) coats the molded plastic body (<NUM>) completely, or the coating (<NUM>) is applied to the molded plastic body (<NUM>) only in a region where the plurality of terminals (<NUM>) protrude from the molded plastic body (<NUM>), including between adjacent ones of the terminals (<NUM>), the coating (<NUM>) comprising a plastic material and satisfying each predetermined electrical, mechanical, chemical and/or environmental requirement not satisfied by the molded plastic body, wherein
the molded plastic body (<NUM>) achieves a first comparative tracking index value that does not fulfil the comparative tracking index requirements for the semiconductor package, and
the coating (<NUM>) achieves a second comparative tracking index value that is higher than the first comparative tracking index value of the molded plastic body (<NUM>) and that fulfils the comparative tracking index requirements.