Patent Description:
One or more embodiments can be applied advantageously to power semiconductor devices.

Various types of semiconductor devices with a plastic package comprise:.

In a power semiconductor device, the current transferred from the high-power section to the output pads of the device can be significant and ribbons or clips are used for that purpose in the place of wires. Wires can still be used to provide electrical coupling to a low-power section (e.g., a controller) in the device.

Ribbons are placed using essentially a wire bonder process.

Clips are placed with a clip attach equipment, and a solder paste is used to connect the clip to pad and die. Solder curing in an oven is applied to provide a solid connection of clips to pad and die.

Conventional clip attachment equipment facilitates achieving an adequate accuracy in chip placement as the clip is applied on die and pad, after which the assembly is transferred to an oven for solder curing.

During this handling and curing process, clips may become displaced from a desired correct position. This may result in a defective final product. Solder thickness and the tendency of the clip to "float" on solder in a fluid state may also lie at the basis of undesired excessive clip tilt.

Documents <CIT>, <CIT> or <CIT> are exemplary of background art in that area. <CIT> discloses a clip bonded in a semiconductor package, wherein lateral shifting of the clip is prevented by a cylinder shaped electrode which is engaged with a sunken pad on the semiconductor chip. <CIT> discloses a clip attached by an array of LASER point welds. <CIT> discloses a clip bonded in a semiconductor device, wherein a positional shift is prevented by engaging male/female parts.

An object of one or more embodiments is to contribute in adequately addressing the issues discussed in the foregoing.

According to one or more embodiments, such an object can be achieved via a method having the features set forth in independent method claim <NUM>, as well as in the corresponding independent device claim <NUM>.

One or more embodiments may relate to a corresponding semiconductor device.

The claims are an integral part of the technical teaching on the embodiments as provided herein.

One or more embodiments may provide one or more of the following advantages:.

In the ensuing description one or more specific details are illustrated, aimed at providing an in-depth understanding of examples of embodiments of this description.

For simplicity and ease of explanation, throughout this description, like parts or elements are indicated in the various figures with like reference signs, and a corresponding description will not be repeated for each and every figure.

In current manufacturing processes of semiconductor devices, plural devices are manufactured concurrently to be separated into single individual device in a final singulation. For simplicity and ease of explanation, the following description will refer to manufacturing a single device.

<FIG> is exemplary of a power semiconductor device <NUM> with a plastic package.

As conventional in the art, the device <NUM> comprises a substrate (leadframe) <NUM> having arranged thereon one or more semiconductor chips or dice. As used herein, the terms chip/s and die/dice are regarded as synonymous.

The figures illustrate by way of example a semiconductor power device <NUM> comprising a low-power section (e.g., a controller die <NUM>) attached on a first die pad 121A in the leadframe <NUM> and a high-power section (e.g., one or more power dice <NUM>) attached on one or more die pads 122A in the lead frame <NUM>, with an array of leads 12B around the die pads 121A, 122A having the dice <NUM> and <NUM> mounted thereon.

The designation "leadframe" (or "lead frame") is currently used (see, for instance the USPC Consolidated Glossary of the United States Patent and Trademark Office) to indicate a metal frame that provides support for an integrated circuit chip or die as well as electrical leads to interconnect the integrated circuit in the die or chip to other electrical components or contacts.

Essentially, a leadframe comprises an array of electrically-conductive formations (or leads, e.g., 12B) that from an outline location extend inwardly in the direction of a semiconductor chip or die (e.g., <NUM>, <NUM>) thus forming an array of electrically-conductive formations from a die pad (e.g., 121A, 122A) configured to have at least one semiconductor chip or die attached thereon. This may be via conventional means such as a die attach adhesive <NUM> (a die attach film or DAF, for instance).

A device <NUM> as illustrated in <FIG> is intended to be mounted on a substrate such as a printed circuit board (PCB - not visible in the figures), using solder material, for instance.

Electrically conductive formations are provided to electrically couple the semiconductor chip(s) <NUM>, <NUM> to selected ones of the leads (outer pads) 12B in the leadframe <NUM>.

As illustrated, these electrically conductive formations comprise wire bonding patterns <NUM> coupling the low-power section (chip <NUM>) to selected ones of the leads 12B and to the high-power section (chip or chips <NUM>. These wire bonding patterns <NUM> are coupled to die pads <NUM> provided at the front or top surfaces of the chips <NUM> and <NUM>.

Conversely, so-called clips <NUM> are used to couple the high-power section (chip or chips <NUM>) to selected ones of the leads 12B acting as (power) output pads of the device <NUM>.

Using clips <NUM> in the place of wires as included in the wire bonding patterns <NUM> (used to provide electrical coupling to a low-power section e.g., a controller <NUM>) takes into account the fact that the current transferred from the high-power section <NUM> to the output pads in a power semiconductor device may be significant. As noted, wires such as the wires <NUM> are still used to provide electrical coupling to a low-power section (e.g., a controller) in the device.

An insulating encapsulation <NUM> (e.g., an epoxy resin) is molded on the assembly thus formed to complete the plastic body of the device <NUM>.

While the device <NUM> as shown comprises two clips <NUM>, certain devices may comprise just one clip or more than two clips.

A device structure as discussed so far is conventional in the art, which makes it unnecessary to provide a more detailed description herein.

To summarize, for the purposes herein, producing the device <NUM> as discussed herein involves:.

In such a bridge-like position, the electrically conductive clip <NUM> has coupling surfaces facing towards the semiconductor chip <NUM> and the electrically conductive pad 12B.

The electrically conductive clip <NUM> positioned in said bridge-like position is soldered to the semiconductor chip <NUM> and to the electrically conductive pad 12B to provide electrical coupling therebetween.

As illustrated, soldering is via soldering material <NUM> dispensed (in a manner known per se to those of skill in the art) at said coupling surfaces. The soldering material <NUM> is consolidated (in a manner likewise known per se to those of skill in the art), e.g., via heat treatment in an oven.

As discussed, clips such as the clip <NUM> are placed using a clip attach equipment and a solder paste <NUM> is used to connect the clip to pad and die. Solder curing in an oven is applied to provide a solid connection of the clips <NUM> to pad (e.g., 12B) and die (e.g., <NUM>).

Conventional clip attachment equipment facilitates achieving an adequate accuracy in chip placement as a clip <NUM> is applied bridge-like between a die such as the die <NUM> and a respective pad/lead such as the pad/lead 12A: this case is considered for simplicity; in certain devices an individual clip <NUM> may be coupled, e.g., to plural pads/leads.

After clip placement the assembly is transferred to an oven for solder curing. During this handling and curing process clips may become displaced from a desired correct position, which may result in a defective final product.

The thickness of the solder <NUM> and the tendency of the clip to "float" on the solder <NUM> in a fluid state may also lie at the basis of undesired excessive clip tilt.

Undesired clip movement (displacement) can be attempted to be countered by adding fixing features in the clip and leadframe design.

Smooth handling may also help along with very accurate clip centering in clip placement.

Selecting solder paste materials countering undesired clip floating properties can also be considered.

None of these solutions appears fully satisfactory, for various reasons.

For instance, certain features added to the clip/leadframe design can be space-consuming, which may suggest reducing pad dimensions and/or using larger package dimensions to gain space, neither of which is attractive/desirable.

Handling of the parts involved is already fairly gentle and further improvements in that directions are hardly conceivable.

Selecting solder paste materials different from those conventionally used may have negative effects in terms of thermal and electrical performance.

In examples as considered herein - prior to soldering- the clip or clips <NUM> are immobilized in the desired bridge-like position via welding (e.g., laser welding) or gluing at one or more dedicated immobilization areas, e.g., as generally indicated by <NUM> in <FIG>.

As discussed in the following, the immobilization areas <NUM> can be suitably formed and shaped, e.g., by coining, in order to be configured to facilitate welding or gluing of the clip(s) <NUM>.

Examples as considered herein are thus based on the concept of "tacking" or "basting" the coupling of the clip(s) <NUM> to chip(s) <NUM> and the pad(s) or lead(s) 12B in the desired bridge-like position so that the clip(s) are immobilized and retained in such a position during soldering, thus countering undesired displacement (e.g., translation/rotation/tilt).

In various examples this involves stepped lateral portions of a clip <NUM> resting on the leadframe <NUM> (e.g., at a lead or pad 12B) and providing flat areas of contact for the laser welding or the gluing of the clip <NUM>.

<FIG> are exemplary of the possibility of modifying a clip <NUM> as illustrated in <FIG> in such a way to facilitate "tacking" or "basting" the coupling of the clip(s) <NUM> by laser welding as exemplified by references LB in <FIG>.

<FIG> are exemplary of the possibility of modifying a clip <NUM> as illustrated in <FIG> at the areas or regions <NUM> in such a way to facilitate "tacking" or "basting" the coupling of the clip(s) <NUM> by gluing as exemplified by the reference G in <FIG>.

In either case (laser welding, gluing) the fact is taken into account that solder material <NUM> is provided to facilitate electrical coupling of the clip <NUM> to the lead 12B.

In order to avoid undesired detachment/displacement of the clip <NUM> with respect to the desired bridge-like coupling position one or more dedicated clip areas <NUM> to be used to apply (micro dot) laser welding LB - <FIG> - or gluing G (via, e.g., the glue available under the trade designation Loctite® <NUM> with Henkel AG& Co KGaA - <FIG>.

In that way the clip <NUM> can be pre-fixed through laser welding after clip attach to the clip bar. Such micro-welding facilitates robust clip positioning until the solder paste <NUM>, after melting, carries out the brazing.

Additionally, the clip(s) <NUM> maintain the die <NUM> well placed during reflow avoiding translation and rotation.

Flat welding areas <NUM> at the clip edges also facilitate clip planarity, which results in improved results in board-level testing (BLT) as compared with conventional processes.

Specific welding areas such as those indicated by the reference <NUM> in <FIG> can be realized (by coining, for instance) when manufacturing a clip bar.

The points above also apply identically to dedicated clip areas <NUM> intended for gluing as represented in <FIG>.

Glue drops as indicated by reference G in <FIG> can be dispensed on a leadframe clip bar in addition to solder paste, with glue curing taking place during preheating in a reflow tunnel oven. Such gluing facilitates robust clip positioning until the solder paste <NUM>, after melting, carries out the brazing.

Additionally, glue bonded clip(s) <NUM> maintain the die <NUM> well placed during reflow.

Improved clip planarity is again facilitated as compared with conventional processes.

In both instances (laser welding, gluing) clip and die are maintained in a desired position during assembly processes.

As illustrated, throughout the figures, clips such as the clips <NUM> exhibit (in manner known per se) a bent shape with a distal portion 18A configured to be coupled to the lead(s) 12B and thus at least slightly downset to allow for the thickness of the chip <NUM>.

Advantageously, the dedicated "tacking" or "basting" (laser welding, gluing) areas <NUM> are provided at such distal portion 18A.

Advantageously, a pair of such areas <NUM> are provided at opposed ends of the distal portion 18A.

As visible in figures such as <FIG> or <FIG>, the area(s) <NUM> at least slightly protrude from the general plane of the (distal portion 18A of) clip <NUM>. This results in containment walls (indicated by reference <NUM> in <FIG>, <FIG>, <FIG>) being formed that facilitate containment of solder paste <NUM> as visible in <FIG> and <FIG>.

In that way, once the area(s) <NUM> are fixed (welded or glued) to the lead 12B, a gap is formed between the remainder of the distal portion 18A and the lead 12B.

As visible in figures such as <FIG> or <FIG>, solder paste <NUM> in a molten state can penetrate into such a gap to provide a solder layer of controlled thickness.

The shape of the areas <NUM> was not found to represent per se a critical parameter.

As illustrated in <FIG>, a round shape was however found to be advantageous for areas <NUM> intended for laser welding LB.

As illustrated in <FIG>, a rectangular (or square) shape was found to be advantageous for areas <NUM> intended for gluing G.

Examples as presented herein facilitate maintaining a precise clip "centering" during assembly processes.

Undesired clip movement and rotation are avoided, and solder thickness can be adequately controlled.

In examples as presented herein, dedicated welding/gluing areas <NUM> are shown formed at the clip end 18A intended to cooperate with a pad/lead 12B in the leadframe <NUM>.

In principle (at least insofar as gluing is concerned) such areas <NUM> could be formed also or only at the opposed end of the clip <NUM>, that is at the end intended to cooperate with the chip <NUM>.

It is noted that the presence of the dedicated welding/gluing areas <NUM> will be noticeable also in the final device, where the clip(s) <NUM> will be fixed in the desired bridge-like position by the joint action of:.

Claim 1:
A method, comprising:
arranging at least one semiconductor chip (<NUM>) on a die pad (12A) in a substrate (<NUM>), the substrate (<NUM>) comprising at least one electrically conductive pad (12B) adjacent to the die pad (12A),
positioning at least one electrically conductive clip (<NUM>) in a bridge-like position between the at least one semiconductor chip (<NUM>) and the at least one electrically conductive pad (12B), wherein, in said bridge-like position, the at least one electrically conductive clip (<NUM>) has coupling surfaces facing towards the at least one semiconductor chip (<NUM>) and the at least one electrically conductive pad (12B), and
soldering (<NUM>) the at least one electrically conductive clip (<NUM>) in said bridge-like position to the at least one semiconductor chip (<NUM>) and to the at least one electrically conductive pad (12B) to provide electrical coupling therebetween, wherein soldering is via soldering material (<NUM>) at said coupling surfaces,
characterised in that the method comprises, prior to said soldering, immobilizing (LB, G) the at least one electrically conductive clip (<NUM>) in said bridge-like position via welding (LB) or gluing (G) to at least one of the electrically conductive pads (12B) and the at least one semiconductor chip (<NUM>).