Patent Description:
Solutions as described herein can increase the reliability of Quad-Flat No-Leads, QFN packages, such as, for instance power QFN packages, for automotive products.

So-called wettable flanks in semiconductor device packages are desirable for facilitating automatic optical inspection of solder joints after assembly on a mounting surface (a printed circuit board, PCB, for instance).

Wettable flanks of increased height are particularly desirable; however, wettable flank height may be limited to half the leadframe thickness if half-etched tie bars are provided during packaging to keep leads in place.

Also, defects in wedge contact between wire/ribbon and a corresponding lead may result in a failure of the whole unit, e.g., due to issues related to wire bonding, notably weld off/weld lift.

Document <CIT> discloses a lead frame comprising a tab whereon a semiconductor chip is mounted, an inner lead which is provided with a wire connection made lower than the tip and is electrically connected with the pad of the above semiconductor chip, at the wire connection through a bonding wire, a sealing body which is made by sealing the above semiconductor chip and the bonding wire with resin, and an outer lead which is projected outward from this sealing body. A wire connection which is a circular recess formed lower than the tip is made in the vicinity of the tip of the inner lead.

Document <CIT> discloses a semiconductor device including: a semiconductor chip having a plurality of pads; a plurality of leads each having a mounting surface and a wire connection surface, each including a thick part and a thin part having a thickness smaller than that of the thick part, and each formed so that the length of the wire connection surface is smaller than that of the mounting surface; a plurality of wires for connecting the semiconductor chip to the leads; and a sealing body formed of a resin. The thin part of each lead is arranged by crawling into the lower part of the semiconductor chip and the leads 1a and the semiconductor chip are connected to each other through the wires by reverse bonding, whereby the package size is brought close to the chip size by reducing the distance from a side face of the semiconductor chip to a side face of the sealing body as much as possible while securing the length of the mounting surface of each lead and a QFN is downsized.

Document <CIT> discloses a method of manufacturing a semiconductor device wherein a lead frame is provided having a trench part formed thereon so as to communicate bottom surfaces of a first lead and a second lead, which are coupled to each other between device regions adjacent to each other. After a part of a coupling part between the first and second leads is cut by using a first blade, metal wastes formed inside the trench part are removed. After the metal wastes are removed, a metal film is formed on exposed surfaces of the first and second leads by a plating method, and then, a remaining part of the coupling part between the first and second leads is cut by using a second blade. At this time, the cutting is performed so that the second blade does not contact the trench part.

Solutions as described herein aim at addressing the issues discussed in the foregoing.

Such an object can be achieved via a method having the features set forth in the claims that follow.

One or more embodiments relate to a corresponding semiconductor device.

One or more embodiments relate to a corresponding assembly (a semiconductor device plus a mounting member therefor, e.g., a printed circuit board, PCB).

One or more embodiments relate to a support substrate (e.g., a leadframe for use in manufacturing semiconductor devices).

The claims are an integral part of the technical teaching provided in respect of the embodiments.

Solutions as described herein provide a leadframe design and wire bonding scheme for wettable flanks (e.g. for QFN packages) that increases the total height of the wettable flanks while also providing an additional and direct electrical path between wire/ribbon and solder joint in case of weld failure.

Solutions as described herein may include leads comprising half-etched, outwards extending recessed portion.

Solutions as described herein contemplate providing a leadframe, mounting a die thereon, forming electrical connections between the die and leads in the leadframe through wire/ribbon bonding.

In solutions as described herein a second bond is formed in the recessed portion of the lead and a wire/ribbon tail extends outwardly of the recessed portion.

In solutions as described herein an encapsulation is molded and partial sawing of leads facilitates forming wettable flanks.

In solutions as described herein, partial sawing extends into the wire/ribbon tail and exposes a flank portion thereof.

In solutions as described herein tin plating of the partially cut leads and exposed portion of wire/ribbon follows.

Solutions as described herein facilitate wettable flanks/solder join inspection, e.g., package flank optical inspection (a wire/ribbon section is visible on package sides) and/or package X-ray inspection (where recessed lead portions are visible).

The edges of features drawn in the figures do not necessarily indicate the termination of the extent of the feature.

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, and unless the context indicates otherwise, 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.

<FIG> is a partial cross-sectional view through a semiconductor device <NUM> mounted (attached) on a support member S such as a printed circuit broad, PCB.

As illustrated in <FIG>, attachment is via solder material SM and various device packages (e.g., for the automotive market) have a geometry feature <NUM> on the lead sides, called wettable flanks.

This feature facilitates formation of a meniscus in the solder material SM when soldering the device <NUM> on a support member, e.g., PCB.

Wettable flanks facilitate automatic optical inspection insofar as the solder meniscus is visible.

As illustrated herein by way of example, a semiconductor device such as the device <NUM> comprises:.

As used herein, the terms chip/s and die/dice are regarded as synonymous.

<FIG> illustrate a power semiconductor device <NUM> comprising a low-power section (e.g., a controller die, illustrated on the right-hand side of <FIG>) attached on a first die pad in the leadframe <NUM> and a high-power section (e.g., one or more power dice illustrated on the left-hand side <FIG>) attached on one or more die pads in the lead frame <NUM>, with an array of leads around the die pads having the low-power and the high-power dice mounted thereon.

The current transferred from the high-power section to the output pads of the device can be significant. As shown on the left-hand side of <FIG>, ribbons or clips <NUM> are thus used for that purpose in the place of wires. Wires, likewise indicated as <NUM> can still be used as shown on the right-hand side of <FIG> to provide electrical coupling to a low-power section (e.g., a controller) in the device.

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>) thus forming an array of electrically-conductive formations from a die pad (e.g., 12A) configured to have at least one semiconductor chip or die attached thereon. This may be via conventional means such as a die attach adhesive (a die attach film or DAF, for instance).

Again, as illustrated in <FIG>, electrically conductive formations are provided comprising wire bonding patterns <NUM> coupling the low-power section (right-hand side of <FIG>) to selected ones of the leads 12B and to the high-power section (left-hand side of <FIG>). These wire bonding patterns are coupled to die pads provided at the front or top surfaces of the chips.

Conversely, so-called ribbons are used to couple the high-power section (left-hand side of <FIG>) to selected ones of the leads 12B acting as (power) output pads of the device <NUM>. Using ribbons in the place of wires accounts for the fact that the current transferred from the high-power section to the output pads in a power semiconductor device may be significant.

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

A single (integrated circuit) chip or die <NUM> mounted on a single die pad 12A will be considered throughout the rest of this description for simplicity.

The leadframe <NUM> is the base material support for the device <NUM>. It is made of, e.g., copper, and shaped to accommodate semiconductor dice and generate pad connections.

Photo-etching can be advantageously used for leadframe manufacturing: a sheet or strip of copper material is etched on top and bottom side to create die pads and leads.

Pads/leads are connected together by sacrificial connecting bars. Wedge-wedge bonding could be used to bond wires. Plating treatments are also applied as desired.

Wedge(-wedge) bonding is a current designation for wire/ribbon bonding technology that use force and ultrasonic power in order to create an interconnection between a surface (e.g., a leadframe <NUM>/a die <NUM>) and a wire/ribbon as designated by <NUM> in figures: in fact, Wedge bonding technology can be applied to both wires and ribbons.

As illustrated, e.g., in <FIG>, the (welded) connection of a wire and/or ribbon <NUM> to a die/leadframe surface includes two sections:.

The proximal wedge section <NUM> carries the current/signal, while distal tail <NUM> does not contribute appreciably to signal transport, because that section is not welded to the die/leadframe surface.

After molding of the encapsulation <NUM>, the device package is sealed. Dice <NUM> and wires/ribbons <NUM> are no longer accessible from the outside.

Any detachment between a wire/ribbon <NUM> and a corresponding lead 12B, for instance, will result in a failure of the whole device <NUM>. Wire bonding related issues may represent a major source of device failure.

A method to provide wettable flanks on a device package is by sawing.

This approach relies on the fact that, in current manufacturing processes of semiconductor devices, plural devices are manufactured together and separated into single individual device in a final singulation (sawing) step.

That is, after a chain of devices <NUM> formed on a common leadframe strip is assembled (dice, wires and molding: see <FIG>), individual devices <NUM> are separated via singulation as represented in <FIG> using a sawing blade SB.

As illustrated in <FIG> and <FIG>, prior to the final singulation, the leadframe <NUM> is partially sawn (engraved) using a circular saw SB' (this may be the same singulation blade SB or a different blade) that partially cuts the common leadframe strip material at pad (lead) level between adjacent devices, that is, at the device flanks. Pads (leads) remain connected together via connecting bars provide to facilitate electrical connection for plating.

Plating as indicated by <NUM> is applied to the whole leadframe <NUM> so that all exposed pad areas (die pads 12A and leads 12B) will be plated.

Final singulation as illustrated in <FIG> is then applied, cutting (e.g., via a saw SB) all the materials starting from the bottom or back surface of the leadframe <NUM> till the top of the package. As a result, each packaged device <NUM> is separated from manufacturing chain.

As illustrated in <FIG>, a device <NUM> thus produced will have an external flank surface plated area extending only on that part of the lead shape that does not derive from cutting during singulation: that portion of the wettable flanks produced from cutting (that is, that portion exposed as result of cutting during singulation, as indicated by <NUM> in <FIG>) will have exposed - and unplated - leadframe material (copper) on the external package side.

As noted, solder SM is applied in order to connect the device package to a board S. Meniscus will appear after soldering.

Electrical connection between the solder SM (the board S) and wire/ribbon <NUM> takes place primarily through the metal (copper) lead 12B. In case of detachment between wire <NUM> and lead 12B, the device will likely fail with no possibility of recovery.

For simplicity a wire/lead is illustrated throughout <FIG>, but the same discussion also applies to a ribbon.

At least in principle, the risk of weld off/lift can be reduced keeping the assembly process as stable as possible, e.g., performing "in process control": input and output key parameters are checked according to a process control plan, and output parameters are monitored and checked (e.g., wire/ribbon pull and shear test). Electrical testing can also be envisaged.

In process control is performed with a specific sample size and frequency: <NUM>% checks are hardly feasible. Visual inspection of the connection between a wire and a lead can be performed before molding step only. Electrical testing can evaluate the interconnection during the test phase only: possible evolution and consequent failure of the interconnection cannot be predicted.

Solutions as exemplified herein contemplate providing a recessed region <NUM> in the front or top surface of the leads 12B and forming the wedge bond to the wire/ribbon <NUM> in such dedicated recessed region of the lead.

Such a recessed area <NUM> can be provided, e.g., by chemical etching, coining, laser machining, or other conventional technologies known to those of skill in the art. For instance, such recessed areas or regions <NUM> can be formed at the leads 12B during leadframe manufacturing.

Such recessed areas or regions <NUM> can have different shapes.

As visible in <FIG> (and <FIG> - that refer to a condition after removal of the sacrificial tie bars 120B) the recessed portions <NUM> can be provided in individual leads 12B as a (e.g., rectangular) recess etched (in a manner known per se) in the metal (e.g., copper) of the sculptured structure of the leadframe <NUM>.

It is noted that <FIG> (and <FIG> as well) refer for clarity of representation:.

It is further noted that the leads 12B visible in the schematic representations of <FIG> (and <FIG>) have shapes that are different from the leads 12B of <FIG> or <FIG> and <FIG>. This is intended to highlight the possibility for solutions as discussed herein to be applied to leads 12B having a wide variety of shapes.

As noted, solutions as presented herein rely on the fact that in current manufacturing processes of semiconductor devices, plural devices <NUM> are manufactured concurrently to be separated into single individual device in a final singulation (sawing) step. That is, after a chain of devices <NUM> formed on a common leadframe strip is assembled (dice, wires and molding: see <FIG> onwards), individual devices <NUM> are separated via singulation as represented in <FIG> using a sawing blade SB.

Advantageously, the bond of the wire/ribbon <NUM> to the recessed portion <NUM> at the proximal section <NUM> (this may be formed, e.g., as a "second bond" in a wire bonding process where a "first bond" is formed at the chip or die <NUM>: see, e.g., <FIG>) extends into a "long" distal, tail formation <NUM> long enough to cause the tail <NUM> to lie at (above) the wettable flank sawing area (see, e.g., <FIG>, <FIG>).

The first sawing step (see SB' in dashed lines in <FIG>) will cut through the (copper) lead 12B plus a portion of wire/ribbon thickness only, which is facilitated by a well-controlled sawing cut and relative wear of the blade SB').

Electrical connection between pads (leads) can still be maintained via a sacrificial connecting bar 120B that is still present.

Plating <NUM> is again applied over the whole (back or bottom surface) of leadframe, with exposed pad/lead areas plated.

Final singulation (see the blade SB in <FIG>) will cut through the material thickness separating the individual devices <NUM>, removing the connecting bars 120B as well.

As visible in <FIG>, a device <NUM> thus produced will again have an external flank surface plated area extending on that part of the lead shape that does not derive from cutting during singulation.

That external flank surface will comprise:.

Only the end portion (e.g., the tail portion <NUM>) of the wire/ribbon <NUM> exposed as result of cutting during singulation will have exposed - and unplated - leadframe material (copper) on the external package side.

As visible in <FIG> (which represent the situation after removal of the sacrificial tie bars 120B) the recessed portions <NUM> can be provided in individual leads 12B as a (e.g., rectangular) recess etched (in a manner known per se) in the metal (e.g., copper) of the sculptured structure of the leadframe <NUM>.

It is again noted that <FIG> refer, for clarity of representation, to individual leads 12B illustrated in isolation from any other neighboring part (e.g., the encapsulation <NUM>).

These leads 12B are shown after a final singulation step (see <FIG>) that results in these etched recessed being cut about their half length, thus giving rise (in mutually facing leads 12B of adjacent devices <NUM> manufactured simultaneously and then singulated) to half-recesses in the form of notches cut in the flanks of these devices.

The final device <NUM> will thus have a plated area shape for wettable flanks (except for a small portion of wire/ribbon) on the external package side.

Solder can thus be applied to connect the device <NUM> to the board S. Meniscus will be produced in the solder SM as a result of soldering.

As exemplified in <FIG>, electrical connection between the solder SM (the board S) and the wire/ribbon <NUM> will occur:.

In case of detachment between the wire/ribbon <NUM> and the lead 12B (connection EC1 fail), the device <NUM> can remain operable through the solder to wire/ribbon <NUM> connection (connection EC2).

To summarize, solutions as presented herein involve arranging at least one semiconductor die <NUM> on a die pad 12A at a first surface of a support substrate (leadframe) <NUM>.

The support substrate <NUM> has a first thickness (indicated as T1 in the figures) between the first surface and a second surface opposite the first surface.

The support substrate (leadframe) <NUM> comprises an array of electrically conductive leads 12B around the die pad 12A.

Terminal recesses <NUM> (that is, recesses located at or in proximity of the distal end of the lead 12B concerned) are formed in (at least some of) the electrically conductive leads 12B in the array at the first surface of the leadframe <NUM> (that is, the surface onto which the die <NUM> is mounted).

At the terminal recesses <NUM>, the leads 12B have a second thickness (indicated as T2 in the figures) that is less than the first thickness T1.

The electrically conductive, elongated formations such as the wires or ribbons <NUM> have coupling ends <NUM>, <NUM> to the electrically conductive leads 12B arranged at the recesses <NUM>.

Partially cutting (engraving) the leadframe <NUM> starting from the second surface as presented by reference SB' is carried out at the terminal recesses <NUM> with a cutting thickness Tx (depth) that lies between the first thickness T1 and the second thickness T2 (T1 > Tx > T2).

Partial cutting SB' thus produces exposed surfaces of the support substrate (see reference 1200A) and the coupling ends (see reference 1200B) of the wires or ribbons <NUM>.

The insulating encapsulation <NUM> molded onto the semiconductor die <NUM> arranged on the die pad 12A at the first surface of the support substrate <NUM> leaves uncovered (as solder wettable flanks) the exposed surfaces of the support substrate 1200A and the coupling ends 1200B of the electrically conductive elongated formations <NUM> produced by partial cutting.

As illustrated, the coupling ends <NUM>, <NUM> of the wires or ribbons <NUM> to respective leads 12B, comprise, arranged in the terminal recesses <NUM>:.

As illustrated, the distal portions <NUM> are exempt from bonding to the leads 12B.

Advantageously, partial cutting SB' at the terminal recesses <NUM> with a cutting thickness between the first thickness (T1) and the second thickness (T2) takes place at the distal portions <NUM>, thus producing exposed surfaces of the support substrate (leadframe, see reference 1200A) and of the distal portions <NUM> the coupling ends of the wires or ribbons <NUM>.

As exemplified in <FIG> and <FIG>, the device <NUM> with the support substrate <NUM> having the semiconductor die or dice <NUM> arranged on the die pad 12A can be mounted onto a mounting member (such as a printed circuit board S) via solder material SM that wets and forms a solder meniscus at both exposed surfaces of the leadframe <NUM> (see reference 1200A) and of the coupling ends (see reference 1200B) of the wires or ribbons <NUM>.

Embodiments as presented herein can be applied to any package that use a wire or ribbon welded through wedge-wedge bonding technology and take advantage from having a wettable flank.

<FIG> is illustrative of a possible variant of embodiments of the present description where double or plural bonding is applied, e.g., to a wire <NUM>.

It will be appreciated that the (first, partial) sawing line SB' can pass through proximal section <NUM> of the bond, thus also in the absence of a "long" distal, tail formation <NUM>.

Wedge(-wedge) bonding as discussed herein can be performed using wires or ribbons made of aluminum, copper, gold or other materials suited to be bonded, e.g., with an ultrasonic process.

Solutions as discussed herein provide improved package reliability, e.g., as a result of solder meniscus SM directly connected to the wire/ribbon <NUM>.

No additional process steps or costs are involved: solutions as discussed herein can be implemented using existing wire bonding machines.

Wire/ribbon tails can be made accessible from the outside of the device package. Wettable flanks may have a direct connection between wire and solder, when mounted on board.

Wettable flanks are a desirable specification of certain automotive products.

Wettable flanks facilitate automatic inspection in order to check the correct welding of the package to the board.

Using wedge bonding technology together with a special lead design (recessed area), facilitates exposing a wire or ribbon portion (e.g., a "tail") outside the package sides. Those portions can be incorporated together with the leads by the solder, when a device is assembled on a board. In that way, the wires/ribbons are connected with respective leads and with the solder as well. In case a first connection (e.g., EC1) fails for any reason (wedge lift), a second connection (e.g., EC2) can compensate the first, thus maintaining device operation. Device reliability is improved.

Visual inspection on the package side (and/or 3D x-ray analysis), possibly facilitated by cross-section and/or decapsulation can reveal the lead recessed areas <NUM>.

Claim 1:
A method, comprising:
arranging at least one semiconductor die (<NUM>) on a die pad (12A) at a first surface of a support substrate (<NUM>), the support substrate (<NUM>) having a first thickness (T1) between the first surface and a second surface opposite the first surface and comprising an array of electrically conductive leads (12B) around the die pad (12A), wherein electrically conductive leads (12B) in the array of electrically conductive leads (12B) have terminal recesses (<NUM>) at the first surface of the support substrate (<NUM>), wherein at said terminal recesses (<NUM>) said electrically conductive leads (12B) in the array of electrically conductive leads (12B) have a second thickness (T2) less than said first thickness (T1),
electrically coupling (<NUM>) the at least one semiconductor die (<NUM>) with said electrically conductive leads (12B) in the array of electrically conductive leads (12B) via electrically conductive elongated formations (<NUM>) having coupling ends (<NUM>, <NUM>) to the electrically conductive leads (12B) in the array of electrically conductive leads (12B) arranged in said terminal recesses (<NUM>), and
partially cutting (SB') the support substrate (<NUM>) starting from the second surface, partially cutting (SB') being at said terminal recesses (<NUM>) and with a cutting thickness between the first thickness (T1) and the second thickness (T2), wherein the partial cutting (SB') produces exposed surfaces of the support substrate (1200A) and the coupling ends (1200B) of the electrically conductive elongated formations (<NUM>).