News Column

Patent Issued for Device with Semiconductor Die Attached to a Leadframe

July 16, 2014



By a News Reporter-Staff News Editor at Biotech Week -- A patent by the inventor Griffin, Garrett (Limerick, IE), filed on January 5, 2012, was published online on July 1, 2014, according to news reporting originating from Alexandria, Virginia, by NewsRx correspondents (see also Medical Device Companies).

Patent number 8766432 is assigned to Analog Devices, Inc. (Norwood, MA).

The following quote was obtained by the news editors from the background information supplied by the inventors: "Such semiconductor devices comprise a semiconductor die and a leadframe comprising a plurality of leads which are wire bonded to corresponding electrically conductive contact pads of the die. The die and portions of the leads of the leadframe are encapsulated in an encapsulating housing formed from a mouldable compound, typically, a thermosetting material, such as epoxy resins, with distal portions of the leads of the leadframe extending through the encapsulating housing for facilitating electrical coupling of the semiconductor device to other components, for example, for facilitating electrical coupling of the semiconductor device to a printed circuit board using various types of solder coupling technologies including surface-mount technology and other more conventional technologies. In the manufacture of such semiconductor devices, it is important that the die and the leadframe are secured together prior to wire bonding of the contact pads of the die to the leads of the leadframe. Various methods for securing a die to a leadframe prior to wire bonding of the contact pads of the die to the leads of the leadframe are known.

"For example, U.S. Pat. No. 5,541,446 of Kierse granted to the assignee of the present Application discloses a leadframe comprising a plurality of leads, proximal ends of which extend beneath a die, and the die is bonded directly to the leads by an electrically non-conductive adhesive. Subsequent to bonding of the die to the leadframe, the electrically conductive contact pads of the die are wire bonded to the corresponding leads of the leadframe, and the assembly is then encapsulated in a mouldable compound. The leadframe is then subsequently trimmed in order to electrically isolate the leads from each other. However, a problem of the method disclosed in U.S. Pat. No. 5,541,446 is that the die must first be coated with the adhesive, which typically is an epoxy adhesive paste. The coating and subsequent handling of dies so coated is difficult and requires specialised coating and die handling machines.

"U.S. Pat. No. 4,943,843 of Okinaga, et al discloses a number of embodiments and methods for encapsulated semiconductor devices. In one of the semiconductor devices, the device comprises a die and a leadframe with an electrically non-conductive pad disposed between and bonded to the die and the leadframe by respective adhesives which are disposed between the non-conductive pad and the leadframe, and between the non-conductive pad and the die. The specification of Okinaga is silent on the type of adhesives, however, the adhesives appear to be provided as sheets of thermoplastic adhesives, which are subsequently heated to effect bonding. A problem of this method is that alignment of the die relative to the leadframe for aligning the electrically conductive contact pads of the die with the corresponding leads of the leadframe is difficult. Another problem associated with the method of Okinaga is that due to the fact that the wire bonding to some of the leads of the leadframe has to be effected at the extreme proximal ends of the leads, where the extreme proximal ends of the leads extend just beyond the non-conductive pad and the die. This only allows a relatively small area at the extremities of the proximal ends of such leads available for wire bonding, and adhesive may seep from beneath the carrier pad along such leads to their respective proximal extremities, thus preventing adequate electrical contact between the bond wires and the leads. In another embodiment of the method disclosed in the specification of Okinaga, a die of an encapsulated semiconductor device is directly bonded to the leadframe by an electrically non-conductive adhesive which appears to be a thermoplastic adhesive, examples of which are given as polyimide resins, silicon rubbers and ceramics.

"U.S. Pat. No. 4,987,474 of Yasuhara discloses an encapsulated semiconductor device comprising a die and a leadframe with an electrically non-conductive pad disposed between and bonded to the die and the leadframe. The non-conductive pad is a polyimide type resin film. The leadframe is bonded to the non-conductive pad by a layer of thermoplastic adhesive such as a polyester amidoimide, and the die is bonded to the non-conductive pad by a bonding layer which is formed by a multi-layer non-conductive adhesive such as silicon rubber, epoxy rubber, epoxy type resin or polyimide type resin stacked on a thermosetting polyimide type resin adhesive. A problem with this method is that alignment of the die with the leadframe for aligning the electrically conductive contact pads of the dies with the corresponding leads of the leadframe is difficult.

"U.S. Pat. No. 5,140,404 of Fogal discloses two methods for securing a die to a leadframe prior to wire bonding electrically conductive contact pads of the die to corresponding leads of the leadframe. In one of the methods the die is attached to a die paddle of the leadframe with an adhesive such as an epoxy paste, an epoxy film, metal or glue. In the other of the methods the die paddle is replaced with a tape which appears to be electrically non-conductive, and the tape is attached to proximal ends of leads of the leadframe, and the die is attached to the tape. The tape is layered on one or both sides thereof with a thermoplastic material, depending on whether the die and the leads of the leadframe are to be attached to the same side, or to respective opposite sides of the tape. The thermoplastic material is provided instead of adhesive for attaching the die and the leadframe to the tape. A problem with this latter method is that it is difficult to align the die and the leadframe for alignment of the electrical conductive contact pads of the die with the corresponding leads of the leadframe. Additionally, the die and the leadframe must be simultaneously attached to the tape which leads to further problems of aligning the die and the leadframe.

"U.S. Pat. No. 5,304,842 of Farnworth, et al discloses a method for securing a die to a leadframe which requires locating a carrier material such as polyimide between the leadframe and the die, and attaching the die and the leadframe to the carrier. A layer of thermoplastic adhesive is disposed between the carrier and the leadframe, and a layer of thermoplastic adhesive is disposed between the carrier and the die. The layer of adhesive between the carrier and the die is selected so that it softens and flows at a lower temperature than the layer of adhesive disposed between the carrier and the leadframe. In this way the carrier can be secured to the leadframe, and then subsequently to the die. However, this method also suffers from the problem that alignment of the die with the leadframe is difficult.

"U.S. Pat. No. 6,706,559 of Jiang, et al also discloses a method for securing a die to a leadframe prior to wire bonding of electrically conductive contact pads of the die to the leads of the leadframe. In the method of Jiang a wet film adhesive layer is disposed between the die and the leadframe, and the adhesive may be applied to the die or to the leadframe, or to both. Examples of adhesives are liquid or paste mixtures of unreacted monomers, polymers or diluted resins, polyimides or polyimide siloxane. The adhesive is then subsequently cured.

"Accordingly, in general, all such prior art methods either require a film of adhesive to be disposed between the semiconductor die and the leadframe or a die paddle for directly securing the die to the leadframe or paddle, or alternatively, prior art methods require a sheet of material, typically, an electrically non-conductive sheet to be disposed between and attached to the die and the leadframe by respective layers of adhesive. Additionally, in prior art methods the adhesives used to bond the semiconductor die to the leadframe or to the die paddle, or to bond the die to the electrically non-conductive sheet, and to bond the electrically non-conductive sheet to the leadframe are typically thermoplastic adhesives. Thermoplastic adhesives, while they remain stable at room temperature after curing, are not stable at higher temperatures, and after curing become unstable at temperatures in the region of their normal melt, cure and bond temperatures. Thus, in virtually all cases the bonding of the non-conductive sheet to the leadframe and to the die must be carried out simultaneously. This causes considerable problems in the assembly of the leadframe, the non-conductive sheet and the die, since the leadframe, the non-conductive sheet and the die must be accurately and correctly positioned and aligned relative to each other prior to subjecting the assembly to an appropriate temperature for raising the thermoplastic adhesives to a temperature sufficient for effecting bonding of the die and the leadframe to the non-conductive sheet. Maintaining the die, the leadframe and the non-conductive sheet accurately aligned and positioned during this procedure is difficult.

"However, even in the method disclosed in U.S. Pat. No. 5,304,842 of Farnworth, which permits sequential bonding of the leadframe and the die to the carrier pad, the problem of alignment of the die with the leadframe still exists.

"There is therefore a need for a method for attaching a semiconductor die to a leadframe which addresses at least some of the problems of the prior art methods and which facilitates handling of the die and the leadframe and in particular alignment of the die with the leadframe.

"The present invention is directed towards providing such a method, and the invention is also directed towards providing a semiconductor device with an encapsulated bonded assembly of a die and a leadframe."

In addition to the background information obtained for this patent, NewsRx journalists also obtained the inventor's summary information for this patent: "According to the invention there is provided a method for attaching a semiconductor die to a leadframe, the method comprising:

"providing the leadframe with a carrier pad attached thereto, the carrier pad having first and second major surfaces, and a first adhesive bonding the leadframe to the first major surface of the carrier pad,

"applying a flowable temperature curable second adhesive to the second major surface of the carrier pad,

"placing the die on the second adhesive on the second major surface of the carrier pad for facilitating wire bonding of the die to the leadframe, and

"subjecting the second adhesive to a curing temperature for curing thereof to bond the die to the carrier pad.

"In one aspect of the invention the second adhesive is applied to the second major surface of the carrier pad by dispensing the second adhesive through a nozzle, and preferably, the second adhesive is dispensed through the nozzle so that the second adhesive forms a pattern on the second major surface of the carrier pad. Advantageously, the nozzle is moved during dispensing of the second adhesive therefrom in a common plane of movement in at least one of an X-axis direction and a Y-axis direction for applying the second adhesive on the second major surface of the carrier pad in the form of the pattern.

"In one aspect of the invention the second adhesive is continuously dispensed through the nozzle while the nozzle is being moved in the at least one of the X- and Y-axes directions to form the pattern of the second adhesive as a continuous pattern on the second major surface of the carrier pad. Alternatively, dispensing of the second adhesive through the nozzle is periodically disrupted while the nozzle is being moved in the at least one of the X- and Y-axes directions form the pattern of the second adhesive as a discontinuous pattern comprising a plurality of discrete portions of the adhesive.

"In one aspect of the invention the nozzle is adapted for forming the pattern of the second adhesive as a line pattern as the nozzle is being moved in the at least one of the X- and Y-axes directions. Preferably, the nozzle is simultaneously moved in the common plane of movement in both the X-axis direction and the Y-axis direction during dispensing of the second adhesive therefrom.

"Preferably, the common plane of movement of the nozzle in the X- and Y-axes directions lies substantially parallel to a plane defined by the second major surface of the carrier pad.

"In another aspect of the invention the die is manoeuvred on the second adhesive on the second major surface of the carrier pad for aligning electrically conductive contact pads of the die with corresponding leads of the leadframe for facilitating subsequent wire bonding of contact pads of the die and the leads of the leadframe, and preferably, the die is pressed into engagement with the second adhesive on the second major surface of the carrier pad.

"Preferably, the second adhesive is a thermosetting adhesive, and advantageously, is an electrically non-conductive adhesive.

"In another aspect of the invention the second adhesive has a cure temperature in the range of 80.degree. C. to 250.degree. C., and preferably, the second adhesive has a cure temperature in the range of 120.degree. C. to 200.degree. C., and ideally, the second adhesive has a cure temperature of approximately 175.degree. C.

"In a further aspect of the invention the second adhesive is flowable at room temperature, and preferably, is liquid at room temperature. Advantageously, the second adhesive is an adhesive sold under the trade name ABLESTIK 84-3J.

"Preferably, the carrier pad is of an electrically non-conductive material, and advantageously, is of a flexible electrically non-conductive material.

"Advantageously, the carrier pad is of polyimide material, which preferably, is stable at the cure temperature of the second adhesive.

"In another aspect of the invention the area of the carrier pad in plan view is substantially similar to the area of the die in plan view. Preferably, the die and the carrier pad are attached to the leadframe so that the die overlays a proximal portion of at least some of the leads of the leadframe, and preferably, the die overlays proximal ends of at least some of the lead of the leadframe. Advantageously, the die and the carrier pad are attached to the leadframe so that the die overlays the proximal portion of all the leads of the leadframe, and ideally, the die overlays the proximal ends of all of the leads of the leadframe.

"In one aspect of the invention the die comprises a first major surface adjacent which an integrated circuit is formed therein, and a second major surface remote from the integrated circuit and defined by a substrate on which the integrated circuit is formed, and the die is placed on the second adhesive with one of the first and second major surfaces of the die abutting the second adhesive.

"In another aspect of the invention the method further comprises wire bonding the die to the leadframe, and preferably, the method comprises the further step of encapsulating the die, the leadframe and the carrier pad bonded together in an encapsulating housing of a mouldable thermosetting material, with distal ends of the leads of the leadframe extending from the encapsulating housing.

"In one aspect of the invention the first adhesive is a temperature curable adhesive. Preferably, the first adhesive has a cure temperature in the range of 120.degree. C. to 250.degree. C. Advantageously, the first adhesive has a cure temperature in the range of 190.degree. C. to 210.degree. C. Ideally, the first adhesive has a cure temperature of approximately 200.degree. C.

"In one aspect of the invention the first adhesive is a thermosetting adhesive, and preferably, is an electrically non-conducting adhesive.

"Preferably, the first adhesive is pre-coated onto the first major surface of the carrier pad, and advantageously, is partially cured thereon, and in another aspect of the invention the first adhesive is partially cured during coating onto the first major surface of the carrier pad.

"In another aspect of the invention the method further comprises bonding the carrier pad to the leadframe, and preferably, the carrier pad is bonded to the leadframe prior to bonding the die to the carrier pad.

"Advantageously, the carrier pad is placed on the leadframe with the first adhesive on the first major surface thereof abutting the leadframe, and preferably, the leadframe is heated for in turn at least partially curing the first adhesive, and advantageously, the leadframe is heated prior to placing the carrier pad on the leadframe.

"In another aspect of the invention the first adhesive is subjected to a curing temperature for curing thereof to bond the carrier pad to the leadframe, and preferably, the leadframe is heated to a temperature sufficient to raise the first adhesive to its cure temperature.

"The invention also provides a semiconductor integrated circuit device prepared by the method according to the invention.

"Additionally the invention provides a semiconductor device comprising:

"a carrier pad having first and second major surfaces,

"a leadframe bonded to the first major surface of the carrier pad by a first adhesive,

"a semiconductor die bonded to the second major surface of the carrier pad by a second adhesive, the second adhesive being a flowable temperature curable adhesive, and having been applied to the second major surface of the carrier pad in flowable form for facilitating alignment of electrically conductive contact pads of the die with leads of the leadframe during placement of the die on the carrier pad.

"Preferably, the carrier pad comprises a carrier pad pre-coated with the first adhesive on the first major surface thereof

"In one aspect of the invention the die comprises a first major surface adjacent which an integrated circuit is formed therein, and a second major surface remote from the integrated circuit defined by a substrate in which the integrated circuit is formed, the die being attached to the carrier pad with the second major surface of the die adjacent the second major surface of the carrier pad. Preferably, the die, the carrier pad and the leadframe bonded together are encapsulated in an encapsulating housing of a mouldable thermosetting material, and preferably, distal portions of the respective leads of the leadframe extend from the encapsulating housing.

"Advantages of the Invention

"The advantages of the invention are many. By virtue of the fact that the carrier pad is initially bonded and secured to the leadframe prior to bonding and securing the die to the carrier pad, once the carrier pad has been secured to the leadframe, the die can then be accurately aligned and positioned on the carrier pad relative to the leadframe, and then subsequently secured to the carrier pad in accurate alignment with the leadframe. Thus, in the method according to the invention it is no longer necessary to ensure accurate alignment of the die with the leadframe, before any bonding commences. Furthermore, by providing the second adhesive which bonds the die to the carrier pad as a flowable adhesive and in particular as an adhesive which is liquid at room temperature, the die can be easily positioned on the carrier pad and aligned with the leadframe, since the carrier had has already been secured to the leadframe by the first adhesive and therefore there is no danger of further movement of the carrier pad relative to the leadframe. Accordingly, the contact pads of the die can readily easily be accurately aligned with corresponding leads of the leadframe while the second adhesive is in the liquid form.

"By providing the carrier pad pre-coated with the first adhesive on the first major surface thereof, the carrier pad can be readily easily secured initially to the leadframe. By initially securing the carrier pad to the leadframe, the die can be placed on and pressed into the second adhesive on the second major surface of the carrier pad, and then accurately aligned in X- and Y-axis directions with the leadframe so that the contact pads of the die are accurately aligned with the corresponding leads of the leadframe prior to curing of the second adhesive.

"A further advantage is achieved when the second adhesive which is applied to the second major surface of the carrier pad is a thermosetting adhesive. Once a thermosetting adhesive has been cured, it is permanently cured and is unaffected by any further rises in temperature or temperature fluctuations, in other words, a thermosetting adhesive remains temperature stable once it has been cured. Therefore, the bonded assembly of the leadframe, the carrier pad and the die remain unaffected by any further temperature rises or fluctuations to which it may be subsequently subjected during coupling of the bond wires to the contact pads and to the leads of the leadframe, as well as during encapsulation of the bonded assembly in the encapsulating housing, and/or during use by an end user. A similar advantage is achieved when the first adhesive which is applied to the first major surface of the carrier pad is a thermosetting adhesive, and additionally, by providing the first adhesive as a thermosetting adhesive, once the first adhesive is cured during bonding of the carrier pad to the leadframe, the cured bond formed by the first adhesive between the carrier pad and the leadframe is unaffected during subsequent high temperature curing of the second adhesive for bonding the die to the second major surface of the carrier pad.

"A further advantage of the method according to the invention is that it permits the bonding of the carrier pad to the leadframe to be carried out as a sub-assembly entirely independently and separately of securing the die to the carrier pad. Indeed, the sub-assembly of the leadframe and the carrier pad can be produced independently of, and indeed, on a separate production line to that of the bonding of the die to the carrier pad. For example, it is envisaged that a third party manufacturer who manufactures leadframes in elongated strips of leadframes which will be well known to those skilled in the art may carry out the operation of bonding the carrier pads to the leadframes of each strip of leadframes. In which case, the sub-assembly of the carrier pads bonded to the leadframes would be supplied in elongated strips, whereby each strip would include a plurality of leadframes with carrier pads bonded thereto. Such strips of leadframes and carrier pads could then be supplied to a separate manufacturer who would then bond the dies to the carrier pads on the leadframes.

"A further advantage of providing the second adhesive in a flowable or liquid form at room temperature is that the second adhesive can be readily easily dispensed onto the second major surface of the carrier pad, and furthermore, the dispensing of the second adhesive onto the second major surface of the carrier pad can be accurately controlled, and in particular, the quantity and depth of the second adhesive dispensed onto the second major surface of the carrier pad can be accurately controlled. Indeed, by providing the second adhesive in flowable or liquid form, the second adhesive can readily easily be dispensed through a nozzle, and furthermore, by moving the nozzle in a common plane in respective X- and Y-axis directions, the second adhesive may be dispensed onto the second major surface of the carrier pad in any desired pattern, which may be a continuous pattern or an discontinuous pattern. This further enhances accurately determining the depth of the second adhesive located between the die and the second major surface of the carrier pad. The ability to accurately control the depth of the second adhesive between the die and the second major surface of the carrier pad is particularly advantageous where the second adhesive is an electrically non-conductive adhesive, and the non-conductive properties of the adhesive are being relied on, particularly, for example, in cases where the carrier pad may not be of an electrically non-conductive material, or the surface of the die which is bonded to the second major surface of the carrier pad is the surface adjacent the integrated circuit.

"A further advantage of the invention is achieved by virtue of the fact that in the method according to the invention there is no longer a need for applying adhesive to either the leadframe or the die. Thus, both the leadframe and the die can readily easily be handled without the need for special purpose handling machinery, and in particular, special purpose pick-and-place machinery. Furthermore, by virtue of the fact that there is no longer a need to apply adhesives to the leadframe and the die, there is no danger of the leadframe or the die becoming stuck in pick-and-place apparatus of production machinery, which would otherwise result from the presence of adhesive on the leadframe and/or the die.

"The invention and its many advantages will be readily apparent to those skilled in the art from the following description of a preferred embodiment thereof, which is given by way of example only, with reference to the accompanying drawings."

URL and more information on this patent, see: Griffin, Garrett. Device with Semiconductor Die Attached to a Leadframe. U.S. Patent Number 8766432, filed January 5, 2012, and published online on July 1, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8766432.PN.&OS=PN/8766432RS=PN/8766432

Keywords for this news article include: Technology, Electronics, Semiconductor, Analog Devices Inc., Medical Device Companies.

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Source: Biotech Week


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