The present invention relates to a method of assembling semiconductor devices including saw singulation and to semiconductor devices assembled using such a method.
Semiconductor devices, such as integrated circuits, include a semiconductor die (or chip) in a package with leads presenting exposed electrical contact surfaces. The devices may be mounted on a support with electrical connections, such as a printed circuit board (‘PCB’), for example. Using surface mount technology the electrical contact surfaces of the leads can be soldered directly to corresponding pads on the support, providing mechanical attachment as well as electrical connection.
A surface mount device typically includes an electrically insulating molding material that encapsulates the semiconductor die so that the device has a top face and a bottom, active face, which are generally rectangular or square, and transversely extending edges. The molding compound may encapsulate the semiconductor device completely or define an air cavity that is then sealed with a ceramic or plastic lid. Typically, the package has a pair of sets of leads on opposite sides of the package (‘dual in-line’) or two orthogonal pairs of sets of leads on respective sides of the package (‘quad’). Each set of leads is formed of discrete elements arranged side by side at intervals along the corresponding side of the active face of the package, the electrical contact surfaces extending perpendicularly to the side of the active face for soldering to the electrical connections of the support. In a kind of package referred to as ‘no-lead’ or ‘leadless’, the ends of the leads terminate at and are flush with the side edges of the packages. Such a no-lead device may have a smaller size than a device with leads projecting beyond the sides of the encapsulation or molding material. The solder process during mounting the no-lead device on the support may form a fillet of solder rising up the ends of the leads, facilitating a visual check of the quality of the solder joint between the contact surfaces at the active face of the device and the electrical connections of the support.
When assembling the device, the semiconductor die may be mounted on a pad or flag formed from the same material as the leads, which is usually a metal, such as copper, which may be plated. The die pad may be exposed at the bottom face of the package to assist cooling the die. Alternatively, the die may be mounted on the discrete leads, the die and the leads being supported mechanically by the encapsulation material. The leads may be connected electrically to bond pads on the die with bond wires, of gold, copper or aluminum for example, accommodating differential thermal expansion of the die and the package materials.
A prevalent technique used in manufacturing such a surface mount device includes forming an array of lead frames in a strip or sheet of electrically conductive material, usually metal, by etching and/or stamping for example. Each lead frame in the array includes the sets of leads, respective supporting frame structures, and any die pad for supporting the die. The discrete elements of each set, which will form the leads of the device after singulation, are disposed side by side at intervals and include respective contact portions presenting respective electrical contact surfaces. The array of lead frames may be a single strip but typically is a two-dimensional array, with the supporting frame structure of the array comprising surrounding bars on the outer edges of the array and intersecting intermediate bars common to adjacent lead frames.
In a typical surface mount semiconductor device assembly or packaging process using lead frames, the semiconductor dies are mounted on and connected electrically to respective ones of the lead frames, with the encapsulation material then being molded over and around the lead frame strip or sheet so as to encapsulate the integrated circuit dies, the leads of each of the lead frames, and the bond wires. The individual devices are then separated by a singulation process, in which the lead frame strip or sheet is cut apart. The singulation may be a punch operation. However, punch singulation normally requires the lead frames to be individually molded, leaving a gap in the encapsulation material between adjacent devices for the passage of the punch tool. Saw singulation enables a smaller distance between each individual package and therefore improves lead frame utilization. Saw singulation also enables the molding compound to be applied over the entire array, being cut subsequently during the singulation process. During saw singulation, a saw blade is advanced along ‘saw streets’ that extend between the discrete electrical contact elements of adjacent lead frames, so as to cut off the supporting frame structures of the lead frames and separate the individual devices from each other.
Application Note 1902 Rev. 4.0, 9/2008,“Quad Flat Pack No-Lead (QFN), Micro Dual Flat Pack No-Lead (uDFN)”, published by Freescale Semiconductor, Inc. at http://www.freescale.com/files/analog/doc/app note/AN1902.p df, U.S. Pat. No. 7,183,630 and the Article “Saw Singulation Characterization on High Profile Multi Chip Module Packages with Thick Leadframe” by Nazrul Anuar and Amalina Taib in the Proceedings of the IEEE 2004 Electronics Packaging Technology Conference reference 0-7803-8821-6/04 describe surface mount semiconductor device lead frame saw singulation processes.
Saw singulation can give rise to defects at the sawn edge of the semiconductor devices. Defects noted in the above cited IEEE article include chipping of the molding compound, smearing of the contact material laterally over the sawn surface of the molding compound with consequent risk of short-circuit between adjacent leads, and formation of burrs of the contact material below the sawn ends of the leads, which can degenerate solder joint reliability and surface mounting quality.
U.S. Pat. No. 6,544,817 discloses a singulation method in which the saw streets are positioned at the electrical lead elements instead of along the common bars of the frame structure between adjacent lead frames. However, saw blade life is still a preoccupation and there remains a risk of damage to the saw blade during singulation.
The present invention is illustrated by way of example and is not limited by embodiments thereof shown in the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.
The present invention provides a method of assembling a semiconductor device. The device is assembled by putting together a semiconductor die and a lead frame and encapsulating the die and lead frame to form a packaged device. The lead frames are provided in arrays, with adjacent lead frames being separated from each other with common connection bars. After encapsulating the die, lead frame and any wires electrically connecting the two together, adjacent devices are separated from each other with a singulation process. In one embodiment of the present invention, a saw singulation operation is performed in which a saw cuts along each side of the common connection bars, but does not cut the bar itself, as is done in the prior art. In another embodiment of the present invention, the leads of each lead frame that are connected and orthogonal to the common bars include recesses or cavities. The saw blade cuts the leads at the cavities, which means the saw blades cut even less metal because the leads are thinner at the cavities. Also according to the present invention, the cavities do not extend across the common bars and into a lead of another lead frame. Rather the common bar is not thinned, and thus the common bars provide good support to the lead frames.
The device 40 shown in
Each lead frame 12 further comprises sets of discrete, mostly elongate electrical contact elements or leads 20 disposed side by side at intervals along respective sides of the active face 46 of the device 40 and extending perpendicularly to the side of the corresponding active face. In the completed quad ‘no-lead’ package illustrated in
As shown in
Before encapsulation, semiconductor dies 2 are mounted on and attached to respective lead frames, either on die pads 18 using tape or thermally conductive adhesive 4, as shown in
In accordance with the present invention, the leads are exposed within the active face 46 of the package body 42. In addition, the outer ends of the leads 20 are exposed within the side edges 50 of the package body 42. As shown, the presently preferred embodiment of the invention has recesses 34 formed in the bottoms of the electrical contact surface portions 28 and the exposed outer ends of the leads 20 of the semi-completed conductor package 40. During solder mounting of the device 40 on its support (e.g., PCB), solder can reflow up into the recesses 34. The electrical contact surface portions 28 and outer end, including the recesses 34, of each lead 20 (which are exposed within the package body 42) may have a plating layer applied to facilitate soldering to the support.
As shown in
A saw street S of the lead frame array 10 extends along the common bars 32 common to each pair of adjacent lead frames 12. The passage of a saw blade along each saw street S separates the adjacent lead frames 12 from each other. Orthogonal row and column saw streets S extend within the two-dimensional array 10.
In the manufacturing process disclosed in U.S. Pat. No. 7,183,630, the saw blade is the same width as each of the saw streets S and straddles the intermediate common bar 32 of the outer frame structure 14 while it is sawing. Thus, during the saw singulation process, the saw blade cuts along each saw street S longitudinally, cutting into and along the common bars 32 of each of the outer lead frame structures 14, which reduces all the metal material of the common bars 32 to swarf, which is discarded, and further removes or severs a portion of each of the leads 20 to form their outer ends at the peripheral edge surfaces of the package body 42, in addition to cutting the molding compound 42. Accordingly, as the saw blade cuts along each saw street S, it is always continuously cutting the metal of the common bars 32 longitudinally as well as cutting the metal of the leads 20, even if the amount of metal cut is less when the saw blade passes through the aligned spaces separating the leads 20 from each other and through the cavities 30 (attributable to the reduced thickness of the metal). However, the continuous longitudinal metal cutting of the saw blade prevents the blade from sharpening itself and notably results in excessive smearing and burrs, with attendant risk of the defects leading to short circuits between adjacent leads 20 and insufficient coplanarity of the bottom, active faces 46 the completed devices 40.
With the dies attached and connected to the lead frames 712, the lead frames 612 are then separated from each other with a singulation process. In this embodiment of the invention, singulating includes sawing through the leads 620 on opposing sides of the common bars 631 and 632 without sawing the common bars 631 or 632 longitudinally. As shown in
Since the saw blades do not cut the elongate common bar 631 or 632 longitudinally, but rather saw beside the common bar 631 or 632, much less metal swarf is produced than as compared with the conventional process that cuts right along the common bars. The process allows the blade diamond grit to preserve its self-sharpening characteristics. Also, the width of each saw blade and of each saw street 607 and 608 can be substantially less than the common bar 631 or 632 and of the saw street S of
U.S. Pat. No. 6,544,817 discloses a singulation method in which the saw streets are positioned at the electrical contact elements instead of along the common bars of the frame structure between adjacent lead frames. However, sawing off the intermediate row and column common bars by sawing in two orthogonal directions can reduce saw blade life, with a remaining risk of damage to the saw blade during singulation.
In one embodiment of the present invention, the saw singulation process includes sawing in a first direction through the leads 620 in saw streets 607 or 608 on each side of each side of each row or column common bar 631 or 632 without sawing the common bar 631 or 632 itself longitudinally so as to saw off material from the corresponding common bars. The saw blades saw transversely through the orthogonal common bars 632 or 631, as well as the leads 720, but it will be appreciated that the amount of swarf generated by sawing transversely through the width of the orthogonal common bars 632 is much less than sawing continuously longitudinally of the common bars 632, as is the case in
In one embodiment of the present invention, the material sawn from the intermediate common bars 631 or 632 in the first direction 707 or 708 is removed before sawing off material from the intermediate common bars 632 or 631 in the orthogonal direction 708 or 707.
In one embodiment of the present invention, the lead frame array 610 is singulated with a jig singulation saw, in which the lead frames 612 are individually supported in a jig, by vacuum suction for example. The sawn-off material from each common bar 631 or 632 is washed away as it is sawn off, by a stream of cooling water directed at the contact between the rotating saw blade and the lead frame array 610. Most of the sawn-off material of each common bar 631 or 632 is washed away completely during the passes of the saw in the first direction. Accordingly, during the passes of the saw in the orthogonal direction, no material is left at the intersections of the saw streets 607 and 608 to form small chips.
The frame structures of the lead frame array 614 also include surrounding bars 635 around edges of the array of lead frames 610. Although
In this embodiment of the present invention, the lead frames 612 are supported during the singulation operation. Singulation includes sawing beside each of the surrounding bars 635 without sawing the surrounding bars 635 longitudinally so as to saw off material from each surrounding bar 635, and removing the sawn-off material from the surrounding bars 635 before sawing through the leads on each side of the intermediate common bars 631 or 632. In this way, the saw blades do not have to pass through the surrounding bars 635 when sawing off the intermediate common bars 631 and 632 and the number of passes of the saw blade through the surrounding bars 635 is reduced.
Cavities may be formed in the ends of the leads 620 to form recesses 34, extending partly into the thickness of the lead frame 612, for example by semi-etching. The cavities could be elongate cavities extending across the common bars 632, like the cavities 30 shown in
The singulation starts at step 810 by supporting the lead frames such as 612 in a jig. Singulation includes at step 812 sawing beside each of the support bars such as 635 of the frame structures 614 surrounding the array(s) 610 without sawing the surrounding bars 635 longitudinally so as to saw off material from each surrounding bar 635, and removing the sawn-off material from the surrounding bars 635 before sawing through the leads 620 on each side of the intermediate common bars 631 or 632. It should be noted that sawing beside the support bars instead of down the middle of the support bars extends the life of the saw blade and thus saves manufacturing time and cost.
At step 814, intermediate frame structure (row or column) common bars such as 631 (or 632) are removed by sawing through the leads 620 in saw streets 607 (or 608) on each side of each row (or column) common bar 631 (or 632) without sawing the common bar 631 or 632 longitudinally before proceeding at step 916 to sawing off the orthogonal intermediate common bars 632 (or 631) in the orthogonal direction saw streets 608 (or 607).
The example of a method of assembling semiconductor devices described above with reference to
In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of the invention as set forth in the appended claims. For example, the connections may be any type of connection suitable to transfer signals from or to the respective nodes, units or devices, for example via intermediate devices. Accordingly, unless implied or stated otherwise the connections may for example be direct connections or indirect connections.
Where the context admits, it will be understood that the semiconductor material described herein can be any semiconductor material or combinations of materials, such as gallium arsenide, silicon germanium, silicon-on-insulator (SOI), silicon, mono-crystalline silicon, the like, and combinations of the above.
Where the context admits, the terms “front,” “back,” “top,” “bottom,” “over,” “under” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.
In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Where the context admits, terms such as “first” and “second” are used to distinguish arbitrarily between the elements such terms describe and these terms are not necessarily intended to indicate temporal or other prioritization of such elements.
Number | Date | Country | Kind |
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201010166155.4 | Apr 2010 | CN | national |