Design and Technological Peculiarities of Flip-chip Mounting in the Production of Highly Integrated 2.5D and 3D Microassemblies
Highly integrated 2.5D and 3D microassemblies are characterized by such peculiarities of design as high density of features placement, multilayer interconnections, large number of leads, as well as high reliability of connections. These design peculiarities impose rigid requirements onto technological processes of assembly and installation with the aim of obtaining devices with high and stable percent of yield. Modern technologies used for obtaining 2.5D and 3D microassemblies make it possible to efficiently reduce the area (volume, mass) occupied by electronics assembly within hardware [2, 3].
Modern assembly equipment (FC150 and FC300 High Precision) used in the research makes it possible to perform operations of chips registration and assembly with high accuracy in the area of a heated table with the dimensions 200 × 200mm and an upper clamping tool with the dimensions 50 × 50mm. The tool and the table may be heated up to 450 °С providing clamping force up to 4000N with 1 gram discreteness, and optical mechanical registration system makes it possible to register objects with accuracy up to 0.5 micron. These parameters of equipment enabled us to solve such task as registration of 20 × 20mm chips with significant design and technological margin, containing array of bumps (Fig. 1а) 10 microns in diameter, their number being 30 thousand pc. per 1cm2. Using contactless optical profilometer ContourGT-K produced by Bruker, an estimation of chip surface structure profile was performed with multiple-height indium microbumps (Fig. 1b). Using roentgenoscopic digital system of microcircuits monitoring with tomography function XD7600NT (DAGE Precision Industries Ltd, Great Britain) we estimated microbumps registration accuracy (Fig. 1c), as well as excess pressure degree of the tool at chips mounting. More accurate confirmation of pressure degree at installation, as well as chips flatness after their installation were determined by scanning-electron microscope FEI Quanta 3D FEG (Fig. 1d) by measuring gap between dies at the four corners of microassembly. Meanwhile, the scatter of sizes constitutes 1.5 micron with a gap equalling 10 microns.
We have conducted research on formation and estimation of bump strength profile (solder balls) 100 and 60 microns in diameter at contact pads surface of test dies plated by Au and at contact pads of switching pads plated by ImmSn. In most cases unpackaged integrated circuits produced domestically with contact pads made of Al were used, which is caused by technology of semiconductor microcircuits manufacturing and their following installation into conventional packages using the method of wire soldering. Bumps installation using solder at aluminium contact pads requires Ni/Au plating. In fact, bumps are solder balls made of SAC305 alloy (Sn = 96.5 %; Ag = 3 %; Cu = 0.5 % — Тliquidus = 220 °С, Тsolidus = 217 °С) 100 microns in diameter (Figs. 2, 3). To determine optimal physical and mechanical parameters, technological modes of Ni/Au chemical deposition at aluminium 100 × 100 microns contact pads were developed and tested. Optimal Ni thickness ranged in value from 3 to 8 microns, Au thickness — from 0.1 to 0.8 microns. As a rule, the technological process of coating by Ni/Au goes on as follows: cleaning, activation of the surface (removal of natural oxide layer), zincate treatment, selective nickel deposition and immersion gold deposition. In the process of ImmSn plating the optimal thickness ranged in value from 0.5 to 1 micron.
In the process of bumps formation adjustment at SB2-Jet system developed by Packaging Technologies, an optimal range of solder balls geometrical dimensions was determined after their installation at dies contact pads and at silicon substrates (Fig. 4). In case of initial bump diameter equal to 100 ± 3 microns its height after installation attains 60–85 microns, and its diameter ranges in value from 110 to 120 microns. These values were obtained in different modes of operation. The equipment makes it possible to install bumps ranging in size from 40 to 760 microns at the rate up to 10 pc. per second, with size repetition being ±5 %.
The research included tests of shear strength of bumps made of SAC305 alloy from contact pads at chips with Au plating and from contact pads on silicon substrates with ImmSn plating. Investigations were conducted at an installation for testing microcircuits DAGE 4000Plus. As a result of performed tests we have concluded that:
plating by Ni/Au coating does not cause stripping from contact pads in the course of bumps shearing; it provides good adhesion, making it possible to perform further dies installation with bumps within highly integrated microassemblies. Mean value of single bump shearing force constitutes 0.7N.
in case of plating by ImmSn mean value of single bump shearing force constitutes 0.5N (Figs. 5a and 5b).
After tests on shearing strength performed for single bumps, shearing force of chips (5 × 5mm in size, thickness 460 microns for Ni/Au plating, mounted using flip-chip technology) from silicon switching substrates (thickness 300 microns) was determined (contact pads with different plating):
for plated by Ni/Au, mean value of chip shearing strength with bumps made of SAC305 alloy (with initial diameter 100 microns) from silicon substrate constitutes 7.5N (Figs. 6а and 6b);
for plated by ImmSn, mean value of chip shearing strength with bumps made of SAC305 alloy (with initial diameter 100 microns) from silicon substrate constitutes 5N.
Using the results of developed technological processes and conducted tests, a method of highly integrated devices assembly has been achieved. Basing on the developed technology experimental samples of 2.5D microassemblies were manufactured. In essence, microassembly is a switching substrate 250 microns thick, made of silicon (interposer) and with planar dimensions 17 × 17mm with dies mounted using flip-chip technology. Microassembly includes two dies 2 × 2mm, one die 3 × 5mm and one die 10 × 10mm. As four dies with different thickness are to be placed in one plane of the microassembly, a tool (technological tooling) for die gripping with a footstall, manufactured separately for specified range of packageless microcircuits sizes and their relative placement on the substrate, was used. Precise assembly with auto-registration of dies with the substrate was performed. In the course of thermocompression profile design, physical characteristics of microbump material, SAC305 (LF45), were used, as well as its correction upon measuring the compression degree by roentgen microscope. In case of initial bump diameter equal to 100±3 microns, its height after compression attained 50±5 microns.
Fig. 7 presents a sample of 2.5D microassembly placed into ceramic-metal package of CPGA type with chip containing over 500 bumps of SAC305 solder and 100 microns in diameter.
Upon assembling packageless microcircuits, we have worked out processes of filling interchips space by superfluid compound (also called underfill), possessing capillarity effect of flowing into narrow gaps and providing resistance to the environment impact. The process was conducted in several runs using AeroJet MUSASHI Engineering (Japan) system for filling gaps between substrate and die. The compound in the course of filling initially accumulates along one or two sides of the component, where subsequently capillary forces displace it to other sides of the component, resulting in complete encapsulation of assembly joints underneath. Relevant parameters were chosen empirically. The control of filling defectiveness was conducted by means of Sonoscan digital acoustic microscope (Fig. 8). The compound hardening was conducted in a drying chamber with specified temperature profile during 3 hours.
The research has made it possible to work out technological solutions for chemical deposition for metals structure of Ni/Au and ImmSn layers on contact pads; for formation of bumps made of SAC305 at contact pads plated by Ni/Au and ImmSn; for precise registration and assembly of dies at silicon substrate using flip-chip method; for filling the space under dies with compound.
We have obtained results of tests aimed at determining design and technological peculiarities and restrictions related to flip-chip mounting in the course of highly integrated 2.5D and 3D microassemblies production, in particular:
optimal Ni layer thickness ranges in value from 3 to 8 microns, Au thickness — from 0.1 to 0.8 micron, ImmSn thickness — from 0.5 to 1 micron;
optimal dimensions of SAC305 bumps with initial diameter 100 microns upon die mounting on contact pads or on silicon substrate were obtained, namely: height 60–85 microns, diameter 110–120 microns; upon die mounting on substrate bump the height attains approximately 50 microns;
average value of shearing force was determined, making 0.7N for bumps made of SAC305 with initial diameter 100 microns from contact pad plated by Ni/Au, and 0.5N for contact pad plated by ImmSn;
average value of shearing force for dies with bumps made of SAC305 with initial diameter 100 microns from silicon substrate with contact pads plated by Ni/Au made 7.5N, for those plated by ImmSn — 5N.
It has been found that plating contact pads by ImmSn for installation of bumps made of SAC305 can be an alternative variant to plating by Ni/Au.
Basing on the developed technology for assembly and installation of packageless microcircuits using flip-chip method, experimental samples of 2.5D microassemblies were manufactured, which are in essence silicon substrates (interposer) 250 microns thick and with planar dimensions 17 × 17mm with four types of dies mounted by flip-chip method.
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