Package Substrates/Interposers


The rapidly growing demand for semiconductor packages like BGA's, CSP's, and SIP's has led to an equally strong demand for substrates and interposers that these packages employ. Substrates are parts that provide the package with mechanical base support and a form of electrical interface that would allow the external world to access the device housed within the package.  An interposer is an intermediate layer often used for interconnection routing or as a ground/power plane. Sometimes the terms 'substrate' and 'interposer' are used to refer to the same thing.


There are several types of package substrates, but the two major categories are rigid and tape substrates.  As their names imply, these two categories differ with respect to their mechanical properties.  Rigid substrates have a fixed shape and form, while tape substrates are thin and flexible. Early rigid substrates were mostly made of ceramic, but today organic substrates have become more widely deployed among various packages. 


Rigid substrates that are composed of a stack of thin layers or laminates are called 'laminate' substrates. There are several different materials used for manufacturing laminate substrates. Two widely used materials for laminate substrates are FR4, a traditional epoxy-based laminate,  and the more advanced and higher-performing resin-based Bismaleimide-Triazine (BT) from Mitsubishi Gas and Chemical. 


BT resin has become the preferred laminate material for many manufacturers because of its high Tg, low dielectric constant and good insulation properties.  In fact, BT is now apparently the standard substrate material for BGA's, and is also being used in CSP laminates. New laminate substrates emerging in the market, such as those introduced by Hitachi Chemical, Nelco International, and Sumitomo Bakelite are composed of advanced epoxies or epoxy blends.  These are expected to give BT laminates some good competition that may lead to more reductions in substrate prices.


Tape substrates are composed of high-strength and high-temperature polymer material such as polyimide. One major advantage of tape substrates is quite obvious: it is compliant enough to be subjected to motion while carrying the circuits built onto them, which is useful in 'moving' applications such as disk drives and printers.  Tape substrates are also light-weight, less costly, and better than laminate substrates in achieving fine-line and microvia features, a fact that CSP's took advantage of in pursuit of fine-pitched wiring. The disadvantages of tape substrates include: 1) more difficult handling during processing; 2) warpage issues; and 3) large differences in CTE with other materials such solder masks. 


Aside from serving as a base for IC packages, substrates are also used to route the chip's I/O system to the application board's interconnection features.  Thus, substrates must somehow have within themselves metal conductors that can accomplish this routing function. These are usually in the form of traces etched from copper foil that's bonded to one or more laminates of the substrate. The copper layers of the substrate are commonly finished with a layer of immersion gold over a layer of electroless nickel. The nickel prevents copper-solder interdiffusion while the gold inhibits oxidation and enhances solderability.


A laminate substrate may have several layers with metal planes or traces that are interconnected to each other by through-hole plated vias, in much the same way as conventional PCB's. BT substrates often have an even number of routing layers. In a 4-layer substrate, for instance, the I/O routing planes are the ones at the top and bottom of the substrate, while the inner layers are used as a ground and power plane. 


Major manufacturers of rigid substrates include:   ASE Material Inc.; CMK Corporation (Nippon CMK); Compeq Manufacturing Co., Ltd.; D. T. Circuit Technology Co., Ltd.; Eastern Co., Ltd.; Hitachi Chemical Co., Ltd.; Japan Circuit Industrial Co., Ltd.; Kyocera Corporation; Matsushita Electronic Components Co., Ltd.; Mitsubishi Electric Corporation; NEC Toppan Circuit Solutions Inc.; NGK Spark Plug Co., Ltd. (NTK); Phoenix Precision Technology Corporation; Shinko Electric Industries Co., Ltd.; and WUS Printed Circuit Co., Ltd.


Major manufacturers of tape substrates include: Compass Technology Co. Ltd.; Hitachi Cable, Ltd.; Mitsui Mining and Smelting Co., Ltd.; NEC Toppan Circuit Solutions Inc.; Shindo Company, Ltd.; Shinko Electric Industries Co., Ltd.; Sumitomo Metal Mining Co., Ltd; and WUS Printed Circuit Co., Ltd.


See Also:  Ball Grid ArrayChip-Scale Package System-in-a-Package  




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