Single sided substrates and packages based on laminate materials

Laminate based substrates have traditionally been of two or more layers of metal because thin Cu clad laminate cores (CCL) have not been available, nor was the equipment/process technology available to circuitize these CCLs. Concomitantly, the designs on both die and substrate level have always assumed the multi-layer nature of the substrate which allowed for wire crossings by using plated through holes (PTH). Thin CCLs, 60 μ or less, are now routinely available and equipment/process technology have evolved to where these thin CCLs can be processed with good yields. Several process flows have been devised to manufacture single sided substrates, termed aS
^3™
substrates as well as corresponding packages. The common structure for these substrates is a single layer of circuit pattern that is supported by a layer of dielectric core material. Blind vias (BV) in the dielectric are co-located over lands of the circuit pattern. These BVs are equivalent to ball pad openings (BPO) in solder mask of standard BGA substrates and serve as solder mask defined ball pads. The circuit pattern on the opposite site has wire bond fingers connecting via traces to the lands which are larger than BPOs. If necessary, the traces and lands can be protected with solder mask. Surface finishes of bond finger and ball pad can follow any option available for standard BGA substrates. Design examples will be given which demonstrate that many two-layer designed substrates can be converted to single layer substrates. To take full advantage of a larger range of packages, the die pin out should be optimized to facilitate single layer routing. The benefit of this new type of substrate is the cost savings which is derived from a simplified process flow and bill of materials. Further, in many cases, a shrink in substrate size has been demonstrated which translates into additional cost savings. Assembly and reliability of aS
³
packages have been demonstrated. The assembly process can follow the usual rules of thin substrate assembly. Reliability for small packages is excellent. Drop test performance shows very high survival rates and moisture sensitivity levels of one have been demonstrated. A significant design feature for wire bond packages is that both fan-in and fan-out are allowed.