Integrated circuitry chips are typically formed into packages, with the packages then being mounted or otherwise connected to other substrates and devices. Many different packaging methods and devices exist for integrated circuitry in the form of a semiconductor chip. One exemplary package mounts a semiconductor chip to another circuit substrate, for example a printed circuit board. The printed circuit board is typically fabricated to have a plurality of conductive traces formed thereon in desired patterns. An insulative layer referred to as a soldermask is then typically formed on the circuit substrate. Such layers are typically patterned to provide openings to locations on the circuit traces therebeneath. The soldermask typically prevents solder bridging on the circuit side of the assembly. The semiconductor chip is typically mounted to the circuit substrate by being adhered to the soldermask with a die attach adhesive. Conductive wire or other bonding is then conducted to connect the circuitry of the chip with the circuitry of the substrate.
Thereafter, in one exemplary packaging process, an insulative encapsulant material is provided to one side of the substrate over the semiconductor chip and soldermask. Such can be formed by a transfer molding process whereby a mold having a void is placed against the circuit substrate and an encapsulant caused to flow therein. The mold is ultimately removed and the encapsulant is allowed to cure.
One type of semiconductor packaging finding increasing use are ball grid arrays. Such can be fabricated as described above and additionally include conductive traces and a soldermask received on the opposing side of the circuit substrate from which the semiconductor die or chip is mounted. Openings are provided in the soldermask on the opposing side to desired locations of the opposing side circuit traces. An array of solder balls are mounted through the openings to surfaces of the conductive traces. The solder ball array serves to provide an electrical connection for the package with another substrate or device.
The current trend towards ball grid array and other semiconductor packaging has created a number of challenges. Among these are cracking of the soldermask on the circuit side of the substrate during encapsulation, and less than desirable adhesion of the encapsulant material to the underlying soldermask. Such can create defects in the package that can cause production yield losses and long-term reliability failures. Typical soldermasks used today comprise a polymeric material that is applied to the outer surfaces of the substrate to, among other things, protect the circuitry, define particular features (for example, solder ball pads), define plated areas and control solder wicking during the reflow of solders. Typical soldermask materials used today are relatively soft with low mechanical strength.
Traditionally, soldermask materials are used to cover all areas of a ball grid array substrate that are not specifically open to reveal some part of the underlying circuit. In the area of the perimeter of the mold body, the soldermask is typically used to protect the circuit traces from the clamping forces applied by the mold body and to form a level surface of the ball grid array substrate so that the mold body can form a good seal during encapsulation. Yet, clamping forces applied by the mold body to the ball grid array substrate can be quite high. In some cases, these forces can be in excess of four tons on a single ball grid array substrate strip. Because of these high clamping forces on the relatively soft character of the soldermask, high shear forces are induced in the soldermask. These shear forces can cause severe cracking of the soldermask. Cracks in the soldermask can cause a functional failure by severing the circuit traces below and, even if not, are a cosmetic defect that may cause such part to be rejected by the consumer.
It would be desirable to overcome these and other drawbacks associated with semiconductor packaging and packaging processes. Yet, the invention is limited only by the accompanying claims as literally worded and as appropriately interpreted in accordance with the doctrine of equivalents without any limitation being read therein with respect to objective or result.