Electronics packaging and mounting reliability issues are almost always associated with thermal-mechanical stresses. These issues are compounded by the industry's ongoing revolution in which the market demands smaller packages and higher density devices delivering more power and greater functionality. One of the weakest links in the hostile environment of a sensor package, for example, is the soldered joint utilized to attach the sensor components to a printed circuit board (PCB).
Several types of solder-based techniques have been used for attaching integrated circuit modules and/or discrete components to a suitable interconnecting forum, typically a PCB. One such traditional mounting technique involves positioning a component encased in a plastic material and having metal leads extending therefrom onto a PCB such that the leads extend into holes in the PCB. A solder joint is utilized to rigidly couple the leads to the PCB holes. Another solder-based mounting method, commonly known as surface mount technology (SMT), involves mounting the electronic components such that the leads are soldered onto the surface of a PCB rather than inserted into holes in the board.
Solder molding compounds have several significant performance characteristics that must be considered by a manufacturer for a given application. These items include temperature of glass transition, coefficient of thermal expansion (CTE), moisture absorption characteristics, flexural modulus and strength, and thermal conductivity.
These and other characteristics may contribute to problems associated with soldered mounting of electronic components including moisture sensitivity, cracked packages, open bond wires, intermittent electrical continuity failures, etc. Particularly when deployed in high-temperature environments, such as many automotive sensor applications, CTE mismatch strains can significantly compromise the strength and reliability of the solder joints in an electronics package. Most of these problems are not present in the devices prior to assembly onto PCBs but are the result of thermally induced stressing during assembly soldering or rework de-soldering.
An alternative approach to soldering PCB components on a common circuit substrate to form a functional unit such as an electronic sensor package is generally known as press-fit technology. A press-fit connection is made, as the name implies, through the pressing in of a contact pin into a PCB through hole. The important consideration here is that the cross-section diameter of the pin is greater that the diameter of the PCB hole. This difference in pin cross section and hole diameter results in deformation of either the PCB hole or the cross section of the pin during the insertion process of pin into a PCB through hole.
The two major press-fit techniques include a solid pin that does not deform in the insertion process and a compliant pin that compresses as a result of insertion into the PCB through hole. The compliant pin approach has proven more popular for achieving a reliable press fit contact for several reasons. First, the reduction in the size of press-fit section exerts less potentially damaging forces on the PCB through hole and greater tolerances can be accepted for the plated through hole. Furthermore, compliant pins result in lower required insertion forces, resulting in fewer logistical complications. Finally, compliant pins enable multiple press cycles into the same through holes.
In addition to being used for connecting components to a PCB, press fit technology is also commonly utilized in multilayer PCB applications in which press fit connector adapters are installed as effective plug-in adapters onto a PCB.
In view of the aforementioned problems with soldered PCB connection techniques, such press-fit technologies are rapidly emerging as a more attractive alternative for connecting multiple components to a common platform. Press-fit compliant solutions offer several advantages over soldering including providing simple, clean, robust and highly reliable interconnections, even in high-vibration and thermal cycling environments. Press-fit connectivity further eliminates the need for special features for PCB ground plane layers to minimize their heat sink effects when soldering. Use of press fit PCB connectivity eliminates the need to solder and consequently eliminates package exposure to the high temperatures associated with conventional soldering and wave soldering applications. The elimination of solder joints enhances long-term reliability by providing a more shock resistant connection. Furthermore, the use of press fit connections eliminates the need to wash the PCB as is required for removing flux and other soldering board residue.
While alleviating problems associated with soldering connectivity, conventional press-fit technologies pose several significant problems. Many of the drawbacks to conventional press-fit relate to the fine tolerances involved in positioning and pressing multiple pins into the very small press-fit holes. For example, the connector pins often protrude through the PCB when pressed thereon, requiring clearance holes or slots be provided at all connector sites. Furthermore, high precision press positioning equipment including tooling pins is required to pre-align the PCB holes with the fixture. Furthermore, the aforementioned compliant pin approach results reduced component re-usability due to the deformation of component leads.
It can therefore be appreciated that a need exists for an improved apparatus and method for packaging and mounting electronic components to a substrate forum that alleviates some or all of the foregoing problems with soldering and conventional press-fit technologies. The present invention addresses such a need.