SUN’s Dynamic Retention printed circuit backplane systems is designed specifically to meet the requirements if MIL-C-28859. SUN’s press fit design completely eliminates the need for contact-to-panel soldering interconnection configurations of up to 400 contacts and more are made possible by the low insertion force performance of the contact. Special ultra low-force tubing fork contacts may be specified that yield insertion forces of 2.25 oz. Maximum (1.8 oz. typical) per contact when mated with SUN’s blade headers. The modular construction of the two-row and three-row 0.100” X 0.100” grid insulators facilitates the design of high-density row arrays that are compatible with LSI circuitry. Sophisticated printed circuit process technology is used to produce a wide range of panel designs, from two-sided to complex multilayer, with heavy-copper inner layers for applications which require high current carrying capability.
PRESS FIT CONTACT SYSTEM
The most critical element in the printed circuit backplane is the contact /plated-through hole interface. The dynamic retention contact is designed to provide this interface with unsurpassed reliability and performance. The integral spring member design of the dynamic retention contact:
Ø insures a high-reliability gas-tight connection.
Ø results in optimum contact retention within the panel.
Ø minimizes panel stress and growth, and distortion of the plated-through hole.
Ø Enables contacts to be easily replaced, with no loss of reliability.
DYNAMIC RETENTION DESIGN
The compliant interface section of the dynamic retention contact consists of two bow-shaped springs, which have a geometry determined in conjunction with the stress characteristics of the beryllium copper contact material. Upon insertion of the contact into the plated-through hole in the glass-epoxy panel these springs interface with the sides of the hole and resiliency flex inward. This resilience creates an inverse vector force which acts on the contact to maintain it in intimate association with four vertical sections of the hole. The flexibility of the spring members also enables the contact to be used effectively in holes with a plated-through tolerance of up to +/-0.003”, insuring optimum performance over a “real world” range of hole sizes. The simplicity of the basic design permits each contact to be fabricated as a virtually exact reproduction of every other contact; further insuring consistent and optimum press-fit performance.
In press-fit systems which do not have the resiliency of the dynamic-retention contact, the printed circuit panel must absorb all of the stress generated by the interference between the contacts and the plated-through holes. The accumulation of the individual stresses results in a total stress which manifests itself in a warped or bowed panel. The panel also exhibits a tendency to grow excessively and unpredictably during contact insertion, often resulting in contact misalignment of more than 0.20 diameter TP at the board surface. Because of normal variations in laminate thickness and composition, and plated-through hole tolerances, it is extremely difficult to predict the magnitude and direction of the panel growth. The resiliency of the dynamic retention contact enables it to share interface stress with the panel, thus greatly reducing stress-related effects, and producing a consistency reliable, high quality, and dimensionally accurate backplane.
STRESS PATTERN ANALYSIS
A variation in panel dimensional accuracy is directly related to variation in contact to panel interface stress. Because of its balanced four-point interaction with the plated-through hole, the dynamic retention contact produces a predictable stress pattern that is minimally affected by variations in hole size and other process variations. A polarized-light stress pattern analysis illustrates the consistency of the dynamic retention contact relative to other press-fit contacts.