Surface-mount crystal unit

#1. Field of the Invention

#The present invention relates to a surface-mount quartz crystal unit, and more particularly, to a surface-mount crystal unit which is suitable for reducing an area necessary to mount the crystal unit on a wiring board.

#2. Description of the Related Art

#FIG. 1 is a cross-sectional view illustrating a conventional surface-mount crystal unit;

#FIG. 2 is a plan view illustrating a conventional crystal blank;

#FIG. 3A is a cross-sectional view illustrating a surface-mount crystal unit according to a first embodiment of the present invention;

#A surface-mount quartz crystal unit according to a first embodiment of the present invention illustrated in FIGS. 3A and 3B comprises planar substrate 8 made of laminated ceramics; concave metal cover 9 provided to overlay planar substrate 8; and quartz crystal blank 2 encapsulated between planar substrate 8 and metal cover 9. Crystal blank 2 used herein is similar to that illustrated in FIG. 2, for example, an AT-cut quartz crystal blank in a substantially rectangular shape.

#Planar substrate 8 is formed of laminated ceramics which include first layer 8a and second layer 8b. Flat substrate 8 is formed with a pair of crystal-connecting terminals 13 on one surface, i.e., on the surface of first layer 8a. Crystal-connecting terminals 13 are connected to both ends of one side of crystal blank 2 to which extending electrodes 6 are extended. Also formed on the surface of first layer 8a is metal film 10 which circumvents along the outer periphery of first layer 8a. Metal film 10 has width W of 0.15 mm, by way of example. Crystal-connecting terminals 13 and metal film 10 are integrally formed during firing of planar substrate 8 made of laminated ceramics. Frame-shaped brazing material 11, for example, gold-tin (AuSn) eutectic alloy is bonded to metal film 10. A pair of mounting electrodes 4 are formed on the bottom face and side faces of second layer 8b of planar substrate 8. Mounting electrodes 4 are electrically connected to crystal-connecting terminals 13 via through-holes formed through first layer 8a and second layer 8b. Crystal blank 2 is held horizontally with respect to planar substrate 8 and electrically and mechanically connected to planar substrate 8 by securing both ends of the one side of crystal blank 2, to which extending electrodes 6 are extended, to crystal-connecting terminals 13 with conductive adhesive 7.

#Metal cover 9 is made, for example, of Kovar, nickel silver or the like, and formed into a concave shape by drawing or the like. Specifically, metal cover 9 has a substantially rectangular bottom wall, and side walls extending upright from the bottom wall. The side walls form an open end face. Metal cover 9 has outer planar dimensions smaller than those of planar substrate 8. The open end face of metal cover 9 has inside dimensions equal to those of metal film 10. Also, metal cover 9 has a thickness, particularly, thickness D of the side walls, smaller than width W of metal film 10.