Patent Application
20050048821
The present invention relates to clocksprings used in automobiles, and in particular, the termination and the method of terminating a flat electrical cable having round conductors therein by soldering to a circuit board in a clockspring.
A majority of automobiles today utilize airbag crash systems. An airbag is typically located on the steering wheel facing the driver and must be in continuous electrical connection with sensors in the car body. The sensors provide an electrical signal to the airbag crash assembly which instantly inflates the airbag in the event of a crash. Clocksprings are found in virtually every vehicle to electrically connect rotating devices in the steering column to stationary components in other parts of the vehicle.
To facilitate the rotation of the clockspring, the electrical cable located within the housing of the clockspring is a flat cable which is wound around a central hub of the clockspring. The flat cable is terminated at a circuit board on the clockspring, for eventual connection to the airbag or other electrical device within the car. These connections are oftentimes made by welding the conductors in the flat cable to metal leads on the insert molded circuit board.
a-1d show a current clockspring termination structure.
The flat cable 24 is generally formed by sandwiching the flat conductors 26 between two insulating layers of plastic or similar material. The insulating layers in
The welding structure of the prior art suffers from the disadvantage that it requires the metal leads to be spaced relatively far apart, resulting in larger clocksprings. The metal leads 22 are formed by a stamping process which requires that they be spaced a distance generally equal to the thickness of the metal leads 22 (in
A possible solution to this problem is to solder the flat conductors 26 directly to the insert molded circuit board 16. Soldering would remove the need for the metal leads 22, which would be replaced by solder pads that could be laid onto the insert molded circuit board 16 without the spacing demands of the metal leads 22. However, soldering flat conductors is not practiced because of the shortcoming of soldering a flat conductor to a flat surface. The interface between a flat conductor and a solder surface are not conducive to solder joints and provide a weak bond between the two surfaces.
Because of this drawback, most soldering is performed using round conductors. However, round conductors have not been used in flat cables because of the difficulty in removing the adhesive residue around the conductors. The presence of adhesive residue in solder joints weakens the joint, compromising its reliability and quality. Flat cables with flat conductors are typically stripped at the ends by grinding the insulating layers and adhesive off the flat conductors, which is effective in removing the majority of the adhesive residue. However, the grinding process cannot be used with round conductors because of the conductor's curvature. There is no way of accessing the adhesive at the round conductor's edges without grinding away portions of the conductor itself. Therefore, flat cables having round conductors have not been previously soldered to insert molded circuit boards.
More recently, a method of manufacturing flat cables without the use of adhesives has been disclosed in U.S. Pat. No. 6,026,563, issued to Tom Schilson (hereinafter referred to as “the '563 patent”) and assigned to Methode Electronics, Inc. of Chicago, Ill. The '563 patent is hereby incorporated in its entirety by reference. The '563 patent discloses a method of ultrasonically welding polyester layers around the conductors of a flat cable without using adhesives.
In view of the foregoing, it would be advantageous to provide a flat cable manufactured without the use of adhesives and having round conductors that may be soldered to an insert molded circuit board. It would be a further advantage to provide a high density flat cable having conductors that are spaced closely together and that are soldered to the solder pads of the insert molded circuit board to provide a flat cable with greater signal capacity.
The present invention is directed towards a solder joint between a flat cable having round conductors and the solder pads of a circuit board in an automotive clockspring. The flat cable is formed by a top and bottom layer of insulating material which cover the round conductors. The insulating layers are bonded to one another using a sonic welding process, which allows the flat cable to be manufactured without the use of adhesives. The ends of the flat cable are stripped to expose the ends of the round conductors, which are then soldered to the solder pads on the insert molded circuit board using a hot bar soldering process.
a shows a bottom view of the exterior of a prior art clockspring housing;
b shows a circuit board used in a prior art clockspring;
c and 1d show a flat cable welded to the circuit board of the prior art clockspring;
a and 2b show a bottom view of the clockspring housing of the present invention, with and without a cable cover, respectively;
c shows a connection module located on the clockspring housing of the present invention;
a-3c show a flat cable soldered to a circuit board of the clockspring of the present invention; and
a shows an end view of the round conductors along line 4-4 of
Referring now to the several drawing figures in which identical elements are numbered identically throughout, a description of the preferred embodiment of the present invention will be provided.
a-2c shows a bottom view of the clockspring housing 100 of the present invention having a connection module 102 with sixteen (16) contacts 106 therein.
The circuit board 104 is shown in greater detail in
The conductors 114 of the high density cable 112 are round which facilitate the soldering of the conductor 114 to the solder pad 110.
An additional benefit of using round conductors is that the width of the solder pads 110 only needs to be as wide as the thickness of the round conductor 114, or only slightly wider. This is because of the grooves 120 at the intersection of the round conductor 114 and solder pad 110 provide the necessary space for the soldering material to bond the conductor 114 and the solder pad 110. In contrast, soldering a flat conductor would require larger solder pads 110, because considerable space, typically 12 the width of the flat conductor, is necessary adjacent the contact point between the flat conductor and solder pad for the solder material to accumulate.
Therefore, flat cables having round conductors that are soldered to the solder pads of a circuit board allow for a higher conductor density, increasing the amount of signal the flat cable is capable of carrying.
The soldering process used in the present invention may use any known soldering method. However, in the preferred embodiment, a hot bar soldering process is used. The hot bar soldering process uses a solder paste 111, which is a layer of soldering material formed over the solder pads 110 that melts and joins the round conductors 114 to the solder pads 110 during the soldering process. The solder paste 111 is shown in
Although preferred embodiments are specifically illustrated and described herein as being used with a clockspring, it should be appreciated that the structure and methods disclosed above may be used in situations not involving a clockspring, and many modifications and variations of the present invention are possible in light of the above teachings, without departing from the spirit or scope of the invention.
