METHOD AND MEANS FOR WAVE SOLDERING OF LEADS OF AN INTEGRATED CIRCUIT PACKAGE

Abstract: Solder coating of lead of IC packages is accomplished by passing IC packages on a pallet through a solder wave. The rectangular or square IC packages are positioned so that they are tilted relative to the horizontal and rotated so that a side or adjacent sides of the package enter and exit the solder wave at an angle that is not 90° or 180°.

Description: This invention relates to a method and means for soldering electrical leads associated with an integrated circuit package. BACKGROUND OF THE INVENTION Description of the Prior Art During production of integrated circuit package assemblies, one step of the process is the solder coating of leads, conventionally made of copper. The solder coating serves to prevent undesirable oxidation of the leads that would adversely affect solderability and electrical conduction during operation of the integrated circuit device. To implement the solder coating of the leads, wave soldering is one soldering technique used. Prior art wave soldering is briefly described on page 24 of "The Story of Microelectronics," published by North American Aviation, Autonetics Division, copyright 1966. The coating process is implemented by positioning the integrated circuit package in a holder, which is then directed into a molten solder wave. The conventional method places each IC package flat on a pallet, with one side of the rectangular or square package facing the solder wave, as the package on the pallet approaches and enters the solder wave. A problem that exists with this method is that after the integrated circuit package and its external leads are passed through the solder wave, the solder material tends to cool and form solder drops or clumps which generally occur at the ends of the leads. Since the fine pitch leads are usually spaced about 0,635 millimeter (0,025 inch) apart, center to center, the solder clumps that form can bridge the gaps between the leads and cause electrical shorting contacts. In the prior art, US-A-3 452 916 discloses an adjunct to a process that shoots a "fountain" of molten solder directly at the tilted circuit boards in order to solder individual components to the board but does not address the problem of solder clumping on closely spaced leads or disclose the advantages of tilting for such a purpose. The process according to JP-A-60-200 559 "gushes" molten solder directly at rotated printed circuit boards in order to solder individual components to the board but does not address the problem of solder clumping on closely spaced leads or disclose the advantages of rotating for such a purpose. The process disclosed in DE-C-3 316 153 does not transport an IC package through a solder wave, and therefore has no relevance to the present invention. SUMMARY OF THE INVENTION An object of this invention is to provide an improved method and means for solder coating leads of an integrated circuit package. Another object of this invention is to provide a method and means for preventing solder accumulation and electrical shorting during the solder coating of closely spaced leads of an integrated circuit package. In accordance with this invention, the leads of an IC package are coated by passing the package and its leads through a molten solder wave. The rectangular or square integrated circuit packages are fixedly positioned on a holder or pallet so that each package enters from the front side and exits from the back side of the solder wave in a tilted position relative to the linear path of movement. Also, each package is set in a rotated position on the pallet relative to the back side of the solder wave so that a corner, not a side, of the package enters and exits the solder wave. By tilting and rotating the package assembly relative to the linear path of movement and to the solder wave, unwanted solder clumping is avoided and electrical shorting is prevented. DESCRIPTION OF THE DRAWINGS The invention will be described in detail with reference to the drawing in which: Fig. 1 is a representational top plan view of a pallet supporting a number of integrated circuit packages, adjacent to a solder wave; Fig. 2 is a sectional side view of the pallet and solder wave of Fig. 1; and Fig. 3 is an enlarged side view illustrating the aspect of a pallet and IC package relative to a solder wave as the pallet exits from the solder wave. Similar numerals refer to similar elements throughout the drawing. DETAILED DESCRIPTION OF THE INVENTION With reference to the figures of the drawing, a pallet 10 supports a number of IC packages 12 seated in an array. In keeping with this invention, the rectangular or square integrated circuit (IC) packages are rotated approximately 45° so that a corner of each IC package is facing towards a solder wave 14. The solder wave is generated by a pumping mechanism, which spurts the solder in a molten state to a given height above the reservoir or container 16 of solder. The IC packages are seated on an angular surface or ramp 18 so that the corner or nose of the IC package facing the solder wave is pointed up at an angle of about 10-20° relative to the horizontal. The packages are maintained in position by crossed rails 20 that enclose and isolate each IC package. Furthermore, a gate or door 22 having fingers 24 is lowered to hold the packages firmly in a stationary position. The portion of each finger 24 that contacts a respective IC package is configured to have an angle of about 10-20°, corresponding to the angle of the ramp below the IC package. The fixed IC package assemblies are thus in a tilted and rotated position, in accordance with this invention, when moved into and through the solder wave by a conveyor 26, for example. The rectangular or square IC packages are positioned in a rotated orientation so that as they enter the solder wave with adjacent sides at an angle to the direction of entry into the solder wave. The IC packages maintain the angular orientation, as well as the tilted position, along the path of movement through the solder wave. A preferred angular rotation is about 45° relative to the path of movement of the pallet. The packages in effect surf through the solder wave so that the solder coats the external package leads without solder accumulation and clumping. As the pallet exits from the solder wave, excess solder drips down and back into the solder wave so that there is no undue loss of solder material during the coating process. By virtue of the surfing action effectuated by passing tilted and rotated IC packages through a solder wave, substantially smooth solder coating of leads is accomplished without the problems of solder clumping and resultant electrical shorting.

Claims: A method of solder coating lead wires of rectangular or square integrated circuit packages (12) for the purpose of preventing undesirable oxidation of the lead wires, the method using a wave soldering technique which creates a continuously flowing solder wave (14) in a solder bath, where the flow pattern of the wave has an umbrella-shaped cross section with an upwardly flowing center and downwardly flowing front and back sides straddling its upwardly flowing center, the method using a means (26) to move packages along a linear path through the front and back sides of the wave, the method being characterized by the steps of: positioning a plurality of integrated circuit packages on a pallet (10) which moves along the linear path through the front side and then the back side of the solder wave, said positioning including tilting up a front nose portion of each of said packages at an angle relative to the linear path of movement of said pallet, and rotating said packages so that sides of said packages are at an angle relative to the back side of said solder wave; and moving said pallet and said packages along said linear path and through the solder wave, so that said packages effectively surf through said solder wave with excess solder dripping from the packages down and back into the solder wave as the packages exit the back side of the solder wave, thereby avoiding undue accumulation of solder. A method as in claim 1, wherein said angle of tilt is approximately 10°-20° relative to said linear path. A method as in claim 1, wherein said angle of rotation of said packages is about 45° relative to said back side of the solder fountain. An apparatus for solder coating leads of a plurality of rectangular or square integrated circuit packages comprising a means for generating an upwardly spouting wave of solder at a top surface of a solder bath, and a means to move the packages along a linear path through said front and back sides of said solder wave, the apparatus being characterized by: a pallet for temporarily supporting said packages on the means to move the packages for transport along a substantially linear path through the solder wave, said pallet including ramps (18) having surfaces on which said packages are seated, said ramp surfaces being tilted relative to a base surface of said pallet for tilting upwardly a front nose portion of each package relative to the top surface of the solder bath in order to effect a surfing action as each package moves through the solder wave. An apparatus as in claim 4, wherein said ramp surfaces are tilted at an angle of about 10°-20° relative to the base surface of said pallet. An apparatus as in claim 4, wherein said pallet has opposed fingers (24) and said ramps are defined on opposed ends of said fingers for making frictional contact with said packages. An apparatus as in claim 4, wherein said pallet includes position maintaining means (20) for holding said packages in a rotated position relative to said linear path, so that adjacent package sides are disposed angularly relative to said linear path. An apparatus as in claim 7, wherein said adjacent sides define angles other than 90° or 180° relative to said linear path.

Inventors: LONG, JON
Applicants: LSI LOGIC CORPORATION
Application year: 1988
Main classification: B23K1
IPC codes: B23K1, H01L23, B23K3