Wave soldering fixture

Details Wave soldering fixture Wave soldering fixture

1999-10-18

2001-05-29

Ryan, Patrick (Department: 1725)

Metal fusion bonding

Including means to force or clamp work portions together...

Work portion comprises electrical component

C228S037000, C228S039000, C228S043000

Reexamination Certificate

active

06237832

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to apparatus for assembling components on printed circuit boards PCBs and particularly to a fixture that supports the PCB through a wave soldering step. In the context of this specification and in accordance with the practices of the art, the terms, pallet, fixture, carrier and rack will be understood to have a common meaning.
BACKGROUND AND INFORMATION DISCLOSURE
The technology for manufacturing circuits comprising components mounted on boards has evolved continuously during the past fifty years keeping pace with the evolution of discrete semiconductor devices to present high density integrated circuits on a chip.
In the earliest techniques for the manufacture of PCBs, components were hand soldered onto the board. The next evolutionary step involved exposing the unsupported PCB to a solder wave. The problem with this technique was that the trailing and leading sides of the PCB had no support, thus causing the middle of the board to bow in the middle from the front to the back. This led to the technique of laying the PCB on a fixture which was a pallet having a “window” opening. When double sided PCBs were introduced having components on both sides, the “selective wave pallet was introduced which was designed to cover and protect components on the bottom of the PCB and allow the solder wave to enter cavities that are milled in the board. Registration of the PCB with the pallet was maintained by milling a depresseion in the pallet having the outline of the PCB so that the board was laid in the depression. Previous technology also used tooling pins for registration in some cases.
With the demand for greater density of components (IC's), exposure of the PCB to the heat of a solder wave at 450° F. became a more important issue. This led to the development of the “mask wave” pallet which was used even though there were no components on the bottom of the PCB. This pallet was a full sheet with only holes corresponding to pin through locations permitting exposure to the solder wave.
Pallets according to present practice combine all of the features of the window frame pallet, the selective pallet, and the masking pallet. All of these techniques, used separately or in combination, present limitations to the continued demand for greater component density and economy of manufacture. These techniques include the machining of pockets in the pallet for nesting the PCB which must be very accurate in order to maintain accurate registration of the PCB with the pallet. Even with accurate machining of the pockets, differences in heat expansion between the pallet and PCB complicate the problem of maintaining registration. The machining of pockets requires that the pallet sheet have thickness that is greater than a minimum thickness necessary for the machining operation. The amount of heat absorbed by the pallet is proportional to the thickness of the pallet and the efficiency of the soldering process is diminished by increased absorbtion of heat. Therefore, common practice is select a pallet thickness depending on PCB design.
Variation of the thickness of the pallet requires that the height of the crest of the solder wave must be adjusted according to the thickness of the pallet. The distance of the PCB surface to the crest of the solder wave must be set very accurately in order that the wave barely “brush” the surface of the PCB. Typically, the accuracy of this adjustment is set equal to one half the thickness of the PCB. Present procedure is to support a PCB on a pallet over the molten solder pool and adjust the height of the surface of the pool to carry out the wave solder step. This procedure is particularly costly because it is a “trial and error” procedure and usually several test “runs” must be performed as a part of the adjustment procedure.
A number of disclosures have appeared related to fixtures for manufacturing of printed circuit boards.
For example in surface mount technology, U.S. Pat. No. 5,785,307 to Chung discloses a frame for supporting a printed circuit board. Spring loaded clamps are mounted around the area with fingers that may be oriented out over the area to retain the board. The board is released when the clamp is rotated so as to orient the finger out of contact with the board. Japanese Patent 48238 to Seisakusho discloses a one touch security arrangement for securing a PCB to a table.
None of these inventions disclose a pallet for a PCB that overcome all of the problems of registration, minimizing warpage due to exposure to heat, components lifted off the board during wave solder and, frequent adjustment of the crest of the solder wave.
SUMMARY
It is an object of the invention to provide a fixture on which is securely mounted a board that is carried through the wave soldering step in the manufacture of a printed circuit board.
It is a further object that the fixture and its use have certain advantages compared to fixtures that are presently used for this purpose. These advantages include:
quick and accurate replacement of each board;
minimizing heat loss from conduction of heat into the fixture by features of the invention that permit use of thinner material for the fixture;
minimizing warpage/bowing of the board during the wave solder step;
minimizing warpage/bowing of the frame during the wave solder step;
precise positioning of the board by the fixture relative to the distance between the surface of the board to be soldered and the crest of the wave so that the location of the crest of the wave need not be adjusted from one board to the next particularly when a thickness of the board is different from one board to the next.
This invention is directed toward a fixture for supporting a board during a wave soldering operation comprising a non-metallic frame characterized by low heat absorption.
The frame has one (top) side serving as a reference surface on which the board is laid and to which the board is secured by spring loaded clips. A metallic elongated member (referred to herein as a “stiffener”) preferably having a Tee cross section is provided. One leg of the Tee (stiffener) has an elongated edge joined perpendicularly to the middle of a second elongated leg. The surface of the second leg opposite the first leg is secured against the reference surface of the frame and partially overhangs the outside edge of the reference surface.
In some instances, stiffeners having other cross sections such as “L” are used.
The stiffener prevents warpage of the frame and also provides a support surface that is coplanar with the reference surface of the frame and may be in supported contact with a rail along the edges of the solder tank over the solder pool. The board slides along the rail over the crest of the solder wave during the wave solder operation. The required distance of the surface of the board to be soldered relative to the level of the pool is thereby accurately maintained regardless of the thickness of the board.
In one embodiment, during the wave soldering step, the crest of the solder wave is adjusted to be in the same plane as the two supporting rail surfaces, each rail being along an edge of the solder tank opposite the other rail. When the board is positioned with the outside surface of the stiffener supported on the supporting rail at the edge of the solder tank, then the reference surface of the board will be in a vertical location where the crest of the solder wave will “brush” the surface of the board without excessive exposure to heat of the wave to the board surface.
In order to maintain flatness of the board during the wave soldering step, an elongated member extends across the frame and a stud secured between the center of the member and the board maintains flatness of the board.
Another feature of the invention is a removable bar extending across the frame and is positioned to confine some components so as to prevent the components from floating off the board when positioned over apertures in the board wherein the components are solder by the wave through the apertures.


REFERENCES:
patent: 4655164 (1

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