Metal fusion bonding – Including means to force or clamp work portions together... – Work portion comprises electrical component
Reexamination Certificate
1999-10-18
2001-07-31
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Including means to force or clamp work portions together...
Work portion comprises electrical component
C269S2540CS, C269S2540DF, C269S043000
Reexamination Certificate
active
06267288
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to apparatus for assembling components on printed circuit boards (PCB) and particularly to a fixture that supports the double sided PCB through surface mount technology (SMT) and wave solder processes.
BACKGROUND AND INFORMATION DISCLOSURE
The technology for manufacturing circuits comprising components mounted on printed circuit boards has evolved continuously during the past fifty years keeping pace with the evolution of discrete semiconductor devices to present high density integrated circuits.
The current “surface mount technology” (SMT) is applied to printed circuit boards (PCB) that have a plurality of layers of wiring with contact points from each layer extending up to “pads” located on the top and bottom surfaces of the board. Components are soldered to the pads.
The process starts with a bare board referred to as a fab which has been manufactured with layered wiring and surface art comprising exposed pads. The fab is loaded into the printing machine utilizing a stencil to print solder paste on the pattern of pads on the fab. Then the fab is conveyed to a “pick-n-place” machine which places components in position on the fab. Then the fab is placed in a “reflow” oven where the solder is melted and then allowed to cool so that the components are now securely soldered to the fab.
A common requirement for all of these processes is that each fab must be supported as it is carried through each step of the process. This is generally accomplished in the industry by mounting the fab on a supporting pallet which is designed for a specific station and is unique to that specific fab.
The state of the art SMT pallets used in the reflow step have an opening framed by a shoulder for supporting the board over the opening. The tolerance between the step down wall of the shoulder and the board is typically 0.015″ to allow for differential expansion between the pallet and board.
The double sided PCB has the problem that, in a reflow operation, components soldered to one side of the board during a first reflow step can fall off the board when the board is turned over for the second reflow step.
SUMMARY
It is an object of the invention to provide a pallet that can support the fab in all three steps of the SMT operation including print, pick and place, and reflow.
It is an object of the invention to provide a pallet that supports a double sided fab in a secondary reflow operation and prevents the components installed on one side of the fab during a first pick-and-place and reflow operation from dropping off the fab when the fab is turned over to perform the second reflow operation.
Another object is to provide a supporting pallet that is characterized by more uniform heat distribution during the reflow operation so that thinner PCBs (typically, 0.020″ thick) can be manufactured that are not subject to intolerable warping.
It is a further object that the pallet and its use have certain advantages compared to pallets that are presently used for this purpose. These advantages include:
quick and accurate replacement of each fab on pallet;
minimizing heat loss from conduction of heat into the pallet by features of the invention that permit use of thinner material for the fab;
minimizing warpage of the fab and pallet holding the fab during the solder reflow step;
manufacturing a PCB that combines surface mount technology and wave solder technology;
This invention is directed toward a pallet having a depression with a shoulder on at least two opposite sides of the depression so that a space is defined between the PCB supported on the shoulders and the surface of the depression. When the pallet supporting a PCB is placed in a solder reflow oven, the “down” side of the PCB (facing the depression) is shielded from the the direct heat of the oven, the surface temperature of the PCB facing the space is therefore less than temperature outside the space. In practice, power supplied to heat the oven is adjusted so that the temperature in the space is below the melting temperature of the solder paste applied to the PCB so that a first group of components mounted and soldered to the downside of the PCB will not fall off the PCB in a subsequent reflow step to secure a second group of components to the topside of the fab.
Another application of this feature is in manufacturing a board requiring a combination of first group of components mounted by surface mount technology in a first area and a second group of components mounted by solder wave technology in a second area. The pallet of this invention used in this situation has a depression formed in a corresponding first area and openings to admit a solder wave in the corresponding second area so that during a wave solder step applying solder to the second group of components, the first group of components is protected by the depression.
Anther set of circumstances applies to a fab that is thin (0.020″) compared to the present standard fab (0.060″). According to an embodiment of the invention, an array of standoffs are formed in the depression which support such fabs and prevent sagging during the reflow operation. The array of standoffs are arranged to avoid interference with components placed on the PCB in an initial surface mount operation and which now face the depression in the second reflow step.
REFERENCES:
patent: 4585157 (1986-04-01), Belcher
patent: 5577657 (1996-11-01), Glovatsky
patent: 5759269 (1998-06-01), Cutting et al.
patent: 5785307 (1998-07-01), Chung
patent: 5820013 (1998-10-01), Ortiz
patent: 6012713 (2000-01-01), Gleason et al.
patent: 6068174 (2000-05-01), Ball et al.
Johnson Jonathan
Ryan Patrick
Smith Robert Samuel
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