Metal fusion bonding – Process – Plural joints
Reexamination Certificate
1999-06-08
2001-03-20
Shaw, Clifford C. (Department: 1725)
Metal fusion bonding
Process
Plural joints
C228S123100, C228S212000, C228S214000, C228S222000
Reexamination Certificate
active
06202916
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to circuit board construction and processes. More particularly, this invention relates to a technique for preserving solder connections of circuit components mounted to a thin laminate circuit board, and particularly surface-mount components soldered to such a board, during wave soldering of leaded through-hole components to the board.
BACKGROUND OF THE INVENTION
Wave soldering is a well-known method by which solder connections are formed on a printed circuit board (PCB) by bringing the circuit board in contact with a wave of molten solder flowing upward from a nozzle. Wave soldering methods typically apply a relatively thin coating of solder, and therefore find widest use for applications in which a solderable contact or metal lead is to be coated with solder to form a solder fillet or solder connection, respectively. One such example is a circuit component having leads that extend through a circuit board, and to which solder is applied to physically and electrically connect the component to the board. During wave soldering, the circuit board, supported at its perimeter with a pallet, passes through molten solder coming up from the nozzle, such that the ends of the leads projecting through the circuit board are brought in contact with the upward-flowing molten solder. In the process of adhering to the leads, the solder forms solder connections or fillets.
While leaded circuit devices are widely employed in electronic applications, and wave soldering is widely practiced to attach and electrically connect such devices to printed circuit boards, a continuous effort to reduce the size of circuit board assemblies has promoted the use of such advanced packaging technologies as tape automated bonding (TAB), chip on board (COB), flip chips, multichip modules (MCM) and ball grid arrays (BGA). These devices are generally surface-mount technology (SMT) components attached to a circuit board with solder, such as solder bumps that are formed on the devices and then reflowed to solder the devices to appropriate conductor patterns on the board.
While highly successful, SMT devices are vulnerable to downstream processes that can damage their solder connections. As an example, because of their rigidity, flip chips are vulnerable to mechanical loads that distort the circuit board, causing fracturing or separation of their solder bump connections. Wave soldering is a notable example of such downstream processing, during which thermally-induced board distortion occurs as a result of molten solder contacting only one surface of the board. If not minimized, this distortion may cause significant quality concerns, including fracturing and/or separation of solder connections, potential destruction of the affected device(s), and reduced overall system reliability. In the past, PCBs have been of sufficient thickness to allow wave soldering without unacceptable levels of board distortion. Preheating PCBs to achieve a more uniform temperature throughout the board has been used to further reduce board distortion, to the extent that an adequate board flatness of about 0.020 to 0.030 inch (about 0.50 to 0.75 mm) has typically been maintained. However, thin laminate PCBs (e.g., thicknesses of 0.032 inch (about 0.8 mm) and less) are more prone to distortion during wave soldering, with the result that fracturing and separation of flip chip solder connections is much more likely to occur. While the incidence of solder connection fracturing and separation can be reduced or avoided by the use of alternative circuit devices and soldering processes, such solutions are often not practical or cost effective.
Therefore, what is needed is a method for reducing the incidence of solder connection fracturing and separation of SMT devices during a wave soldering operation.
SUMMARY OF THE INVENTION
The present invention provides a method and assembly for preserving solder connections of components mounted on a circuit board during wave soldering of leaded components to the circuit board. In such a circuit board assembly, surface-mount components may be present on either surface of the board, while leaded components are located on one surface of the board with their leads extending through through-holes in the board to the opposite surface of the board. The method of this invention generally entails supporting the circuit board on a pallet such that the surface of the circuit board with the exposed leads contacts the pallet. The pallet is configured in accordance with the invention to include raised pedestals that contact surface regions of the circuit board directly opposite to one or more surface-mount components whose solder connections are at risk of damage during wave soldering. The pallet also includes accesses directly opposite the leaded components so that their leads remain exposed. The pallet and board assembly are then placed on a wave soldering apparatus and, while applying and maintaining a force to the board that ensures contact with the pedestals, the circuit board is wave soldered through the accesses in the pallet so that the leads of the leaded components are soldered to the circuit board. Importantly, the circuit board is held firmly against the pallet by the force in such a manner that maintains board flatness throughout soldering, so that the solder connections attaching the surface-mount components to the circuit board do not fracture or separate. As an example, a suitable flatness (i.e., edge-to-edge deflection) for a 4×4 inch (about 100×100 mm) circuit board has been determined to be not more than 0.010 inch (about 0.25 mm), and preferably about 0.003 inch (about 0.08 mm).
According to the above, the pallet of this invention has enabled wave soldering of thin laminate circuit boards while maintaining a board flatness that is far better than that practiced or achievable with conventional wave soldering practices. Without such flatness control, thin laminate circuit boards having surface-mount devices such as flip chips are unable to undergo wave soldering without adversely affecting the reliability of the devices as a result of damage to their solder connections.
Other objects and advantages of this invention will be better appreciated from the following detailed description.
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Coles Michael Thomas
King Richard Scott
Updike Theresa Ann
Cooke Colleen P.
Delphi Technologies Inc.
Funke Jimmy L.
Shaw Clifford C.
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