Metal fusion bonding – Process – With measuring – testing – indicating – inspecting – or...
Reexamination Certificate
1999-12-30
2001-03-27
Shaw, Clifford C. (Department: 1725)
Metal fusion bonding
Process
With measuring, testing, indicating, inspecting, or...
C228S104000, C228S119000, C374S137000, C374S208000
Reexamination Certificate
active
06206267
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to ball grid arrays (BGAs) provided on circuit boards and more particularly to profiling ball grid arrays, particularly as part of a rework process.
BACKGROUND OF THE INVENTION
The implementation of ball grid arrays has become more extensive. Ball grid arrays have been used to provide high input/output on printed circuit boards with less space while providing a rugged package that eliminates handling problems associated with fine-pitch devices.
PCB assembly processes, that employ ball grid arrays, typically start with a screen print operation with stencils to place solder paste on pads. The board typically passes to a pick and place machine which places components on paste at the bottom side of the board. After the board goes through a reflow oven, it then goes through a top side paste operation and then a top side pick and place operation. The board is placed in a reflow oven after each pick and place operation and subjected to a soak period in which the board is ramped up to a temperature (e.g. 126° C.). In a subsequent zone the temperature is brought up to the reflow temperature to form the solder joints. Next a cool down process is provided, particularly a slow cool down to provide good metallurgical properties to the board. There are many more operations which are typically provided, including testing. However, the operations noted are the basic operations of printed circuit assembly.
Problems can result with the final product including various different defects. The pins can be rotated, there can be poor joints and elements can be skewed. In such situations, a rework process may be followed to take the board off the ball grid array. Ball grid array that are removed from the board are typically not reused.
During rework, temperature profiling allows the ramp up of the ball grid array case and joints to temperatures of 100° C. via a thermal couple based event. After soaking at this level form one to three minutes, temperatures reach 120° to 135° C. at which the upper nozzle temperature and flow rate are set so the joints are a brought to 175° C. sea before reflow. This makes it easier to quickly ramp to the temperature without exceeding maximum allowable case temperatures or exceeding the 90 sec. limit above liquidus temperature. Profiling is also used to achieve the joint temperatures of 208° to 218° C., exceeding the case temperature, which is usually 230° C. to 235° C. Once the profile is optimized, several electrically rejected grid array should be installed and removed to verify the integrity of the process.
The publication “SUCCESSFUL GUIDELINES FOR BGA REWORK” describes rework operations and how they should replicate the original assembly process as closely as possible. A fully populated circuit board together with a minimum of five electrically rejected BGAs are provided for profiling. Since most ball grid arrays will be reworked on a populated board, a populated board is preferred over a bare board. Thermal couples (thermocouples) with wire diameter of 0.005 in. are placed into 0.018 in. drilled holes on the BGAs. The junction of the BGA is just above the pad surface, which is the joint created during the rework process. Junctions (Thermal couple probes) should be less than 0.006 in. above the board to insure the rise through the paste is prevented when micro-stenciling on to the site. The thermal couple is firmly attached to the board using tape. The thermal couple junction is often re-inspected to using magnification to position and center in the thermal couple on pad. If not centered, it may be necessary to remove the thermal couple and reapply the tape. This process can be quite tedious. However, it is absolutely essential to provide a good profile as part of a good rework process.
As the rework process represents additional thermal cycles a proper profile should be created ideally using the same profile board as used in the reflow oven. Sensing critical location temperatures should be provided to insure the vendors maximum allowable case temperatures are not exceeded during the rework process. However, the sensing involves the use of thermal couples strategically positioned wherein precise measurements are required. This can be problematic with regard to physical and system difficulties in obtaining a good profile.
SUMMARY AND OBJECTS OF THE INVENTION
It is an object of the invention to provide a ball grid array circuit board rework process which can used to replicate the original assembly process as closely as possible based on providing a good profile of temperature characteristics during rework in an efficient reproducible manner.
It is a further object of the invention to provide a profiling operation which allows extremely accurate positioning of thermal couples (thermal couple probe ends or junctions), allowing highly accurate and reproducible profiling of ball grid arrays.
It is a another object of the invention to provide a profiling assembly including a holding element with a registration feature which facilitates a quick and accurate positioning of the element relative to the ball grid array. The holding element holds thermal couple probes (i.e., thermocouple junction) connected to it in a selectable manner, relative to the registration feature. The registration feature allows a quick and accurate positioning of the element, and in turn a positioning of the thermal couple probes in a highly accurate way, in order to take measurements at critical locations relative to the ball grid array.
According to the invention, a process for profiling solder joints of BGAs on a circuit board is provided. The process includes forming or providing a sheet element, particularly a thin tape element with length and width dimensions generally corresponding to the length and width dimensions of the BGA and with a registration portion. Thermal couple probes are positioned on the element at locations corresponding, based on said registration portion, to critical locations of the BGA and board requiring profiling. The element is disposed on the BGA and it is secured it in position with the element in registration with the pads on the circuit board with the reflow machine and with the plurality of thermal couples connected to the element such that the thermal couples are positioned corresponding to the critical locations requiring profiling.
The thermal couples may then be connected to a reader machine. Readings may then be taken at the reader machine to produce the temperature profile.
The tape element may be KAPTON tape. The adhesive of the tape may be used to secure the element to the BGA. A reflow machine may also be used to attach the element. The registration, with respect to the circuit board pads, is preferably provided by forming a plurality of holes in the element. Each hole is of a size to go over a solder bump on the BGA.
The BGA solder joint profiling arrangement according to another aspect of the invention includes a thin sheet like element or tape element with length and width dimensions generally corresponding to the length and width dimensions of the BGA and with a registration portion for registering the position of the element with the solder pads on the circuit board. A plurality of thermal couple probes are attached to the element. The thermal couple probes (i.e., thermocouple junctions) are positioned at locations corresponding to critical locations of the circuit board requiring profiling based on the registration portion. The element is disposed on the BGA and secured in position based on the registration portion such that the thermal couples are positioned corresponding to the critical locations requiring profiling.
The registration portion is preferably a plurality of holes. Each hole is formed in the sheet element to be of a size to go over a solder bump on the BGA. Thermal couples are connected to the element such that the thermal couples are positioned corresponding to the critical locations requiring profiling.
A reader machine is provided for connection with the thermal couples. The ther
3Com Corporation
Cooke Colleen P.
McGlew and Tuttle , P.C.
Shaw Clifford C.
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