Tutorial #98
George Riley
November 2009
Micro-Leadframe Packages (MLP) are increasingly used to squeeze more components onto high-density circuit boards. Packages such as the QFN (Quad Flat No-Lead) save space by placing the solder pads on the bottom of the package, with no horizontal protrusions. These packages are soldered directly to the board in the same operation and with the same pre-dispensed solder used for the other surface mount components. This approach avoids the expense and layout complexity of soldering ball-grid array (BGA) packages.
While MLP have cost and time advantages in initially populating and soldering the board, they raise formidable barriers to rework. The original package can be removed by conventional means, but soldering a replacement package to the board is difficult. In the original assembly, solder paste to simultaneously attach all components was stenciled onto a bare board. Replacing one component requires soldering onto a board still filled with those other components.
Stenciling solder onto the board pads is precluded by the space limitations. Direct dispensing or jetting of solder paste onto the board pads is possible, but it requires dedicated, high-volume equipment not generally available for rework.
A possible solution is to reflow solder onto the package pads before mounting the replacement part. However, this requires a separate pre-mount reflow step, with precise alignment of the package to a transfer plate. It also subjects the replacement package to an added high-temperature operation, with accompanying thermal stress upon the package.
Another approach, pre-pasting the package by manually dispensing the solder, might be possible, but handling a microscopic component with paste on one side while inverting and supporting it for machine pickup is challenging, especially for fine-pitch packages.
FINETECH has introduced a better solution by developing a Direct Component Printing (DCP) module as an accessory to its leading line of rework stations. The module permits easy, accurate stenciling of the paste onto the package, while inspecting, aligning, placing, and re-soldering the replacement component with the automated controls of the FINETECH rework station.
The rework process is simple. It begins with the rework station heating and removing the component being replaced, and completing non-contact removal of all excess solder on the board. The DCP module is prepared with a holding fixture and micro stencil designed for the component.
The new component is picked up with the soldering nozzle and aligned to a micro stencil that matches the layout of the component. The component is then placed and locked in the DCP module, which is next flipped 180 degrees so that the die is face-up, exposing the pads of the component. Figure 1 shows the fixture being flipped.
FIG 1. Fixture being flipped to face stencil up. (courtesy FINETECH)
Now that the component is locked into the DCP Module and flipped 180 degrees, solder paste is distributed over the stencil surface with a squeegee, printing the bond pads on the die.
The fixture is flipped again and the top lever is lifted to unlock the component. The component is now sitting on the stencil, which allows the pickup tool to lift the component off the stencil, exposing the solder-printed surface for inspection. Figure 2 shows solder paste on the die bond pads. The component is next aligned to the substrate bond pads and placed in its location on the board. It is reflowed with a standard reflow profile, including cooling, to complete the rework task.
FIG 2. Close-up view of solder paste on die pads. (courtesy FINETECH)
Advantages of Direct Printing
The FINETECH DCP module integrates the stencil-printing process for MLP into the normal rework flow. It eliminates the drawbacks of a second solder reflow step, and the hazards of manual handling. The module can be used with any rework system for saving time and improving yield on high-volume rework of QFN and other micro-leadframe packages.
FOR MORE INFORMATION
URL http://www.finetechusa.com
Phone (+1) 480-893-1630