Before parts are wire bonded, the bare die chips must be diebonded. Although the concept is simple by putting some conductive epoxy on the board and place the die on the epoxy, there are many complex issues involved with die bonding. The placement must be accurate or else it will hinder the wire bonding operation.
First step is called the touch down and pre-deformation at which the bond wire is brought down flat onto the bond surface by the bonding wedge. Depending on the programmed parameters and the dynamics of the bonder, the mere act of bringing the wire into contact with the wire bond surface will cause the wire to be squashed or pre-deformed to some extent. This pre-deformation plays an important part in determining the quality of the subsequent welding process. The lattice structure of the bond wire and/or the bond surface will be changed significantly if the pre-deformation is too high and the quality of the subsequent bonds will suffer accordingly.
The second step is
called ultrasonic stage and welding. By applying an ultrasonic frequency
to the transducer, the wedge , which is connected to the transducer, vibrates
along the wire. The amplitude of the vibrations, 1 to 5 mm, is very small
compared to the diameter of the fine wire typically used. Initially the
wedge and the wire move together, creating friction at a constant pressure
on the interface between the wire and the bonding surface. After a short
time, the wire begins to deform and heat up and welding occurs. Both effects
are crucial for the quality of the weld.
Guidelines for Print Performance
As the squeegee blade travels across the stencil during the print cycle, solder paste fills the stencil apertures. The paste then releases to the pads on the board during the board/stencil separation cycle. Ideally, 100 percent of the paste that filled the aperture during the print process releases from the aperture walls and attaches to the pads on the board, forming a complete solder brick. To ensure that the edges of the aperture in the stencil are always positioned within the solder lands, the dimensions of the aperture should be about 10% smaller than those of the lands.
Another way of applying solder paste to the PCB is by dispensing syringe. This method uses an air- or mechanically-driven syringe to deposit paste to each solder lands. Although it is comparatively slow, it allows precise measured amount of paste to be deposited at each position.
The reflow oven thermal profile depends on the versatility and capacity of the oven. Factors that influence the profile are the temperature controllers, the mass of the product going through the oven, the efficiency of the reflow supply and heat transfer mechanism, and the speed at which the product passes through the oven. The reflow profile has a direct bearing on process yield; solder joint integrity, microstructure and reliability of the assembly.
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