Buletin Boards

CHIP-ON-BOARD TECHNOLOGY

Wire Bonding
Wire bonding is a method used to connect a fine wire between an on-chip pad and a substrate pad. The purpose is of course to connect a bare semiconductor to the outside world. This involves ultrasonically welding thin wire to a pad on a bare semiconductor die-chip and the other end of the wire to a conductive pad on a substrate. The wire is usually gold or aluminium and is usually very thin, often 0.001 to 0.0013 of an inch. If the wire bond substrate is a circuit board, it is called Chip on Board (COB). The Printed Circuit Board can be either ceramic (thick film) or fiberglass (FR4 or green).

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.

Bond Process Control
Ultrasonic bonding of aluminium wire is a friction weld process, whereby two metals are pressed together at room temperature and are rubbed together ultrasonically at the same time. Essentially there are two steps to the process.

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.

SURFACE MOUNT TECHNOLOGY

Design Guidelines for Print Performance
Three major performance issues exist for solder paste printing. First, the aperture size (width and length of the aperture) and stencil foil thickness determine the potential volume of solder paste applied to the PCB or substrate. The second issue is the ability of the solder paste to release from the stencil aperture walls. The third issue is positional accuracy of exactly where the solder brick is printed onto the PCB or substrate.

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.

Reflow Profile
In the reflow process, a PCB or hybrid assembly with solder paste and components placed on it is exposed to a heating process. The solder paste is heated to a point where it becomes liquid and then forms solder joints between the components and the board. The critical statistics in a reflow profile are pre-heat time, pre-heat temperature, time above liquidus(TAL), peak temperature and ramp rate. Pre-heat time and temperature precede the reflow zone of the profile and are critical for heating the assembly evenly and activating the flux. TAL is the period in the profile during which the solder is fluid and the joint is created. Peak temperature is the highest temperature reached by any point on the assembly. Ramp rate is the rate, expressed as temperature/time, at which the assembly is heated and cooled.

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.