Computed Tomography

A CT scan stands for Computed Tomography scan. It is also known as a CAT (Computer Axial Tomography) scan. It is a medical imaging method that employs tomography. Tomography is the process of generating a two-dimensional image of a slice or section through a 3-dimensional object (a tomogram).

The medical device (the machine) is called a CTG scanner; it is a large machine and uses X-rays. It used to be called an EMI scan, because it was developed by the company EMI. Undergoing a CT scan is a painless procedure.

The CT scanner uses digital geometry processing to generate a 3-dimensional (3-D) image of the inside of an object. The 3-D image is made after many 2-dimensional (2-D) X-ray images are taken around a single axis of rotation - in other words, many pictures of the same area are taken from many angles and then placed together to produce a 3-D image.

The Greek word tomos means "slice and the Greek word graphein means "write".

How does a CT scan work?

A CT scanner emits a series of narrow beams through the human body as it moves through an arc, unlike an X-ray machine which sends just one radiation beam. The final picture is far more detailed than an X-ray one.

Inside the CT scanner there is an X-ray detector which can see hundreds of different levels of density. It can see tissues inside a solid organ. This data is transmitted to a computer, which builds up a 3-D cross-sectional picture of the part of the body and displays it on the screen.

Sometimes a contrast dye is used because it shows up much more clearly on the screen. If a 3-D image of the abdomen is required the patient may have to drink a barium meal. The barium appears white on the scan as it travels through the digestive system. If images lower down the body are required, such as the rectum, the patient may be given a barium enema. If blood vessels are the target images the barium will be injected.

The accuracy and speed of CT scans may be improved with the application of spiral CT. The X-ray beam takes a spiral path during the scanning - it gathers continuous data with no gaps between images. For a spiral scan of the chest, for example, the patient will be asked to hold his/her breath for a few seconds.

X-Ray Radiography

X-Ray Radiography is a nondestructive semiconductor failure analysis technique used to examine the interior details of the package. It operates on the principle of dissimilar transmission of X-Rays through different materials.  The ability of a material to block X-Rays increases with its density. It is this dissimilar transmission of X-rays through different materials that is utilized to create an image of various contrasts.  X-Ray imaging may be accomplished on film, by fluoroscopy, or by using image intensifying video systems.

  

A typical modern X-Ray inspection equipment has a filament that produces an electron beam used to excite a target into producing X-Rays. The X-Ray emissions are then directed to and transmitted through the specimen. The transmitted X-Rays are collected by a detector, translated into electric signals, amplified, and transformed into an X-Ray image.

           

The varying densities of the various materials comprising the specimen allow different amounts of X-Rays to pass through, resulting in varying grayscale levels on the X-Ray image.  The quality of the X-Ray image formed therefore depends not only on the proper operation of the X-Ray equipment used, but on the composition of the specimen as well. Some materials used in semiconductor assembly, such as aluminum wires, are transparent to X-Ray, and are therefore invisible in X-Ray images.           

                              

X-Ray radiography is commonly used to inspect for wiresweeping and other wirebond problems, die attach voids, package voids and cracks. It is excellent for determining leadframe outlines as well. Traditional x-ray systems use photosensitive films to record the x-ray image. Since X-Rays are not easy to focus, this method produces low-resolution images, which greatly limits its usefulness. 

                            

Today, X-Ray systems use a microspot source, real-time detection and automated manipulation of the sample to achieve higher resolution and throughput. These systems are capable of detecting much finer package details and defects.