Microfocus X-ray images of BGA short and chip connect short

Acoustic microscopy of delamination under flip chip

AOI image of solder connects

Thermal image of PCB assembly

In recent years the complexity of PCB assemblies has been increasing. The need to increase the population density of components has driven the move to more complex surface mount assembly technology and has led to increased use of area array components such as BGAs and other small footprint devices. This has placed greater demands on NDT examination for quality assurance. For Ball Grid Arrays (BGAs) and other similar types of assemblies, an array of solder balls is placed beneath the component and the connections made are between the component and the substrate, rendering these connections visually inaccessible. Testing the integrity of these joints and the PCB assembly is a problem because of lack of access and the increasingly complex fine pitch connections. This situation will be additionally complicated by the introduction of lead-free soldering through the potential to create further defects and its different surface finish and joint profile.

Currently most NDT is applied off-line in a batch situation, generally using only one technique. If a serious defect is found, it is possible that the whole batch will be scrapped. Once a fault is found with one NDT technique, it is frequently necessary to confirm this with optical or scanning electron microscopy and metallurgical sectioning. This type of methodology adds greatly to the cost of each PCB batch.

The NDT method most commonly used for in-line (production and assembly) applications is automated optical inspection (AOI) but this is only effective for the detection of surface flaws and is defeated by more complex designs. BGAs also present particular problems for AOI as the outer rows of solder balls of the array obscure the inner connections, therefore other NDT techniques are required to supplement this method. This technique will also be challenged by the surface finishes and profiles found on lead-free joints and new technology needs to be developed to accommodate these new materials.

Acoustic and X-ray detection are currently used non-destructive test (NDT) techniques for locating internal flaws, however they are not always optimal for the flaws that can occur in modern PCB assemblies and they are not generally suitable for in-line inspection due to their slow processing speed. Inspection is normally carried out manually by batch or sample testing off-line which greatly increases the cost of PCB assembly production. Thermal imaging (IR) is a technique, which has only been recently applied to PCB assembly inspection.

For BGA assemblies, a series of connection balls beneath the component body are soldered to the substrate. However with the interconnections located beneath the component, inspection can no longer be carried out completely with current AOI techniques. BGA assemblies can be examined using X-rays. However current technology limitations mean that this is usually carried out offline.

Cracking and delamination can occur in the substrate after the components have been soldered due to the effects of heat and soldering. These types of defects can cause the failure of electronic circuitry but are extremely difficult to pinpoint using AOI or X-ray methods. Acoustic methods of examination can highlight these types of defects and, although currently slow and difficult to use, they offer a potentially realistic non-destructive way to identify these failures and are considered complementary with respect to the types of hidden defects they can detect when compared to X-ray systems. In addition the application of thermal or infrared (IR) techniques have shown promise in this field.

The current complex quality assessment/control processes used in industry (e.g. AOI methods to detect visible defects followed by batch testing using X-rays for hidden defects and in some cases microscopy) adds considerable cost (greater than 25%) to the assembly of PCBs without necessarily locating all PCB defects.

A literature and patent search has shown that there are no commercially available in-line systems available using AOI, acoustic, X-ray and thermal inspection systems for the inspection of PCB assemblies. There are also no European standards for manual or automated acoustic and radiographic inspection of PCBs.

Access to such a tool would provide significant impetus to the PCB industry in Europe.

Limitations of current techniques are (1) their limited defect detection ability, (2) manual nature, (3) Inability to be used in in-line in a PCB production line and (4) Lack of ability to detect defects in lead free solders. This project will over come all these limitations.