TWI Collaborative Research Knowledge Summary

'MICROSCAN' - development of a comprehensive, in-line quality control system for printed circuit board assemblies

by Phil Wallace and Ian Nicholson

Summary
Introduction
Objectives
Technical approach
Collaboration
Further information

EC collaborative project; title:
Development of comprehensive in-line quality control system for printed circuit board assemblies

Summary

The goal of the Microscan project is to produce a powerful in-line system for the total quality control of printed circuit board assemblies.

Introduction

Printed circuit boards (PCBs) are widely used in electronic goods and many applications that require microprocessor controls; e.g. personal computers, white goods, and motor vehicle engine management systems. The market for PCBs is euro 25.2bn in Europe, over 36% of the world market. The projected increase in this market is 5-10% per annum in the next two years with greater growth after this time, with SMEs representing 35% of this sector.

Assembled PCBs can suffer from many defects including solder bridging, track delamination and component placement errors. It is highly beneficial for these defects to be detected at the production stage, as failure in the field can result in massive financial costs for the manufacturer in terms of fault diagnosis, repair costs, lost reputation and loss of production time, etc. Additionally, European environmental legislation, to be implemented in 2006, banning the use of lead, mercury, cadmium and hexavalent chromium, PBB and PBDE (two types of brominated flame retardants) may increase the number and types of defects seen in PCB assembly, as manufacturers grapple with new material properties and process conditions.

In today's competitive market, it is always important to maximise the yield from any production line. Any loss of quality will substantially affect the yield and create significant revenue loss for the manufacturer/assembler. It is commonly accepted that the A3-sigma businesses will spend 24% of revenue on fixing quality issues. It is, therefore, extremely important to use in-line examination procedures as part of quality control, thereby improving the overall quality level of the manufacturing operation.

Objectives

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 Ball Gate Arrays (BGAs) and other small-footprint devices. This has placed greater demands on non-destructive examination for quality assurance. For BGAs and other similar types of assembly, an array of solder balls is placed beneath the component and the connections made are between the component and the substrate, rendering these connections inaccessible. Testing the integrity of these joints and the PCB assembly is a problem because of lack of access and the increasingly complex connections.

Currently, most non-destructive testing (NDT) is applied off-line in a batch situation. Generally, only one technique is used. 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 inspection using other NDT techniques. This type of methodology adds greatly to the cost of each PCB batch.

The main objective of the project is to overcome current problems encountered in PCB inspection by delivering four NDT inspection prototypes that can be fully integrated with a PCB production line. Figure 1 shows typical defects in PCBs and their images using current technologies.

Fig.1. Defects in PCBs and their images using current technologies

Defects in PCBs Defects in PCBs Defects in PCBs Defects in PCBs Defects in PCBs
Microfocus X-ray images of BGA short and chip connect short Acoustic microscopy of delamination under flip chip Automated optical inspection image of solder connects Thermal image of PCB assembly

The result will be an ability to detect rapidly an out of control process with respect to defects, and hence real-time control. This will enable an operator or system to intervene rapidly in a continuous PCB production process to restore control before the production of a large quantity of scrap or defective systems being installed in the field. Reducing in-service failures is particularly important in safety-critical applications, such as those found in the automotive and aerospace industries where systems failure may have catastrophic results. The prototypes will also find applications in other product production lines, e.g. inspection of integrated and hybrid circuits and surgical equipment.

Technical approach

A concept drawing of the in-line inspection system is shown in Figure 2. The system will comprise digital radiography, thermal inspection, automated optical inspection (AOI) and acoustic inspection techniques. The system will be modular, so that different techniques can be combined for different PCB production lines. For example, it may not be appropriate to conduct all the inspections at the same time during the PCB manufacturing process. This will enable the end-user to choose the most appropriate techniques, while keeping their costs to a minimum. Development goals will include the following:

  • Development of an X-ray system for the real-time identification and quantification of defects in PCBs.
  • Development of acoustic systems for potential in-line inspection of PCBs.
  • Improvement of AOI systems and defect recognition algorithms.
  • Development of thermal/infrared systems for potential in-line inspection of PCBs.
  • Provision of feedback mechanisms for real-time control of the PCB assembly process.
  • Development of features which allow the NDT systems to be complimentarily integrated.
  • Development of the above systems for use on the new lead-free solder alloys.
  • Increase Probability of Detection (PoD) of defects in lead-free solder by 40%.
  • Increase PoD in standard solder by 20%.

Fig.2. Conceptual drawing of in-line inspection system
Conceptual drawing of in-line inspection system

Collaboration

  • Co-ordinator and manager: TWI Ltd.
  • Small to medium enterprises: X-Tek; LOT Oriel; Machine Vision Products; MICROTEL Tecnologie; Beta Electronics; USL.
  • End Users: Goodrich.
  • Research providers: TWI Ltd; Fraunhofer Institute; Kaunas Technical University.

Further information

Microscan website: www.microscan.eu.com
TWI's collaborative projects
Non-destructive testing at TWI
NDT technology group at TWI, email: ndt@twi.co.uk

Last Reviewed 2008 / Copyright © 2006 TWI Ltd