Printed Wiring Assemblies Research Articles

Theoretical foundations of post-solder cleaning process for printed wiring assemblies

The theoretical foundations of post-solder cleaning process in the microelectronic industry have been developed taking into consideration the geometry of a printed wiring board (PWB) and the nature of its surface. Optimal technological conditions (spray pressure, orientation of PWB, temperature, and the nature of the cleaning or rinsing liquids) have been found and recommended for different consecutive steps. The first step of the post-solder cleaning process, which has been analyzed, is penetration of a cleaning solvent, solution, or emulsion into the gap under a surface mounted component (SMC). Criterion for the prevention of air entrapment under an SMC has been formulated and analyzed for two types of mechanical conditions: spraying and immersing. The influence of orientation of the PWB (horizontal or vertical), spray pressure, or rate of immersing on the possibility of air entrapment during this cleaning process, as well as influence of wettability of a solid surface have also been analyzed. Corresponding optimal conditions have been found. The second consecutive step of cleaning process is dissolution of rosin or other residues. It can be accelerated by an increase in the equilibrium value of solubility of the rosin residue in a cleaning liquid and by an increase in the rate of its dissolution. This can be achieved by recommended optimization of the chemical composition of a cleaning solution (emulsion) and/or its temperature. The third step of the process is the replacement of the rosin or other residue by a clean solvent. It can be accelerated by (a) an increase in pressure during the rinsing of the PWB by a solvent or water; (b) a decrease in the viscosity of the rinsing solution. (c) The rinsing liquid should not contain any components which decline the solubility of rosin in the cleaning solution during replacement. (d) There are two cases depending on the wettability ratio of the solid substrate by the rinsing and cleaning solutions. If the rinsing liquid wets the solid surface better than the cleaning one, a higher interfacial tension is more desirable. In the opposite case, the rinsing liquid should be miscible with a solution (emulsion) for cleaning, or both rinsing and cleaning solutions should have low interfacial tension.

A rule based system for printed wiring assembly process planning

A prototype computer-aided process planning (CAPP) system, PWA_Planner (printed wiring assemblies planner), is presented. PWA_Planner is a rule based system for preparing assembly plans for the man...

Automation of Solder Joint Inspection Procedures Utilizing Laser Induced Infrared

In the rush to automate assembly flow in the fabrication and assembly of electronic equipment, great strides have been made in all areas with the exception of solder joint inspection and rework. This paper focuses on an attempt to automate, at least in part, the inspection of printed wiring assembly (PWA) solder joints. The use of laser and/or X-ray equipment is causing quite a stir in the industry due to the fact that there finally appears to be a technique that may allow for automation of one of the most labor intensive, inexact, and costly tasks in any PWA assembly operation--the visual inspection of solder joint quality. As the complexity of PWA's increase, the number and density of solder joints are increasing. This trend is resulting in decreased efficiency in the inspection process due to the shear number of joints to be inspected, operator fatigue, and the subjectivity of the entire inspection process. In areas where absolute quality is critical, such as military products, this declining inspection effectiveness is being compensated for by the incorporation of multiple inspection-rework loops. These techniques statistically should result in the identification of all defects on a given assembly. However, the impact of this multiple inspection process is one of total disruption to the assembly flow in an automated shop. Laser induced infrared (IR) inspection has been under investigation by Texas Instruments' Defense Systems and Electronics Group for the past several years. Our thrust has been aimed at the incorporation of this technique into the inspection of PWA's that are fabricated for military customers. Extensive efforts have been expended in the determination of the strengths and weaknesses of the technique in an attempt to augment the human element in the inspection process.

PWA_Planner - A rule based system for printed wiring assemblies process planning

Abstract Using rule based approach, printed wiring assemblies (PWA) process planning can be automated. In this paper a prototype computer-aided process planning (CAPP) system for PWA, PWA_Planner, is presented. The system takes a PWA design as input and generates process plan and part programs for the production on automatic component mounting machines. The modelling of the problem and the final results are discussed in the paper.

Troubleshooting Wave Soldering Problems with Statistical Quality Control (SQC)

In an attempt to reduce the incidence of wave soldering defects on printed wiring assemblies, Boeing Electronics Systems Division (BESD) has been investigating and implementing SQC (Statistical Quality Control). The concept is introduced in its historical context, explained and its application by this company to troubleshoot wave soldering problems outlined. Detailed results are provided to illustrate the value of the technique and indicate the important variables influencing soldering quality.

Quality Assessment

In striving to produce higher yields and fault‐free printed wiring assemblies, the PWB processor usually finds solder shorting or bridging to be one of the most difficult faults to eliminate. Finding himself faced with a high incidence of solder bridging, he usually attacks the problem by examining his own process parameters. Fluxing, preheating, solder composition and temperature, conveyor speeds, etc., are all assessed and adjusted so that the problem is minimised as much as possible. But still it appears, though at a lower level of incidence (l‐3%). With nothing else in his arsenal of weapons to attack this persistent problem, the process engineer is left somewhat helpless, wondering what further he can do to aid his course.

In-Circuit Test Systems-An Evolution

This paper surveys the development of in-circuit testing over the past decade and examines the reasons as to why this production floor technique for test and fault diagnosing of printed wiring assemblies has taken hold. Both controller based and minicomputer based in-circuit test systems are discussed, and reasons are advanced as to why minicomputer based in-circuit systems are becoming popular. Some of the reasons presented are their ability to employ the simple programming languages such as Basic. Moreover, users may readily add functional testing to a minicomputer based in-circuit test system by interfacing some of the programmable instruments now on the market with the input-output bus and the in-circuit test fixture. It is also pointed out that minicomputer based in-circuit test systems provide the capability to generate in-circuit test Programs by an operator, unschooled in minicomputer programming, who merely enters data from engineering documents. A test program generator generates the tests in the proper testing sequence. The paper Concludes with a discussion as to how in-circuit and functional testing can be combined effectively for optimizing test and fault diagnostic capability.

Printed Circuits and Microelectronics

In the two decades since the concept of ceramic based "printed circuits" was first suggested for an Army Ordnance application, substantial changes have taken place in the techniques of electronic circuit construction in the United States. Today, such ceramic circuits, in simple and complex networks, are complemented by printed wiring assemblies in several variations. Both technologies, now well established and in mass use on production lines, represent the current plateaus in miniature circuit construction for general commercial and military usage. Other construction philosophies and technologies are now shaping in the country's industrial and military laboratories, all aimed at new orders of size reduction of electronic equipments. In the several microelectronic techniques under development, the elemental electronic part appears destined to lose its logistic identity completely, and yield its classical position as a building block to black boxes called "circuit functions." The paper provides a summary review of the evolution of current ceramic printed circuits and printed wiring practices and, in the light of today's microelectronic activities, frames the trend in equipment design in the years ahead.