Search results

The 1988 Education Reform Act has profoundly affected the organisation of education in England and Wales. In common with changes in several other jurisdictions, the power of both the central government and the schools has been increased and that of the local authorities decreased. The major changes involve (a) introduction of local management of schools by school governing bodies; (b) introduction of both a national curriculum and a national scheme for assessment of pupils; (c) reduction of the operational role of local education authorities, which will become more involved in planning, policy making, and monitoring; (d) more parental choice of schools that their children attend; (e) the possibility that individual schools can apply directly for national funding; and (f) establishment of city technology colleges. Several implications of those changes are presented, together with positive and negative reactions. The applicability of some of those changes to other systems, especially in Canada, is discussed.

The amount of gas blowing into the molten solder in plated‐through‐holes during wave soldering is dependent upon the moisture content of the board and the strength of the copper barrels to withstand the pressure of gas. This strength is dependent, in turn, upon both the thickness of the copper electroplate and the quality of the original electroless copper deposit. The problem of blowholing may be overcome by improving the copper quality and/or by baking the PCB prior to soldering. This paper gives a scientific framework linking the important parameters involved in the control of blowholing. Full data are given to enable the allowable moisture content of a PCB to be calculated in respect of the pressure generated during soldering and of the strength of the copper barrels. If baking is required to attain the required low moisture levels, then certain data are required to quantify the effects of the storage times and the environment conditions between that bake and the subsequent soldering. These data are given.

One family of defects in soldered electronic assemblies that is almost invariably re‐worked is that due to outgassing, manifested as visible blowholes and solder blow‐out. It is known that re‐working can be very detrimental to the service life of an electronic assembly and should be avoided whenever possible. This paper describes work aimed to determine whether outgassing faults such as blowholes are harmful to service performance or whether more harm will be done by re‐working such faults. Standard test plated‐through‐hole (PTH) assemblies with controlled degrees of outgassing faults have been subjected to mechanical testing, thermal shock testing, mechanical fatigue, low cycle thermal fatigue and corrosion testing. Measurements in all these régimes have been carried out quantitatively with baseline controls. No evidence of significant loss of solder joint performance has been found, even for severe cases of solder loss. On the contrary, in cyclic fatigue testing, solder fillets with outgassing faults exhibit statistically significant performance enhancement. The conspicuous nature of blowholing and solder blow‐out undoubtedly over‐emphasises the problem during visual quality control inspection. Provided the copper barrel has been wet by the solder, outgassing faults should not be re‐worked. These faults should be used as process indicators and to draw attention to processes and the need for process control.

Exemplary data are given that demonstrate unequivocally the inapplicability of commonly used visual inspection criteria for judging the long‐term service reliability of soldered joints. The data are obtained from a controlled model system. Plated‐through hole (PTH), wave soldered joints are used in this work, having a controlled degree of voiding, blowholing and solder blowout, covering the full range of normal visual assessment, from perfect to very bad, for this particular type of visible fault. The joint performance is assessed in terms of its joint pull strength, thermal shock, mechanical high frequency fatigue, low cycle thermal fatigue and propensity as a corrosion initiation site. No evidence was found that the visible outgassing faults degraded the solder joint performance. Indeed, in all the fatigue tests, the visually unacceptable solder joints performed significantly better. Furthermore, in a related research project, manual reworking of joints has been found to be demonstrably detrimental to solder joint reliability.

The importance of the quality of the plated‐through‐hole copper barrel in double‐sided and multilayer PCBs is considered with regard to the problem of voids and blowholes in the solder fillet. The thickness of the copper, and its integrity and adherence to the drilled surface define its ability to withstand the pressure burst of gas from the outgassing laminate during the few seconds of the thermal spike induced by the molten solder prior to solidification. The ability of copper electroplate to bridge over areas devoid of electroless copper and produce a barrel free of pinholes is shown to be crucial to this problem. In addition, the use of a nickel layer is shown to enhance greatly the impermeability of the barrel to the evolving gases.

Data from laboratory and production scale tests are given that show that the efficiency of catalyst adsorption controls the coverage by the electroless copper deposit in plated‐through‐holes in FR‐4 laminate, and that this, in turn, governs the outgassing performance of the finished board. The nature of electroless copper nucleation and growth is discussed and the reasons for the formation of voids in the deposit are identified.

For a given batch of printed circuit boards of a particular quality and with a particular level of moisture absorption, the parameters of a wave soldering machine may be tuned to minimise the amount of visible blowholing. This paper shows the inter‐relationships of blowholing with other soldering faults such as icicling, bridging and poor pull‐through. The inter‐relationships result from adjusting each machine parameter in turn. This is important since blowholing cannot be minimised without regard to these other effects. The effect of the quality of the printed circuit boards on the response to machine parameters is predicted from a model, and measurements are found to be in agreement with the predictions.

The drilling and preparation of a hole in FR‐4 laminate prior to the deposition of electroless copper is considered in relation to the quality of soldering achieved on the finished printed circuit board. Data pertaining to the drill speed, drill feed, and stack position are presented and the effect of drilling temperature is demonstrated. The variability of laminate is discussed in relation to outgassing during soldering. Finally the importance of the post‐drilling treatment of the hole‐wall is shown. The relative effects of baking after drilling, ultrasonic cleaning and chemical treatments such as alkaline potassium permanganate are illustrated.

The three European Community manufacturers of commercially available wetting balances for measuring the solderability of electronic components have participated in an intercomparison assessment of surfaces of standard solderability that have the potential for calibrating the balance instruments. The development of the standard surfaces, as well as the small adjustments needed to improve the uniformity of design of the different instruments, and the standardisation of the procedures for their use, have been described in the preceding papers in this series. This paper gives details of the experimental procedures used for the intercomparison and the significance of the comparative data from each balance type.

The wetting balance has been identified as a technique suitable for the establishment of a quantitative basis for the measurement of the solderability of electronic components. This paper deals with those parameters of the instrument and those procedures of the measurement that fall within the choice of the operator. It is important to know the amount of attention that must be paid to each parameter to achieve the precision required.