Search results

This paper aims to describe some of the European Space Agency (ESA) workmanship standards which are based on industrial practices. Coordination with National Aeronautics and Space Administration (NASA) Centres on workmanship issues is also to be described.

The first ESA (then ESRO) workmanship standard was published in 1972. It applied to the hand soldering of electronic assemblies and became a contractual document, either replacing, or supplementing a vast number of company standards that had existed in European manufacturing companies. Since then some 40 standards related to Materials and Processes have been developed by ESA, the majority of which are to be found in the European Cooperation for Space Standardization (ECSS) series of European standards.

It is shown that, where appropriate, the standards are backed by a number of ESA‐approved skills training schools which provide training courses that result in the certification of operators, inspectors and instructors.

The workmanship standards are periodically revised in order to take into account newly developed technologies, the results of failure analyses (lessons learnt), laboratory and field test results, as well as some particular requests from industry.

This paper aims to evaluate the morphology and thickness of oxides that form on the surfaces of tin whiskers. The problems related to the growth of tin whiskers are stated, and the relevance of oxide layers adhering to whiskers is discussed.

Modern laboratory methods including focused ion beam sectioning, energy dispersive spectroscopy and X-ray photoelectron spectroscopy have been used to characterise the composition of oxides present on the surfaces of 48-year-old whiskers. These very old whiskers had nucleated and grown on electronic equipment stored at ambient temperatures. They were compared to the oxide layers on newly grown 2-week-old whiskers.

A dual oxide film, consisting of stannous and stannic oxides, was found present on both the old and the new whiskers. Measurements of oxide thickness were established for both generations of whiskers and these were noted to be similar to those films present on pure, cleaned bulk tin.

Only very new and very old whiskers, and their oxide films, were the focus of this investigation. However, sufficient data were gained to predict the effect both kinds of oxide films would have during whisker bridging between conductors and the risk of short circuits. Thick oxide films (order of 30 nm) may have a greater resistance to shorting, but they will be more difficult to remove during solder dipping (with respect to whisker mitigation).

A knowledge of the oxide thickness on growing/gyrating tin whiskers will provide the electronics industry with data useful for establishing the risk of short circuits. It will also be useful during the forensic work associated with component and assembly failure analysis.

The data resulting from this study are unique. They are of value to others who may require knowledge of the morphology, composition and thickness of oxides present on tin whiskers of different vintage.

The purpose of this paper is to present a process (chemical stripping) that can be used to remove pure tin plating from component leads and propose this activity as a mitigation strategy for tin whisker risk reduction associated with spacecraft hardware. The problems associated with the nucleation and growth of tin whiskers are stated. Present contractual requirements, having the purpose of avoiding the selection of pure tin plating, are highlighted to those not familiar with spacecraft standards.

The key issue here is that the process must remove pure tin from the entire length of terminations present on a wide range of component types. A resilient component can be dipped into liquid solder directly to the component's body in order to remove pure tin by alloying it with tin‐lead. However, delicate heat‐sensitive devices and those with glass‐to‐metal seals can suffer from the effect of thermal shock associated with immersion of their leads/terminations into molten solder. Solder dipping was performed on chemically stripped terminations but, in the specific case of the delicate components, the solder was prevented from wetting along the complete length of the exposed base metal or barrier layer metal. Any negative effect of the chemical stripping solution was assessed by inspection of each components' markings, their construction materials and, after tin‐dipping, a wide range of standard tests were performed on re‐processed devices in order to check that they had not degraded either physically or electrically.

The majority of components that were re‐processed by chemical stripping, and subsequently solder‐dipped, suffered no degradation from either of these processes. In order to avoid the removal of identification markings it was found that the bodies of certain components should not be completely immersed into the chemical stripping solution. Devices sensitive to heat or thermal shock could be stripped and their leads partially re‐tinned by immersion into liquid solder to a predetermined depth. A short length of exposed bare termination substrate adjacent to a seal is considered, for the purpose of this study, to be acceptable.

The European space industry has not previously utilized chemical stripping as a viable method for removing pure tin from components and other space hardware. The complete procedure for stripping and solder‐dipping will be of benefit to companies manufacturing products for industries that have exemption from the RoHS lead‐free directives.

This paper aims to present the results of a 32-year-old laboratory study of whisker growth from tin electrodeposits that was originally undertaken to gain an increased understanding of the phenomenon of tin whisker growth.

Whisker growth was evaluated using electroplated C-rings (both stressed and un-stressed) that were stored throughout in a desiccator at room temperature. Analysis has recently been undertaken to evaluate whisker growth and intermetallic growth after 32 years of storage. Scanning electron microscopy analysis has been performed to investigate whisker length and, using polished cross-sections, the morphology, thickness and type of intermetallic formation.

Normal tin-plated deposits on brass and steel with a copper barrier layer nucleated whiskers within five months, and in each case, these grew to lengths between 1 and 4.5 mm. For normal tin electroplated onto brass, a one- or two-month nucleation period was needed before whiskers developed. They reached a maximum length of about 1.5 mm after six months, and little or no further growth occurred during the subsequent 32 years. Very few whiskers grew on the tin-plated steel samples and no intermetallic formation was observed. None of the fused tin plating samples nucleated whiskers during the 32-year period.

Knowledge about vintage whiskers is important to take steps to increase the resiliency of space missions. Similarly, such knowledge is important to engineers engaged in products reaching their nominal end-of-life, but where, for reasons of economy, these products cannot be replaced.

This study represents a unique insight into whisker growth and intermetallic formation over an extremely long time period.

The more severe cases of mental subnormality have shown a fairly random social distribution, but mild subnormality is typically a lower‐class phenomenon. Various deterministic formulations have been offered in explanation of this, ranging from hereditarian notions of “bad stock” among the lower classes to environmentalist em‐phasis upon the debilitating effect of poor social circumstances. This article argues that such attempts to account for the social distribution of subnormality in terms of aetiology are premature, because the processes whereby people come to be labelled as subnormal are largely unexamined.

Reports that have reached us of the installation of Sir Philip R. Morris as President of the Library Association on January 28th assure us of the contribution he may make to the Association. As the retiring President, Mr. Oldman said, and as we know, his main interest has always been education and, as the Association has many projects in that field and some problems yet unsolved, he welcomed Sir Philip especially in that direction; but our new President has much experience of libraries in spite of his disclaimer of qualifications in our direction. He is a Carnegie Trustee and, unofficially, he connects us again with the body to which our profession owes so much and, as for lack of experience, one who has been Director of Education for Kent and therefore the ultimate official chief of the great County Library system there, cannot lack it. From what we hear of this speech—which we hope will be published in its complete but all too short length in the L.A. Record—we look ahead with confident pleasure to the Address he will give us at the Southport Conference in September.

This issue is a selected bibliography covering the subject of leadership.

The transition to surface mounted device (SMD) technology in electronics manufacturing has placed new demands on the post‐solder cleaning process. For spacecraft electronic systems it is of the utmost importance that all flux residues be removed. This paper reports the results of an investigation of the impact of component stand‐off heights and the distance between solder joints on the cleaning process efficiency. The capability to clean beneath large chip carriers was evaluated for four different cleaning methods using isopropanol or CFC‐113 (Freon TMS) as cleaning liquid. The results show that the cleaning efficiency decreases considerably if the stand‐off height is less than 240 µm for 100 mil pitch chip carriers. For 50 mil pitch chip carriers the stand‐off height needs to be greater than 240 µm to achieve high cleaning efficiency. The cleaning efficiency beneath chip carriers with small stand‐off heights can be increased by using ultrasonic cleaning. However, a very thin layer of white residues is left where the flux has been removed if isopropanol is used as the cleaning liquid.

ELSEWHERE in this number we list libraries which have Esent us copies of their annual reports which we are glad to have. Now and again we are able to elaborate on these, but in the present issue that has not been possible. We would say, however, that these reports are deserving of the attention of librarians generally, and of students at the library schools. They are records of work in progress, and they do suggest the development of library policy. The best of them are of textbook value.