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Organic solderability preservative (OSP) coatings are not new. They have been used successfully with aggressive water soluble flux for assembly of through‐hole only PWBs. However, the multiple heating cycles required for mixed technology assembly and use of no‐clean low solids flux (LSF) for wave solder assembly have placed a greater demand on the solderability protection provided by OSPs. Wetting balance and float testing were used to evaluate numerous OSPs as well as the potential for these surface finishes to be used for ‘No‐Clean’ assembly. Although these laboratory evaluations revealed that OSPs are not as robust as SnPb, they did indicate the assembly processes and materials which could work with OSPs. Additional simulated assembly trials with test vehicles confirmed that thick OSP pre‐flux coatings interfere with soldering and that the solderability of surfaces with thin OSPs degrades when heated in an air environment. Since none of the OSPs evaluated outperformed the imidazole currently in use at AT&T, a no‐clean LSF assembly production trial with a mixed technology telecommunication circuit pack was conducted to compare imidazole with hot air solder levelled surfaces. The production trial and laboratory evaluations resulted in the development of an application model. The elements of the application model are not complicated: (1) use thin OSPs, (2) avoid baking, (3) use as aggressive a flux as possible, (4) apply as much flux as possible, (5) apply the flux where you want solder to wet, and (6) use nitrogen inerted processes whenever possible. Combination of these elements has led to the successful implementation of OSPs for no‐clean assembly. Funding for this effort was obtained through the National Center for Manufacturing Sciences (NCMS) Printed Wiring Board Interconnect Program.

Substitution of lead‐free solders in electronic assemblies requires changes in the conventional Sn:Pb finishes on substrates and component leads to prevent contamination of the candidate solder. Options for solderability preservative coatings on the printed wiring board include organic (azole or rosin/resin based) films and tin‐based plated metallic coatings. This paper compares the solderability performance of electroless tin coatings versus organic azole films after exposure to a series of humidity and thermal cycling conditions. It is shown that the solderability of immersion tin is directly related to the tin oxide growth on the surface and is not affected by the formation of Sn‐Cu intermetallic phases as long as the intermetallic phase is protected by a surface Sn layer. For a nominal tin thickness of 60 ?inches, the typical thermal excursions associated with assembly were not sufficient to cause the intermetallic phase to consume the entire tin layer. Exposure to elevated temperatures, in the presence of humidity, promoted heavy tin oxide formation which led to solderability loss. In contrast, thin azole films were shown to be more robust to humidity exposure; however, upon heating in the presence of oxygen, they decomposed and led to severe solderability degradation. Evaluations of lead‐free solder pastes for surface mount assembly applications indicated that immersion tin significantly improved the spreading of Sn:Ag and Sn:Bi alloys compared with azole surface finishes.

The emergence of new interconnection technologies involving double‐sided surface mounted components has put stronger restrictions on the method of preserving the solderable finish on printed circuit (PC) boards. The popular Sn/Pb coatings have come under strong scrutiny due to environmental hazards of lead and also because they do not provide flat, planar surfaces for SM assembly. Organic solderability preservative coatings (OSP) are emerging as strong contenders for replacing Sn/Pb surface finishes. Benzotriazole based organic coatings have been successfully used in the past by several electronics manufacturers. However, assembly technologies involving multiple thermal operations have necessitated a fundamental understanding of the thermal stabilities and the mechanism of corrosion protection provided by the OSPs. This paper reports the results of an investigation of the thermal stabilities of two organic corrosion protection coatings. Although both are organic azole based, they operate in two distinct regimes: one forming thin films (∼100 Å) and the other forming thick films (∼5000 Å). The mechanism of surface protection has been studied using direct surface analytical techniques such as X‐ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), scanning transmission electron microscopy (SEM/TEM) and Fourier transform infrared spectroscopy (FT‐IR). The solderability of the copper was measured by wetting balance techniques and correlated to the amount of copper oxidation. The results indicate that, although the thin films provide excellent protection for storage and handling operations, they decompose under heat, thereby causing oxidation of the copper. The thick films appear to withstand multiple thermal cycling. However, the underlying copper substrate can still be oxidised by oxygen diffusion through pores or cracks, or the film may undergo chemical changes that render the copper unsolderable.

When implementing chip‐on‐board (COB), the surface finish on the printed wiring board needs to be chosen with several key considerations in mind. The choice of bonding technology dominates the selection of finishes. Wire bonding, tape automated bonding and flip chip assembly all have their own special requirements for effective assembly. The considerations must also examine the presence of intermetallics in the solder joints. These issues are discussed along with several advanced techniques for applying solder coating and organic solderability preservatives on bare copper PWBs.

The purpose of this paper is to present an overview of the evolution of organic solderability preservatives (OSP) technology from a temporary anti‐tarnish to an approved process for the lead‐free assembly of high‐reliability printed circuit boards (PCBs).

As this is an informational paper, the main approach was to compare and contrast various types of OSP molecules and their fitness for use.

The active ingredient of an OSP is one of the most important ingredients that can help to improve the heat resistance, which becomes much more important in conjunction with lead‐free solder. The latest OSP shows excellent stability, even at more than the peak reflow temperatures used with lead‐free solders. The success of any OSP process is highly correlated to process control and maintenance and OSP formulations are not created equal. The thickness of the film is only one difference. However, the key differentiators for OSP coatings are the overall ability of the organic film to resist oxygen penetration and humidity; and withstand the higher temperatures of lead‐free assembly. The author suggests that original equipment manufacturer (OEMs), contract manufacturers and PCB fabricators perform testing and qualification of OSPs using industry‐accepted reliability and solderability testing methods.

This work represents the author's extensive experience with OSP technology, as well as the results of a collective collaboration with Shikoku Chemical Corporation. The findings represent reliability and solderability testing carried out in various manufacturing sites including cooperation with certain OEMs that are users of circuits fabricated with OSP technology.

This paper details how critical advances in OSP technology have enabled improvements in solderability and reliability over earlier commercial generations and have further enhanced the solderability of lead‐free assemblies.

The purpose of this paper is to investigate the influence of Bi additions on the wetting properties of SnZn7Bi alloys (Bi=1 and 3 per cent by mass) on a copper substrate and printed circuit boards (PCBs) with lead‐free finishes (SnCu, immersion Sn, Ni/Au, organic solderability preservative) in the presence of fluxes. The practical implications of the results is the main purpose of these investigations.

A wetting balance method was used for wetting measurements at 230 and 250°C in nitrogen and air atmospheres in the presence of ORM0‐ or ROL0‐type fluxes. The PCBs were investigated ‘as received’ and after accelerated aging. The analysis of variance (ANOVA) analysis was performed in order to explain how the main factors of the experiments (the Bi content in the alloy (1 or 3 per cent), the test temperature and the test atmosphere) influenced the wetting ability of SnZn7Bi on Cu substrates.

As expected, a higher temperature and a higher Bi content in the alloy favoured the wetting of the copper substrate in the presence of the ORM0‐type flux in a nitrogen atmosphere. These results were confirmed by ANOVA analysis. Very good results were also obtained for the SnZn7Bi3 alloy's wettability on “tin coatings” on PCBs (SnCu and immersion Sn) both “as received” and after aging, in the presence of the ORM0‐type flux, for all the applied testing conditions (in both temperatures and N₂ and air atmospheres). The less active flux (ROL0) caused a worsening of the alloy's wettability properties; however, the PCBs with SnCu and immersion Sn finishes maintained their wettability, even after aging, at very good and good levels, respectively.

It is suggested that further studies are necessary for confirmation of the practical application, but they should be limited to the soldering of SnZnBi3 on PCBs with “tin coatings” and the quality of the solder joint performance.

The best SnZn7Bi3 wetting results on PCBs with “tin coatings” (SnCu and immersion Sn) at 230 and 250°C and in N₂ and air atmospheres suggest the possibility of a practical usage of the tin‐zinc‐bismuth alloys for soldering in electronics using both the ORM0‐type flux and the even less active ROL0‐type flux, which are currently used in industrial lead‐free soldering processes.

The wetting balance method, combined with ANOVA was used as the quickest way to determine the wettability properties of SnZn7Bi on Cu substrates. Wettability measurements were also performed on the SnZn7 and SnZn7Bi alloys with different lead‐free finishes, in different experimental conditions.

That the introduction of the Control system should have given rise to a considerable amount of criticism, both appreciative and adverse, was naturally to be expected. The appreciative remarks which have appeared in the press, and those also which have been privately communicated to the directors, indicate that the subject has been intelligently considered, and in some cases carefully investigated and studied. The opinions given are worth having on account of the position and influence of hose who have given them, and on account of the obvious freedom from bias which has characterised them. This is so far satisfactory, and goes to show that the success which has attended the working of the Control system abroad may well be expected to attend it in this country as soon as it is sufficiently well known to be appreciated by the manufacturers and vendors of good and genuine products, and by the general public, whose best interests it cannot but serve.

A pæan of joy and triumph which speaks for itself, and which is a very true indication of how the question of poisonous adulteration is viewed by certain sections of “the trade,” and by certain of the smaller and irresponsible trade organs, has appeared in print. It would seem that the thanks of “the trade” are due to the defendants in the case heard at the Liverpool Police Court for having obtained an official acknowledgment that the use of salicylic acid and of other preservatives, even in large amounts, in wines and suchlike articles, is not only allowable, but is really necessary for the proper keeping of the product. It must have been a charming change in the general proceedings at the Liverpool Court to listen to a “preservatives” case conducted before a magistrate who evidently realises that manufacturers, in these days, in order to make a “decent” profit, have to use the cheapest materials they can buy, and cannot afford to pick and choose; and that they have therefore “been compelled” to put preservatives into their articles so as to prevent their going bad. He was evidently not to be misled by the usual statement that such substances should not be used because they are injurious to health— as though that could be thought to have anything to do with the much more important fact that the public “really want” to have an article supplied to them which is cheap, and yet keeps well. Besides, many doctors and professors were brought forward to prove that they had never known a case of fatal poisoning due to the use of salicylic acid as a preservative. Unfortunately, it is only the big firms that can manage to bring forward such admirable and learned witnesses, and the smaller firms have to suffer persecution by faddists and others who attempt to obtain the public notice by pretending to be solicitous about the public health. Altogether the prosecution did not have a pleasant time, for the magistrate showed his appreciation of the evidence of one of the witnesses by humorously rallying him about his experiments with kittens, as though any‐one could presume to judge from experiments on brute beasts what would be the effect on human beings—the “lords of creation.” Everyone reading the evidence will be struck by the fact that the defendant stated that he had once tried to brew without preservatives, but with the only result that the entire lot “went bad.” All manufacturers of his own type will sympathise with him, since, of course, there is no practicable way of getting over this trouble except by the use of preservatives; although the above‐mentioned faddists are so unkind as to state that if everything is clean the article will keep. But this must surely be sheer theory, for it cannot be supposed that there can be any manufacturer of this class of article who would be foolish enough to think he could run his business at a profit, and yet go to all the expense of having the returned empties washed out before refilling, and of paying the heavy price asked for the best crude materials, when he has to compete with rival firms, who can use practically anything, and yet turn out an article equal in every way from a selling point of view, and one that will keep sufficiently, by the simple (and cheap) expedient of throwing theory on one side, and by pinning their faith to a preservative which has now received the approval of a magistrate. Manufacturers who use preservatives, whether they are makers of wines or are dairymen, and all similar tradesmen, should join together to protect their interests, for, as they must all admit, “the welfare of the trade” is the chief thing they have to consider, and any other interest must come second, if it is to come in at all. Now is the time for action, for the Commission appointed to inquire into the use of preservatives in foods has not yet given its decision, and there is still time for a properly‐conducted campaign, backed up by those “influential members of the trade” of whom we hear so much, and aided by such far‐reaching and brilliant magisterial decisions, to force these opinions prominently forward, in spite of the prejudice of the public; and to insure to the trades interested the unfettered use of preservatives,—which save “the trade” hundreds of thousands of pounds every year, by enabling the manufacturers to dispense with heavily‐priced apparatus, with extra workmen and with the use of expensive materials,—and which are urgently asked for by the public,—since we all prefer to have our foods drugged than to have them pure.

Over the last few years, the emergence of new European draft legislation has focussed electronics industry attention on the likely ultimate proscription of lead in electronics assembly. Much work has already been undertaken to identify the possible alternatives to conventional tin‐lead solders and to evaluate their performance benefits and limitations in comparison with the traditional materials. Although, some companies are already offering products manufactured using lead‐free products, there is still a widespread lack of activity in many areas. With this none‐too‐distant deadline rapidly approaching, Envirowise has sponsored this paper as part of its coordinated activities to assist the UK electronics industry and to promote environmental efficiency and best practice. This paper details the current situation with respect to the drivers towards the adoption of lead‐free assembly before giving an overview of the current situation. This paper concludes with details of sources of further information.