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Taking the Baseline
Soldering Processes Before you start with a task such as changing from lead-bearing solder to the use of lead-free solders, it is wise to consider what is actually happening in the company. Only once the actual facts have been put on the table can one begin to set out a route to pursue. That appears to be rather simple as we are usually pretty sure that we know what is happening in our house, aren’t we? Don’t we walk the hallowed halls nearly every day and by doing so pass all the relevant soldering machines? So, what’s doing? However, there are these psychological traps one tumbles in occasionally as daily routine may have taught us to ‘expect’ rather than to ‘see’. Looking properly is hard as the view is clouded by the expectation of what one wants to see. Our friends across the ocean have coined a word for it: ‘job-blindness’. Or is it a different phenomenon such as ‘familiarity breeds contempt?’ However, lets get serious about the effort and start walking about. We soon learn that for example the touch-up operators are using wires representing three different alloys:
There might even be four, a tin-silver. And we haven’t even considered whether there are also different fluxes within these solder wires. This should indicate to us that not everything is, as we thought it would be, and that a deeper analysis is in order.
Reasoning As we assume a) That most factories will not be able to work with just one alloy after a switch to lead-free technology and b) That the changeover must be complete, that is to be extended to the furthest corners and vestiges of the plant, and we will have to identify all the soldering activities that happen here. It will be unacceptable that any lead-bearing solders are still lingering on (except for the repair of old equipment under warranty – and which the law provides for). An accidental mixing of the lead-free with the lead-bearing solders in any of the joints may lead to catastrophic failure of the product.
How to Proceed How would one make sure – in a systematic and logical way – to ensure that all soldering processes are registered? Identifying the different soldering processes can be done in a variety of ways. If the company is very well organized it may be sufficient that the relevant personnel assembles in a conference room and exchange and tabulate their knowledge about the different soldering operations and their respective locations within manufacturing. On the other hand one may opt to follow each and every single product throughout its manufacturing cycle to register every time it is being soldered. We have to do this with all the different product, as there may be substantial changes from one manufacturing cycle and one product to the next. We may rely on the documentation established during the ISO 9000 certification or by physically marching up and down the different production lines. The best choice may be to do both. On the other hand one may refer to the layout of the plant and just add or highlight the areas where soldering takes place. Referring back to purchasing is another avenue open to track soldering processes. Theoretically, all flux and solder must pass through this office and most likely the storeroom. Following the path of materials may lead you to the places where soldering takes place. Whoever orders or accepts solder or flux will use it or knows at least where and who and why it is being used. Following these tracks may reveal quite a few hidden applications. As man, however, with respect to his inventiveness is very resourceful, we recommend that a physical examination of the plant be undertaken anyway – in addition to all the methods mentioned above. There may be surprises hidden that nobody suspects. Well-intentioned touch-up – not following the established procedures – may only be the tip of the iceberg. We have uncovered secret repair soldering operations even in storage rooms and tinning operations that were to re-establish solderability of components that nobody officially seemed to know about. Nevertheless, those too are soldering process that wee need to know about, as we cannot let them slip through our conversion system. Maybe your company also purchases perfluorinated ethers – nearly a sure sign that condensation soldering is used in your company. Such operations regularly fall through the cracks of investigation as they occupy niche technologies frequently in the ‘blind’ corner of your eye.
Different Solders When establishing the different soldering processes and their specific use you may already establish that several – lead-bearing – solder alloys are in use. It is essential that you find out: why? It is entirely possible that the touch-up and repair sections still use solders with some copper content – harking back to the times when solder iron tips were made of copper. The copper in the solder was to protect these tips. Although for more then a decade the tips no longer are made from copper, one may have ‘forgotten’ to change the alloy specification. In other operations tin-silver(-copper) may be used as certain joints are subject to more current flow and higher temperatures. During selective soldering operations some more exotic alloys may be used to meet certain product requirements. Do note down the reasons for the existence of the different solders as that may give you important clues when selecting ‘alternative’ solders later on. It is important to know whether certain practices are simply followed because of ‘tradition’ or ‘laziness’ or whether there are technical reasons or customer conditions attached to them.
Additional Questions As solder is delivered in a multitude of forms, we should make sure to mention these details about the soldering processes at the same time. There may be ingots, pieces, performs, paste or wire (cored and no-core) that arrive. Mark: not all replacement alloys are available in all these forms). How is a solder pot filled for the first time? {molten solder? chips? bars?}. How often are solder analyses performed? {e. g users have already noted the running-away of the copper content in SnAgCu solders during flow soldering operations}. Who collects and how is dross collected? {Dross will be much more expensive}. Where is the dross collected? {Normal dross consists of approximately 95% good solder and only 5% oxide and flux residues – bad collection practice may make recycling much more costly}. Do we have data on the amount of dross and its cost? {Such information will be very helpful when deciding on the use of nitrogen}. Is nitrogen already being used? {If so, the cost of installation will not have to be added to the general expenses when introducing lead-free alloys}. How do you recycle your dross? {If additional cost for separation has to be added, the recycler will charge extra}. How do you handle paste leftovers? {Some companies can successfully ‘recycle’ their old pastes in the flow soldering machine}. Do you have individual collection facilities for different metals? {Collecting lead-free alloys will have to be alloy-specific}. How do you buy solder – following which standard? {Most of the lead-free solders have not yet been added to industrial standards. No specification is available on the purity of virgin solders or on recommended contamination levels during use. Solder manufacturer extrapolate the existing standards of lead-containing solders to deliver their product. So far it is unknown whether this practice is acceptable or not. We still have a lack of scientific information}. How do you store your solder? {Different alloys have to be stored separately to ensure that no accidental mix-up can happen}. Has personnel on the line been informed about the change and that different solder alloys will be in use. Do they understand why? {It is always better that manpower understands the reasons for the process rather than to follow blindly some instruction}. We elaborated in brackets why some of these questions and the answers to them will make themselves heard during conversion. Several will be quite critical others may be violated only to produce some additional expense. Fluxes too will pose a challenge when converting to lead-free technology. Even now during SnPb soldering we often find that companies apply different fluxes on the same assembly: during flow soldering, during reflow soldering, during selective soldering and during touch-up and repair. The result may be a real cocktail – or speaking frankly: a chemical time bomb whose detrimental behavior cannot be predicted. We do not approve of such processes even for lead-bearing applications and we definitely abhor it for lead-free. This trap may snap more easily for lead-free, as the fluxes we probably will have to employ will leave more residues. Though we will treat this topic in more detail in one of the later articles, here just so much: these residues will be subjected to higher temperatures and should it become necessary to clean, removing them will be so much more difficult. We will have to define those fluxes used in the different soldering processes much more clearly. The EU-Directives [DIRECTIVE 2002/ /EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL on the restriction of the use of certain hazardous substances in electrical and electronic equipment] does not only limit the use of lead but impacts the Printed Wiring Boards [PWB] by forbidding the use of halogenated flame-retardants. It is, therefore, highly recommended, that additional information be collected at the same time. A first survey of all the PWBs used in production may be in order. Later on, we will deal with the properties of the laminate material and the final metallic surface, as they will have to be compatible with the lead-free solders. Many European firms employ a high-mix/small charges production environment – quite in contrast with the single-product/large run manufacturing commonly seen in the US and Far East. Hence European companies have special challenges to face as controlling such variety requires other means of management than the US or Far East situations. And it is rather common that boards are purchased from a multiplicity of board producers. Unbeknownst to many users there is also the good possibility that PWBs ordered from one of the European board shops may actually originate in the Far East. Price pressure exerted on the PWB manufacturers has necessitated cooperative contracts with shops in the Far East. Questions with regard to metallization or solder resist or even about the cleanliness level of the product are frequent. As the European partner may have difficulties controlling – or even dictating such detail to - his Far Eastern associate on that level, there are few answers. To make matters worse, the PWB industry did not yet arrive at an agreement on such important questions as the clear definition of FR4-Hi-Temp substrate material. The more we know about our PWBs – and we will have to treat of this topic separately again – the better we are equipped to discuss such details on a technical level with our PWB purveyor. It may even be necessary that we spell out the precise properties to his technical staff. |