| Component Storage & Humidity matters | ||
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About a contribution to the storage of building parts
About the improvement of the dehydrating process The absorbation of humidity in plastic components from the air is a wellknown problem for the operators of SMT production lines. Deposit and a troublefree manufacturing of the hydrophile materials need an effective and exact process controlling. The storage and dehydrating of non hermetical boxes from humidityabsorbing stuffs are discussed for very long years in order to prevent Micro Cracks. For five years already the international IPC-JEDEC norms 020 and 033A classify eight degrees of humidity or respectively times of storage. As regards the quality security of SMT production lines they offer binding guide lines for the storage and dehydrating of very sensitive construction parts. An analysis of the weight increase (IPC-JEDEC-norm 020) defines the estimated time of storage. The analysis of the decrease of weight however calculates the necessary dehydrating time in order to eliminate the excessive humidity in the construction part. J-STD-033 defines in detail drying temperatures and times. The security field for the process of soldering amounts to 30 per cent reception of humidity respectively 0.1 per cent in the relationship to the weight of the building parts as the following illustration shows:  Soldering without plummet Up from 2006 the topic of the storage of the building parts will have an increasing significance. Up from this date a law will be in function which regulates the contents stuffs of the paste of soldering newly. Plummets and halogenes should no longer be contained. A disadvantage for the user : the plummet-free soldering paste can be only transformed with higher process temperatures (temperature 245 - 260 °C). The soldering temperatures with plummet containing soldering pastes are respectively for every process and material approximately around 220° C. This signifies for the quality security that the humidity in the IC building parts during the soldering process with higher temperatures goes on still quicker and explosively exceeds the package peak temperature (PPT) and goes over to the gas phasis. We are again at the beginning of the process engineering with the plummet change and have to ask ourselves how the production machines can be adjusted to the construction particularities and the conductor plate materials in order to meet the changed demands. Humidity-poor deposit optimizes the process development - IPC-JEDEC J-STD-020C The phenomenon of the micro cracks or the so called "popcorning" appears very often during the production of IC-building parts which contain humidity. It is true that the accrued use of MSL building parts as fine-pitch and ball grid array has drawn the attention to these disturbance mechanisms, but the topic is still not taken very seriously. Not all micro cracks are recognizable with the bare ear. The concerned IC-building parts will be further treated and through the delaminations and crackings in the box penetrate without disturbance humidity or other contaminents in the semiconductors. Corrosion of the conductor plates as well as conductor break and other incidences of breakdown as for instance a complete system disturbance can be observed over a longer period of time - this is regularly connected with a negative feedback of the customer. All different processing processes are concerned from the disturbance mechanisms independently from the soldering procedure. A damage of the IC components does not have to be visible as the chart shows. An effective quality security should therefore permanently supervise the effect and the height of the soldering temperatures and switch on different examination mechanisms as the sight and sound examination as well as electrical examinations.  The most frequent sources of mistakes are internal seperations delamination of the plastic from the founding form as well breaks in the founding form. A seperation from cable frames as well as the damage of the wire adhesive place and internal crackings that do not go through the surface belong also to the observable consequences of the soldering. Sometimes crackings can be seen at the surface and in the worst case the building part breaks during the soldering process ("popcorning"). Storage is not equal to storage - for the correct handling IPC/JEDEC J-STD-033A privides for The MBB is a humidity resistant bag in which the construction parts with the help of a dehydrated means and a humidity indicator card can be stored. It is especially well suited for the storage on the long run of IC building parts. A humidity stop provides for that no further air humidity can penetrate. The handling of the MBBs is workintensive and timeintensive. The drying in the oven is not unproblematic. Deficits are the high consumption of electricity, the risks of oxidation and the problems of soldering. Furthermore for every specific building part only one baking cycle is allowable. (T = 50° C, 60% rF, until seven days drying) The dehydrating over seven days has the advantage of a gentle dehydrating, but is because of the long interruption inconvenient for the very long interruption of the development of production. B) Short term baking procedure (T= appr. 140° C, 60% rF, six hours of dehydrating) An advantage of the short term baking process is the quick period of dehydrating; disadvantageous are the high running costs, oxidation of the wires and therefore the damage of the soldering and the therefrom resulting reduction of the life period of the building part. Nitrogen dehydrating cupboard The storage with nitrogen has the big advantage that the building parts are stored in a protected atmosphere. But the use of nitrogen is expensive, it produces high running costs, is maintenance intensive and a very low humidity can be guaranteed only difficultly. - Air dehydrating cupboards For several years there are switch cupboards that dehydrate with the dehydrating means zeolit or silikagel. Its running costs are lower and they are maintenancefree because the dehydrating means regenerates itself through warmth exchange. Its performance depends on the way and the quantity of the used dehydrating means, the efficiency of regeneration system and the insulation of the cupboard. Normally 1% can be maintained at closed doors. In the following we give you an approximate rule of thumb according to the measures of the IPC-JEDEC-norm 020C by which you can calculate the dehydrating storage time. Tl = Storage time (in hours) The time that one needs in order to store the building parts in the storeroom. To = Open time ZA time in which the components are exposed to the normal atmosphere after expiry of the admitted guarantee time. k = storage factor (IPC-norm=5) Tl [Std] = To* k (30 °C /60% rF) Final remarks If we look back to the history of the building part storage in particular for flat building groups, we would like to familiarize you here with some facts about the use and the application of the dehydrating storage cupboards. In Japan the dehydrating storage cupboards are for almost 16 years in production lines, in the USA for eight years and in Europe for almost four years. What does this mean for us as a service provider in the SMT-production? We are already for four years concerned with the topic "popcorn" effect and try to introduce since 1999 the necessary IPC-norms. Unfortunately only few firms investigate in the QS-management. Until now the production industry has investigated more time and money in the hightech process technology, as for instance the stocking, filter pressure, soldering and so on and has neglected the topic or has applied conventional packaging and storage. However this topic is only a low part of the whole problem that should still be solved. Building part storage systems are only a part of the whole process. Other questions need also innovative answers: How is for instance the leadfree transfer logistically solved? How should the stockkeeping look like? How is the situation as regards consumption respectively the end of range model of certain building parts? We are very curious to know the future solutions. Düsseldorf, November 10, 2004 Hoang-PVM GmbH www.hoang-pvm-enineering.com Author: Dipl.-Ing. QT. Hoang |
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