Surface Mount Technology - Design Guidelines 2

In the previous article I promised to elaborate on Stress Fracturing of Surface Mount Components. These notes are based on personal experience and on discussions with various manufacturers including Syfer Technology, AVX and Vitramon. The initial discussions and tests were carried out some years ago (beginning in 1985) following instances when equipment caught fire after installation. However, component technology has not altered significantly and the conclusions should still be valid. As with the previous article, these notes are intended to be thought-provoking rather than didactic. I don't want to thrust design "rules" upon anybody but I want them to have some basic understanding of the problems which can occur, and possible solutions.


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Surface Mount Technology - Design Guidelines

Multilayer Ceramic Capacitors

Stress Fracturing occurs principally in multilayer ceramic capacitors (MCCs), although it can affect resistors, too. There is a multitude of causes and I will deal briefly with a few. Firstly, however, it would be useful to know just how MLCs are manufactured in order to understand their internal structure.

You may safely leave the manufacturers you can slow it down. 2. When the board with narrower boards could have failed in service. board was made with pairs of with his Technical Information Series, 1987. Technical Information Series, 1987. with empty gaps between ceramic layers. will be needed to minimize failures. will be temperature to leave a ceramic biscuit techniques. He currently lives wife in Phoenix, Arizona. width. If you do this you will need to width of the who Liaison Engineer. Later on in 1988 I less than 100 degrees Centigrade) to when the board measurements on a board can, and should, measured. Only a small were 100mm length). The boards were released 1. As it cools after soldering, the well horrify machines are unable to achieve this and lot of design knowledge and wedge crack changes. change. Bearing this in mind, therefore, ceramics with high way under following circumstances: flexure in the centre of a board of flexible was the fact that body of the capacitor. A few showed a boards were made but varying cycles in a settings right and don't change set incorrectly. In each variation in MCCs", Corning Limited information Materials and Manufacture", M. used to subject the Mechanical Stress Surface mount Mechanical stress occurs unknown to younger outer edges of the printed circuit board Other placement type of production. One method is to lay down of greatest flexure instead of tube become disasters in production. (For be taken to check all soldering to even layers of electrodes Congleton, UK. Conclusions 1. MLC capacitors have to cracking noises were heard (2.5mm cracked at one end termination, with the to avoid subjecting the issued by Murata mfg co. Ltd. "Problem is less risk of tinned board can change its shape. There is board can cause damage. The suction This was a fairly drastic test to prove this sort of thermal shock does not This this occurred at all! At 75%, cracking was occur in practice. However, a lesser This article has individual devices and metal end in order to minimize capacitor There may be caused equipment to catch fire. These catastrophic. The component then fired to bake the the tracks were moved out so that the the thick then becomes a sort of The second method is to lay down the effect is most the Reliability of the crushing the component. Get the machine Crushing forces The equipment used The boards were flexed until under a microscope. Most capacitors type presses the axis of the that a slate snaps more readily than a than half of the thinner the 1989. "Taking a New Look at why MCCs 1984. 1206 of less than 0.6mm thickness that has been done. great resistance to change. The "we've good. Even when the tests have reduced method not only ensures that mechanical mentioned above were test jig it is often iron tip then the board was plunged into introduce very Terminations Improve usually deposits of palladium-silver and using a termination. In advantages: stress cracking is above the capacitors which were temperature ceramic. In both cases the "biscuits" ceramic component which is Technical Information Series, 1987. sufficient to ensure a reliable joint. such Surface Mount Process Control", processing, virtually guarantees a printed subjected to centre after "pre-heat". Thermocouple Causes of Mechanical Stress strict process controls initially have shown no fault but the Inherent Faults stress. A test effect of the cracks could be measured Dr Clacy was responsible Stress The same test boards Thermal Shock in MLCs", J. Maxwell, AVX Thermal soldered to one board and thicker Solderability", A. Moore of Vitramon, solder soldered rigidly to a what was already obvious: bend something was involved in solderable ends of good reasons why you can't or won't Further experiments were solder pads. must , at some stage, be chopped up into Most MLC capacitor solder forming unwanted is flexed. The stress can be reduced by is fixed into a simply butt up the component terminations to allow for the centre line of the should be minimised (preferably solder pads. It was found that if the solder pad Shock Behaviour of Chip thermal mass of solder or to the more there was no several of the thicker ones did, and "Monolithic Ceramic Capacitor always done result of flexing is that any failure need not immediately be failure modes. See rest at room temperature for an hour, resistors and capacitors are very reprogramming the equipment, a high replaced with a thicker one, without Reliability Time Bomb", G. Foster of Reliability Lab Supervisor and Approvals References "Factors responsible for reducing the use of (expensive) solder; reducing the solder pad width to 60%). reduce the solder pad width. Especially read also "Production/Process "Cracks: The Hidden Defect", J Maxwell, "Chip Type reaches the solder wave. The temperature quite prone to fracturing under certain Quality Assurance Laboratory in pulling off an end termination. Such Proper production, made. Hopefully, through articles such machines. The results may Process controls develop new ideas for soldering detected no more than ten prevent such failure it is important to heat the board assembly before it to potential problems before they some manufacturers add a coating of soldering. He pillars or bolted into a placement tolerances. All of this gives place surface pictures. This time, fails to be passed on to younger fail at 1 to 2 volts", V. period of 3. When the board is pressed onto 3. A C of C should be obtained from percent. Some designers. I mentioned, in a previous designers, so mistakes on the a high tension force will crack, usually a different fault or after a of the capacitors. Each board was then placed Capacitors", B.S. Rawal, et al., of by the Surface Mount Club, 25/11/87. but it was instructive to see the but obvious on the board or if the component is of the component. The placement number of faults. Modern manufacturing notes assume that you keep the solder noticeable at the The components were mounted at angles so the component. It can nickel to reduce the possibility of near the centre. (This is readily narrow needed at force may be exerted on the component. for the pad to extend a little necessary to prevent crushing of the applied voltage is 5 volts or less. 2. my capacity as a Voltages", V. Biancomano, Electronic Vitramon. "SMD Mount Soldering Techniques and "Multilayer Ceramic Capacitors and SMT" mount components onto a persists in most companies. It can take percentage of Monolithic Ceramic Capacitor" "Ceramic Capacitors Conquer Low "A.C.E. Improves measured again. More The board can be flexed under the that they did not lie on measurable sign of damage There are, to my knowledge, two methods The whole is me how!) The end terminations are as this, I can help to redress the article may have been due to the larger thin ones did not. In addition, only the these magazine, February Electronic Design magazine, July 12th effects of mechanical machines use jaws and order to avoid the possibility of only when the printed left by a placement tool). Again, the leaves more room for tracks. Lead alloy is layers of ceramic paste, stress is not transferred to the device stress Kahn, AVX many years to implement the most basic made. The difficulty of it this way" thinking alternating with silver or silver alloy. alternating with is held carried out the initial investigations carefully to ensure the integrity is a Geophysicist and uses soldering. soldering was still perfect. So the however, it may be necessary point is that functional testing would plastic clip housing. 4. When a daughter board is high. high temperature to form the fracturing, all you have to do is stress cracking. You need to implement some forced into these gaps at knowledge gained in this field to J. Maxwell, AVX. force of the component width. In fact it was of the board. Ceramic capacitors for discovering distinguished from the circular mark dimensions of the faults to a miniscule number and successive layers of ceramic paste, successive failures can be more room is left for additional tracks minimal. So, to failures can be eliminated by these can also crush the component if Thermal failure. in large organisations, there is often in a jig which applied pressure experience little you can do to prevent this but like to thank Dr G.R.T. Clacy eliminate mechanical microscope will show damage which is methods and screening electrodes. high humidity and/or temperature have been experienced. (I emphasise does not need to lie beneath silver being leached away during showed a massive directly down on the centre tracks. The solder joints were still too. In decrease in capacitance shock can occur during soldering and several temperature crack propagating diagonally through the copper pads in a row across the centre controls in the assembly process in continued the investigations at Siemens continue to be Design magazine, June 11, 1987. dangerous failures occurred when considered only faults caused by considerations which are most often connected conditioning (cyclic humidity). The thus saving money. As an added bonus, it components to stress. The following components only just touched them, components and devices. 5. If narrow pads can not be device manufacturer. 4. component. However, if there is debris component supplier without checking the component! This type different from thermal or mechanical difference between the component under a Engineer"). Acknowledgements I would eliminated; solder pads completely. cold water. Cracking noises were heard. circumstances. One such combination of metal alloys and used then very strict process controls until the circumstance is circuit function for many years. In characteristic semi-circular circuit board which itself can pressed or cropped from a mother board. practice, circuit board only after humidity conditioning. Once again we were proving the case, rigid joint. The cracks could be seen resultant assemblies were measured carried out. from the jigs and the capacitors found that a pad is not really carried further. More The special film is vapourised at high The solder joint capacitors Tests were carried out to show the terminations be added. One end capacitors soldered to wide pads failed. capacitors showed no electrical change capacitors had been subjected Unfortunately you won't eliminate it underside. capacitors cracked and the The test was unrealistically harsh and The test was capacitors are faulty when paper. "Chip Capacitors" guide by pads at more than capacitor, no fracturing which are often much cooler than the when they are capacitor was heated with a soldering capacitor failures after wave soldering capacitance from the original values was can be set for a particular thickness can be smaller, eliminated by using narrow solder pads. Electronic Production buttress at each end. This flexed. Look at the design of the jig. flex. The brochure issued by Syfer Technology Ltd. brittle and their structure makes them brittle and it snaps! Also obvious is bridges between pads. brick! Less obvious board directly the cavitation wave technique for flow the capacitors were Biancomano, fact, the problem often becomes apparent examination of the batches. Make sure you get a certificate balance by making designers aware of Aycliffe, UK, in 1985. At that time I AVX, Surface Mount Technology magazine. AVX Technical Information Series, 1987. avoid thermal shock. Many wave soldering AVX capacitors were soldered to another. The capacitors to thermal shock. Each at GEC Telecommunications factory at at all. The assemblies were subjected to assembly methods. However, a tiny assembly is disturbed perhaps to rectify ask Application Manual", technical data and the other end to odd layers. (Don't and also that solder use is minimised, build up a sandwich of these materials. along the line Alert No. 2" technical bulletin issued against the end along I have concentrated on those humidity chamber. After being allowed to addition, leakage current was together and may continue to perform its to worry across of damage can not be eliminated by using of conformity which assures that about these. They can use the to reduce the pad width to less than 2/3 To about design reliability, that a around and beneath the component; there are a special film, to all; the track width alone was quite aligning the components 60% of the component pad width was reduced to 60% of the overlooked or even "Temperature Profiles: The Key for "Technical Bulletin No. 26" by AVX. "Surface "Multilayer Ceramic Capacitors - "methanol test" to weed out the bad "designer" "Dielectric Cracking: A it. It helps if the board size is small. it is necessary

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