Over the last ten years there has been a number of reasons why chip capacitors have generally been found to fail during production and in the final products, these will be outlined first. The design and method of component manufacture can affect the component's ability to stand up to modern manufacturing methods. In cases where components are selected at the top of the capacitance range for a given package size, due to the number of plates the components can be more sensitive to fracture failures. Often the component manufacture will suggest de-rating parts for a given process; this is common in many designs. The internal construction and lay up of the internal electrodes may also affect the robustness of the parts. This does vary with different part suppliers and should be evaluated during initial component assessment. Components, when they are supplied, may be damaged prior to removal from the packaging; this has certainly been seen in the past. Internal cracking may be present and is then exaggerated by the assembly process. Examination of selected components in their packaging may provide examples of defective parts. Machine centring and placement of components on to the board has been a problem area over the years. Again this is exaggerated where the parts are less robust. (In America SMT magazine in the early days called Nutshell News the editor called the machine chip crackers, boy was he right). Recently it has been the use of board support pins for double sided work on placement and screen print. Poor positioning of pins under components does not allow the board to move when the component is placed. The incorrect height of board support pins can also exaggerate the problem. If the pins are adjusted too high when the board is clamped for placement the board is actually bowed upwards during placement. Minor cracking may occur and is then shown up during soldering and test. Wave and reflow soldering temperature settings again have been shown to affect component reliability. Minimising the temperature of solder baths to 240oC and controlled board pre-heating has mostly eliminated the wave soldering problems in the industry. Any good quality part suitable for use in board designed for manufacture should be able to stand immersion in a solder bath at 260oC with no pre heat. Refer to the IPC deign rules and process compatibility document for components. The IEC documents have the same guide. It is uncommon for the reflow soldering process to cause failures unless parts have already been damaged. If the quality of the capacitor is poor with internal voiding or stress cracks the reflow process is less likely to cause problems. In the case of wave or reflow soldering, if the board is poorly supported and the board is allowed to sag the board may be warped after soldering. When the board cools the board is left in a warped state. If any subsequent process straightens the board it may cause component cracking. A possible example is in-circuit test where the board is straightened flat during pin contact. Hand soldering and rework has also been shown to damage parts if correct procedures are not followed or poorly trained staff are used. It is common where components come into direct contact with a solder bit rather than using the solder to provide effective heat transfer. During any hand soldering operation using soldering irons the solder is used to transfer the heat to the board and component not the bit. Today it is more common to use hot air pencils to rework surface mount component, this minimises physical contact with parts. If the components are correctly specified with a solder coating and solder coated boards are used then normally only flux is required to make a joint. As sufficient solder is already present to make a connection on the board and termination. This is by far the best process. A lot has been said about pre heating components prior to rework, this is no going to happen in the real world. If you can't rework parts using standard technique don't design it into the product its called GOOD DFM, design for manufacture. Board flexure during board handling, break out, in-circuit test or final mounting of the board into the product is more likely to cause failure. This is again true if the component is not correctly specified and tested during initial assessment of the supplier. Most of the cases currently being examined by myself for customers are flexure failures. It is considered that board flexure is one of the main cause of failure. If you break out multi panels using bolt cutters across a bench or just snap V scored boards what stress is being applied??? Alternative laminates or higher Tg materials may help to provide a flat surface for assembly. This is only of value if it coincides with balanced layout during design and correct specification of the solderable finish. The more stable laminate will not help if the board is flexed during assembly. Nine times out of ten look for board flexure and you find the cause of the problem. Bob Willis Process Engineering Consultant Electronic Presentation Services 2 Fourth Avenue, Chelmsford, Essex CM1 4HA. England. Tel: (44) 01245 351502 Fax: (44) 01245 496123 Home Page: http://our world.compuserve.com/homepages/bwillis Email: [log in to unmask] *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * ***************************************************************************