| | | I am in need of possible causes and solutions to capacitors that are failing in the field. | | | Are there issues in our process that need to be | | | checked? | | | Some people said the problems are due to improper | | | storage. Others say it is thermal stress in our | | | reflow (we don't believe this is the case). | | | Could you please offer suggestions as what to look | | | for. | | | Best regards. | | In addition to the other information provided, I have experienced plenty of cracking because of three things. | | 1) Unacceptable design rules as SMT pads or neckdown features causing cracks because of thermal stress after assembly (during operation) | | 2) As the above with cracking after excessive mechanical stress or shock | | 3) Cracking during or after repair or modification operations wherein hand soldering is improperly performed | | Much luck | | Earl Moon
| I never would have thought about pad size for this problem as you mention in item #1. What issues have you seen with pads size and neckdowns ? I thought no one would ask. The problems encountered mostly have been with R/F designs wherein engineers establish rules requiring device placement on large traces (to .250" wide), with no neckdowns, to ensure required electrical performance. In these cases, the copper circuitry, having a TCE of 17-19 ppm (compared with 5-7 ppm for ceramic devices), moves under thermally induced conditions during operation. Also, larger pad sizes can do the same thing with smaller or thinner device types. Additionally, when performing touch up or repair on designs having larger than "normal" neckdowns, the solder obviously drains off (as it does during reflow) often making the hand solder operator keep the iron on the solder termination areas too long (in excess of 3 or 4 seconds). They do this hoping to overcome the drain off but induce component cracking. In these cases, the situation is not much different than using LCCC's though solder joint failure is more prevalent than device damage. We all are aware of the problems so well stated by all others on this question. The thermal excursion and TCE mismatch problems are less known except to military/aerospace/RF types. Earl Moon
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