You can not use Tin or Tin/Lead as a contact material. In humidity and air an oxide film grows which will continually raise the contact resistance over time. ATT years ago did a study of metals for physical contacts and found those two platings unsuitable. WE used some contact connectors on PWAs that had solder coated fingers. The vendor claimed that a hermetic seal would form in the contact from the connector pushing into the solder. We had all kinds of electrical problems. The solder contact had to be perfectly flat and flat within a specified area in order for the contact to maintain good conductivity and this was in an air conditioned office environment. We had to do Ammonium Sulfide and Nitric Acid vapor exposure tests to guarantee that we had a hermetic seal. -----Original Message----- From: Ed Holton [mailto:[log in to unmask]] Sent: Thursday, March 16, 2000 3:06 PM To: [log in to unmask] Subject: Re: [TN] Electromigration demonstration... My first experience with dendretic growth involved a capacitor on a board in a highly humid environment (automotive application). We had dendretic growth underneath the capacitor. There were a few variables that helped aggrevate the situation. I did a simple experiment to show everyone what was happening, but it also showed that even if your board was perfectly clean, as long as there was water and voltage, you will get a dendretic growth. I hooked wires onto each end of a 1206 capacitor. These wires were then hooked to a 12v power supply (to simulate a car battery). A drop of DI water was placed ontop of the capacitor and the power supply turned on. After a few moments, a dendrite jumped from one end of the capacitor to the other. It was pretty cool to watch. This little experiment earned me the nickname Mr. Wizard for the next few months. We had the video camera from our microscope system focused on the capacitor so everyone could watch it on the monitor. We did find in later repeat experiments that it was sometimes necessary to add water, as it sometimes evaporated quickly due to the conditions. (I need to thank John Maxwell, he help me set the experiment up originally) Ed Holton Manufacturing Engineer and Group Leader Hella Electronics Telephone (734) 414-0944 Fax (734) 414-0941 KK Chin <KK.Chin@ARTE To: [log in to unmask] SYN.COM> cc: Sent by: Subject: Re: [TN] Electromigration TechNet demonstration... <[log in to unmask] ORG> 03/16/00 02:13 PM Please respond to "TechNet E-Mail Forum."; Please respond to KK Chin Steve, Dr. William Kenyon responded to my technet posting last year with the demonstration method which was of great help to me. The passage is pasted below. The article was also posted in his column in the SMT magazine, April 99. K.K. Chin Artesyn Technologies Fremont, CA ---------------------------------------------------------------------------- ------------------- There is a simple EmR (electromigration resistance) test you can do -- I put it in a recent (April) SMT column that I write each month on cleaning issues. Here it is, just copied and pasted from the last edit. Fun with Dendrites- Readers have expressed interest in the reliability concerns caused by dendritic growth, or cathode-anode filament formation. Whatever the name, it usually results in catastrophic in-service failure of the PCB in service. Since many PCB fabricators are reluctant to make any process changes based on the possibility of an obscure failure mechanism, as long the bare and assembled PCBs pass the ionic cleanliness test, it seemed appropriate to provide a simple demonstration of the phenomenon that assemblers can use to get the attention of their PCB suppliers. There Here is a simple, inexpensive test that anyone using hot-air solder leveled (HASL) or infrared fused PCBs can do. First, obtain a scrap board that has been through PCB supplierUs HASL process. Find two parallel traces on the PCB that are at least 5 cm long. Cut the traces at both ends to isolate them from the rest of the circuitry. Solder some 5 cm. wires to the positive and negative terminals of a standard 9 V battery, then attach mini-alligator clips to the ends of the wires. Using a sharp knife straight-edge razor, peel up the opposite ends of the cut traces, so you can attach the mini-alligator clips to the bent-up ends of the traces. (One could solder the battery wires to the traces with type-R rosin, but this could introduce doubt in the results. The clips or a connector if the design has wires going to a set of Fax To: Amanda OUBrien @ SMT Magazine Page 4/5 connector fingers is better.) Obtain some DI water. and a decent 10-30x microscope. If a microscope is not available, a powerful magnifying glass can be used. Put the isolated traces under the microscope and bring them into focus. Arrange the PCB to see both traces and the laminate gap between them. Next, hook the 9 V battery terminal wires to the opposite ends of the isolated traces; so one trace is positive and the other is negative. Now, very carefully place a drop of DI water on the isolated traces so it bridges the gap between the traces in the field of view of the microscope. See how long it takes for the dendrites to grow from cathode to anode, timing the progress with a stop watch if desired. Usually one sees the drop of water start to bubble a little, and then a metallic tree-like growth will cross the gap from cathode to anode. The resulting metallic dendrites are quite brittle, which probably accounts for the well-known fix of banging a defective PCB on the table and having it spring to life again. What has happened is that all the dendrites (current leakage paths) have been shattered. Will it happen again over time, especially with exposure to elevated humidity conditions? Absolutely. This test will make believers out of those who think all one has to do is pass the ionic cleanliness test and that pure water never hurts anything. The problem with HASL or infrared fused tin/lead plated PCBs is was typically the organic fluid that forms the basis of the traditional HASL or reflow fluids. It only took takes a little on the surface to give the current leakage, resulting in the dendritic growth problem. The only pass/fail value available that is based on test work is about 1 microgram/sq. cm per 0.001" of trace or pad separation. So a 0.050S space can tolerate 50 mg., while a 0.010S space can tolerate 10 mg. According to this source, most PCBs will have hundreds to thousands of mg./sq. cm of such fluid residues on them. Literature data are is available on the effects of various flux residues and cleaning methods on test PCBs that had such fluids on them or had any traces of these fluids removed prior to the soldering and cleaning tests. Newer fluids either eliminate or minimize this problem, but the test is so easy to do on scrap boards that it is good insurance prior to accepting a lot of PCBs and releasing them to production. ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. If you need assistance - contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ############################################################## ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. If you need assistance - contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ############################################################## ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. If you need assistance - contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ##############################################################