Hi! Apart from what Bev correctly says, there are a zillion variables that can enter into the equation. SIR testing, depending on your instruments, equipment and how you use them can give you almost any answer you want. This is why you must follow a standardised protocol. However, are you really measuring the true SIR or are you trying to detect electrochemical migration? There is an enormous amount of confusion between the two, due mainly to a historical legacy of bad terminology and the fact that the test methods are fairly similar. True SIR testing is done without bias, using voltage gradients generally less than 50 V/mm for the test voltage which is typically applied 30 seconds before the measurement is applied and cut immediately afterwards. ECM resistance testing is done with a bias of a value designed to provoke failure mechanisms. The mechanisms which cause the measured R value to rise are generally either evaporation of condensate, as Bev suggests, or sublimation/evaporation of contaminants. These may be flux residues (e.g. adipic acid in some "no-clean" fluxes) or hygroscopic residues from any of a number of processes, such as those using PEGs, derivatives or similar which actually are ab/adsorbed into the laminate surface by van der Waals' forces. For this reason, it may be advisable to use temperatures <40°C in your test chamber. Lowering R may be due to the e.g. the laminate itself being hygroscopic, due to ionic species within the resin but I would suspect condensation on the test coupons as being a prime possibility. Test coupons must be held vertically with the comb pattern vertical, in a position where nothing can drop onto them. They should be edge-on to the air current flow. The test chamber humidity should be checked with a psychrometer (W&D or dew point): do not rely on types which depend on the electrical characteristics of an absorbent polymer, they are rarely better than +/-20%! Connections should be made with a pure rosin flux at the bottom of the test coupons and the flux residues must never be allowed to bridge the spaces between the conductors. The wires should be PTFE insulated solid conductor, for preference. The test chamber must be organically sterile and clean (nothing like a microorganism growth to bugger up a test!). Use only 18 megohm water for humidification. This very non-exhaustive list is just an insight on a few items that can affect the results. Brian wmlee1 wrote: > Hello,Can anyone help to answer the following questions about Surface > Insulation Test: 1) I found that all SIR test results showed > dramastically decrease in resistance reading after placing the test > coupons into the chamber. However, i found some test results showed > increase in resistance gradually after a period of time the coupons in > chamber, may be after 24 hours or 2 days. While some test results > showed continuously decrease in resistance. Can anyone answer > this? 2) I have been advised that we must take afford to prevent > condensation inside chamber during testing, why? 3) I had experienced > that the flux used for wire bonding would degrade the test result, so > sometimes, i would not use flux for bonding when it is solder finish. > However, if it is bare copper, problem would happen, can anyone advise > some method for this? Thanks Leecp ############################################################## 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 Gayatri Sardeshpande at [log in to unmask] or 847-509-9700 ext.5365 ##############################################################