Dear Leecp,
1)& 2) From what you describe I presume you are not allowing the chamber to equilibrate before you are taking your measurements?  Most SIR/EM standards require that you incubate your coupons (usually one to four days, depending on the test conditions) before applying voltage to them.  If you do not do this, then you can have water condense on your boards and you can form ionic solutions, which of course conduct electricity.  The gradual increase in resistance could be due to this initial condensation evaporating slowly in the high humidity conditions of the oven - or it might be due to the character of what you are testing!

3) What kind of flux are you using to solder the test wires to your coupons?  What kind of spacing do you have been the wire connection pads/holes?  How are you orienting your coupons in the chamber?  Usually test coupons have larger spacing between the edge pads/holes placed there for the test wires than the spacing between the comb fingers.  You do not want to place your coupons so the flux used for soldering the wires to the board can ooze down into the comb area.  Always use a known, good rosin flux to solder the wires to the board.

Get a copy of IPC-TM-650  Test Methods.

Good luck,

regards,
Bev Christian
Materials Engineering and Environmental Lab
Nortel Networks

-----Original Message-----
From:   wmlee1 [SMTP:[log in to unmask]]
Sent:   Monday, November 22, 1999 11:00 AM
To:     [log in to unmask]
Subject:        [TN] SIR

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