I dont' know if this sort of question and answer session will be of interest to you all. Please let me know. Its a question page from a magazine I produce. Dear Bob I want to profile my reflow oven to try and reduce reflow temperatures where should I place my thermocouples? Positioning of thermocouples comes down to experience. They should always be soldered in place using high temperature solder. First position a probe at the centre of the board and one at the corner edge of the board. This should theoretical give you the lowest and highest temperatures. Next consider where the largest mass of components are located on the board and place a probe at one termination point. In a convection oven you should see a maximum temperature differential of 15-20oC. Experiment again with the same board placing the probes in different positions noting the temperature changes. Dear Bob If I change my extraction system or my settings do I need to perform further profiles? Yes depending on the type and position of the extraction points it can have an effect. If the ducting is poorly designed other equipment in the factory can effect the extraction rates and your board temperature during reflow. Dear Bob How can I justify the use of nitrogen gas during reflow? You can justify it on quality grounds, some people have justified it through moving to no clean due to the saving on CFC materials. No clean materials can be run in air so this is not a good justification. Its better to justify it on cost savings if for example you use copper based board finishes. This will improve other manufacturing yields and save enough money per board to pay for gas usage. You may find the new SMART Group video on inert gas soldering useful. Dear Bob If I have flexible circuit to assemble how can I hold them flat during reflow? Yes this is a nice one, as we increase convection rates during reflow the flexibles tend to flap, moving components out of position. Depending on volumes the best way is to hold the circuits flat on jigs. A two part laminate jig can sandwich the flexible, solder paste is dispensed at high speed, components placed and then reflowed. Using a cover plate over the flexible as part of a two piece jig prevents screen printing from being used. Dear Bob What is the benefit of using 62% tin 2% silver alloy paste during reflow? Today the benefit is a slightly lower temperature for reflow. The eutectic alloy, 63% tin 37% has a reflow temperature between 183-184oC. The silver bearing alloy reflows at 179oC. The four or five degrees can make a difference allowing upper reflow temperatures to be lowered. Dear Bob Can I use the same temperature profile for second side reflow on a double sided product? Yes you can, but when does reflow take place on your existing process? You must fully understand your process and when does your product reach reflow temperature. In the case of double sided reflow the first side components can soak up heat and slow down reflow. This may still allow reflow using the same profile but the joints may only just reflow causing a lot of voiding. Dear Bob I have changed my solder paste to a finer particle size due to my fine pitch requirements. I now have problems with solder balls which I did not originally have. Just remember that when you reduce the size of solder particles in a paste for fine pitch printing you are increasing the metal surface area. In combination with a low residue paste it may be just too much for the paste activators. Try running some comparisons of you new and old paste using your existing reflow settings. Dear Bob What are the main problems with reflowing copper finish boards? Despite the literature from the protective finish manufacturers heat does effect the copper circuit. It increases the time to obtain full wetting hence increasing the possibility of poor yield. Double sided reflow again increases the surface oxides and poor yield. You need to confirm the quality of the basic coating with your supplier and consider the other process stages which effect solderability. Cleaning the board, washing off poor paste prints, poor board handling, the original surface preparation of the copper before treatment all can effect reflow yields. >From the desk of Bob Willis in the Old Country, England. Tel: 01245 351502 Fax: 01245 496123 Email: [log in to unmask]