| | | What oven temperature should be used to reflow solder (Sn63Pb37) on a PC board (0.075" to 0.100" thickness) in 5 to 10 minutes? | | | | Profiling - tedious but important. As said, start with the paste manufacturers recomendation which is usually published on the paste spec sheet. Some need a slow ramp to liquidus and others need a ramp to soak and a spike to liquidus. Specific time vs temperature will be given. You will have to adjust your zone temperatures and belt speed to replicate the manufacturers profile. I.E. trial and error and usually time after time with a board that you expect to scrap (but save it to periodically recheck your profile). Rate of rise and cooling are also important. If it is too fast you will damage your components. You will need some way to measure the time vs temperature and graph it. There are several systems that ride along with the board and many ovens have built in profile monitoring. A search of this forum and a look through back issues of Circuits Assembly or Electronic Packaging and Production magazines will get you more info. Some of the paste manufacturers and profiler manufacturers have technical bulletins as well. good luck | | John Thorup | | | | | Judy, | | As usual, John is dishing out the best advice but not wanting to be left out I thought I'd complicate matter's by adding my thought's. | | 63/37 solder reflow's at 183 degree's C sorry my head just wont work in your degree's F, it's eutectic i.e. there's no pasty region so that helps. ( if you add you 2% silver you bring it down to 179 but hey what's 4 degree's between friend's right?mmmm) | The ramp rate's for almost all paste's is about 2 degree's per sec assumming that you've got mixed components i.e. passive's and actives, if it's just passive's ( res & cap's ) you can get away with more but it's best to get into the habit of consistentcy and using a standard set of rules for your profiling. | If your running an IR system you's need to have a soak regoin on your profile to allow eveything to get to a nice even temp of about 150 degree's C for 30 - 60 sec's before shooting it into reflow. | Time above reflow is critical, if your really interested in doing profiling properly there are a few thing's to check, | 1 How long is my paste allowed to be in reflow, some are 40 sec's some are 90 sec's so check your paste spec for max / min setting's and don't forget about the max temp!, most paste's don't like going over 220 degree's C | 2. what about my component's? people forget that component's have a thermal profile, actives especially or certain cap's loose their potting compound's after 45 sec's in reflow refer would be an example. so check out the costly stuff!!! | Cool down rate is as important as ramp, cool to quick and your grain structure is no use and voiding will be a problem, to slow and your dendrite's are way to big, aim for between 2-4degree's per sec. | Other thing's that are gong to effect your temp's is the copper loading on your board, the mass of material ie how many component's and how big they are, how clean is your oven ? flux build up over time is a killer and will seriously effect your profile, the best time to profile if you can is right after the oven is cleanned, then do it again right before it's due to be cleanned then you'll get a max / min tolerance window for your profile that will be usefull later in life if thing's start going wrong with boards or component's. Also the temperature between the front and back rail's of the ovan can vary by anything up to 20 degree's c! so a good calibration tool is needed, but maybe that's another forum question...how do we all calibrate the ovan's and what about thermally matching the ?, mmm the fun of it all... | well hope that's been heplful, I guess it's just a case of try, learn and improve | | all the best | JohnW |
Hi All, Just would like to add some Material Issues during profiling and hope that this would help especially for the newbies:
PCB's can be damaged by excessive or extended exposure to Heat. The glass transition (Tg) temp of the pcb mat'l must be known to help create the reflow profile. Most laminates have a Tg between 120C and 130C, however, laminates with a higher Tg are available. If the material is heated above Tg, two damaging things can occur. The material becomes plastic, so the pcb becomes soft and losess its rigidity. Also the expansion rate of the material increases dramatically. This Z-axis expansion can damage the plated through-holes by cracking the barrel. Flux has 2 key attributes. First, it must remove contamination and second, it must protect the solderable surfaces after contamination removal. A common mistake is to use time/temp profile that consumes the flux before the solder melts. Ideally, the flux would be consumed just as the solder begins to melt. Activation time should range from 30 to 90 secs. Flux usually becomes active at around 120C. Components can be damaged by the incorrect application of heat. All components have a heat exposure limit. Most surface mount components can tolerate a peak temp of 220C for 20 to 70 seconds. Thermal shock, caused by the rapid application of heat can crack certain components especially capacitors. The general rule is a maximum temp increases of 2 deg. C per sec. Component lead finish will affect solderability. A number of lead finishes are being used today, including tin/lead, gold, tin and palladium. It is important to select a flux and solder alloy that works well with the lead finish being used. The solder alloy should be heated to a temp that is 25 to 40 deg. C above it's melting temp. The dwell time above the melting temp should be 20 to 70 secs. Proper reflow temp and dwell time permit proper wetting to occur. Solder joints that are cooled at a reasonable rate achieves a small, fine grain structure. This grain structure provides stronger and more reliable solder joint. Cooldown rates of 1 C to 2 C per sec are preferred.
The basic rules of reflow soldering must be carefully observed and followed !
Dreamsniper
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What oven temperature should be used to reflow solder (Sn6...
- Aug 04, 1999
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