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Quick Purge of Reflow Oven for N2 Operation

Mark D.

#13187

Quick Purge of Reflow Oven for N2 Operation | 28 December, 1998

We have a Vitronics SMR400N oven and many times need to use N2 inerting. My problem is that it takes close to an hour of operation to purge the system to achieve the desired O2 levels (~40PPM). This is not only time consuming but results in an inordinate amount of N2 consumption. We currently attempt to run the purge by increasing the cell fans to around 3000rpm and applying around 350 sfmh of flow to the nitrogen baffles for approximately 15 mins. After this time, we back off the cell fans to around 1300 rpm and wait for the O2 count to reach the desired 40 PPM. Does anyone have a proven method of purging a system so that the desired O2 levels are accomplished quickly and without a large N2 consumption? Any help would be appreciated. Thanks!

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Earl Moon

#13188

Re: Quick Purge of Reflow Oven for N2 Operation | 28 December, 1998

| We have a Vitronics SMR400N oven and many times need to use N2 inerting. My problem is that it takes close to an hour of operation to purge the system to achieve the desired O2 levels (~40PPM). This is not only time consuming but results in an inordinate amount of N2 consumption. We currently attempt to run the purge by increasing the cell fans to around 3000rpm and applying around 350 sfmh of flow to the nitrogen baffles for approximately 15 mins. After this time, we back off the cell fans to around 1300 rpm and wait for the O2 count to reach the desired 40 PPM. Does anyone have a proven method of purging a system so that the desired O2 levels are accomplished quickly and without a large N2 consumption? Any help would be appreciated. Thanks! | Purging within an hour of interrupted operation sounds great to me. Continuous operation would be ideal, but not always possible. You've read the O2 sensor and it does not lie? What else can you do? It sounds as if you have control of the process.

Earl Moon

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Marc P.

#13189

Re: Quick Purge of Reflow Oven for N2 Operation | 4 January, 1999

| We have a Vitronics SMR400N oven and many times need to use N2 inerting. My problem is that it takes close to an hour of operation to purge the system to achieve the desired O2 levels (~40PPM). This is not only time consuming but results in an inordinate amount of N2 consumption. We currently attempt to run the purge by increasing the cell fans to around 3000rpm and applying around 350 sfmh of flow to the nitrogen baffles for approximately 15 mins. After this time, we back off the cell fans to around 1300 rpm and wait for the O2 count to reach the desired 40 PPM. Does anyone have a proven method of purging a system so that the desired O2 levels are accomplished quickly and without a large N2 consumption? Any help would be appreciated. Thanks! | I agree that 1 hour seems a bit long. Some thoughts:

1)The typical flow to purge the "average" nitrogen oven is from 1500-2500 scfh. (Standard cubic feet per hour) And the running or production flow should be around 800-1200 scfh.

The above numbers can vary due to factors such as board size and opening height of the tunnel but if yours is an "average" oven it should fall into this range.

(Your units of 350 sfmh are new to me. Please spell out so I can learn this one)

2) Has anyone tested the oven for leaks? Sometimes there can be pin leaks in the gasketing or RTV that is used to seal up the tunnel and heater modules. These leaks while small can wreak havoc in an inerted chamber.

The following method might be considered cheating but it works 95% of the time. While purging the oven (with no product running in or through the oven), fully cover the entrance and exit openings of the oven.

The oven should purge down much faster because you have eliminated the interference from the O2 in the room.

Clearly, this is not a sound method for production but it will tell you one or more of the follwoing:

a)If the oven purges faster, it is likely that there is a "net flow" going through the oven. Oxygen is being sucked in through the entrance or exit openings so the nitrogen is constantly combatting this newly introduced O2. The oven chamber can be baffled to balance the flow and this problem can be eliminated.

You can also test for this by using a smoke test. Take a piece of string, light the end with a match and blow it out. Move the smoking string along the entrance opening of the oven. If the smoke blows into the oven at any time there is a net flow into the oven. Repeat this for the exit side of the oven.

b)If the purging time does not change much, you probably have a leak or 2 or 3. The oven should then have a thorough once over for leaks.

3)Sometimes O2 sensors do lie. Your nitrogen company offers small cannisters of "calibrated" (known ppm) gas called span gas. Have them send you a cannister and recalibrate the analyzer. Some analyzers also calibrate on the O2 level in the room so check your analyzer manual for calibration instructions.

I hope this helps. Let me know how it goes!

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