The cooling zone is where the quality of the solder joint is defined, with the cooling slope influencing the joint strength, and overall longevity. These two qualities are often at odds with each other because strength often comes from slower cooling rates, while longevity results from faster rates. Different cooling slopes have been tested to try to find what rate produces the best combination of strength and long life when subjected to accelerated thermal-cycling. These studies have concluded that slow cooling rates (1 to 2 °C/sec) allow too much time for intermetallic alloy growth, a strong but often brittle alloy prone to cracking when stressed. Faster cooling (5 to 7 ºC/sec) can form a softer solder joint with less overall strength, not to mention possible component damage. Cooling slopes between 3 and 4 °C/sec were found to be the best at producing a solder joint with both good strength and overall longevity.

So… don’t forget the cooling zone when developing the best thermal profile for your solder process.

References:

“Cooling Rates in Lead-free and Tin/lead Reflow”
SMT Magazine

by Denis Barbini, PhD.; Ursula Marquez

“Accelerated Thermal Fatigue of Lead-Free Solder Joints as a Function of Reflow Cooling Rate” Journal of Electronic Materials

by Qi Y; Zbrzezny A R; Agia M; Lam R; Et al

“Proceedings of 2005 International Conference on…” Asian Green Electronics
by Qiang Hu; Zhong-suo Lee; Zhi-li Zhao; Da-le Lee

1. (Max preheat) How hot the board is just before it hits the wave
2. (Dwell time) how long do you spend in the wave
3. (Contact temp) Temperature of the solder at the contact surface with the board
4. Conveyor speed

All the rest of the many measurable parameters are secondary to these in my opinion. Let’s talk a little about each of these as measured by the WaveRIDER SPC software:

Figure 1: WaveRIDER SPC Software

Max Preheat temp (Preheat Max Temp) is really the temperature of the board (solder joints) just before it hits the wave so you can understand the “shock” the solder joint will get when contact with the wave forces it to melt the solder in a quick few seconds. This temp may NOT be the max preheat temp applied because most wave machines have a large gap between the pre-heater and the solder wave so the board will loose some of the heat during that space/time.

The Dwell time is how long the solder will be above liquidous, all said and done, and this time needs to be consistent across the width of the wave (AKA: parallelism), but you can see consistency across the wave just as easy by setting a min/max spec for the three dwell times. How long you must dwell is a function your solder joint requirements and components ability to withstand the wave temp. Most component specs say they can take 10 seconds or less and most solder joints need only a second to form. The longer you spend above liquidous the more time you give to form the brittle “intermetallic” alloy between the not molten metals. So the only reason you dwell at all is to get the heat to travel up the lead to the board top (on through-hole parts), which may take a few seconds depending on the thermal conductivity of the board and its thickness. A pot of liquid solder has plenty of heat to force into a board, so it depends mostly on the thermal conductivity of the board material itself. You will have to watch/trial your assembly (board) to see just how long that takes so you can set a minimum dwell time. The Dwell time is usually about 2.5 seconds for most 0.062″ (1.6mm) thick boards and this can vary +/- 10% to 15%.

Figure 2: Wave crest pattern

Pot temp at the solder contact surface (Contact temp) does vary depending on the load (your board) places on the surface of the solder wave. It is not the same as the solder pot temp, since that temp is controlled by a Thermocouple deep in the core of the solder pot and not the surface where the board skims across. This temperature should be maintained at least 20ºC or more above liquidous and no more then the maximum temperature allowed by the component specifications.

The conveyor speed is measured by dividing 9.75″ (distance between C and Speed sensors on the WaveRIDER pallet, see Figure 4) by the time measured between the C and Speed sensors hitting the main wave. It is easy to see why this is an important measure since ALL other values will be altered if the conveyor speed changes. Most solder machines can maintain speeds within +/-0.1 ft/minute (+/-1.2 in/minute, +/-3cm/minute)

Immersion depth is only important to prevent solder from spilling over the top of the board during wave contact. The absolute setting is not critical provided it is NOT spilling over the board and its depth is affording the dwell time one needs. However, it is easier to adjust the conveyor speed to control dwell time then the board’s immersion depth and risk a spill over. This is usually controlled by pump RPMs and how full the pot is of solder. If these change over time, the dwell times slowly change and thus is be one of the reasons for dwell changes. However, neither can be directly measured by WaveRIDER which has no chance to access the pump’s RPMs or the amount of solder in the pot.

Conclusion:
Focus your efforts on the critical WaverRIDER measurements by setting reasonable specifications. The many other things the WaveRIDER measures are not as critical, and can be used to diagnose a problem if there is one rather then used to prove there is NO problem. Use the WaveRIDER SPC software to set Spec limits and plot X-Bar R charts for these critical parameters. (Figure 3)

Figure 3: X-Bar R charts

Figure 4: Keep the WaveRIDER pallet contacts clean using a fine wire brush. This will reduce oxide build up on the sensors which can cause the Dwell times vary beyond normal.