2. Widely used two profile modes now

Warm-up and warm-keeping mode (traditional profile)

Explanation: Temperature rises from its initial value rapidly to preheating value within the range of 140~170'C and then is maintained.

TPHH-TPHL depends on reflow oven's capacity (about 10'C). Temperature is kept unchanged for about 40~120S as preheating zone, again rises quickly to reflow zone, finally cools rapidly to cooling zone (temperature change rates are kept below 4C/second).

Feature: Usually, low preheating temperature is chosen, thus heat affects components slightly (low stress is caused to components), accordingly heating time can be prolonged to activate the flux. Meanwhile, temperature rise is relatively violent from preheating zone to reflow zone, so soldering flow is easy to worsen and migrate and flux's activation temperature may be too low.

Gradual warm-up mode (preferred profile)

Explanation: Temperature rises to about 175C with a low rising rate of 0.5~1C/second and to about 180C with a gradient of 20~30 seconds, jumps to about 220C with a rate of 2.5~3.5C/second, and finally cools down quickly with a rate of no more than 4C/second. The key point to control is to maintain a certain temperature rising rate and ensure that preheating's end point comes near to tin's melting point.

Feature: Components are not affected by violent temperature change and flux's activation temperature can be set higher, but flux's activation time is too short and high preheating temperature affects components easily.

By comparing the above two reflow profiles, the main difference between them is that the latter has no plateau structure (i.e., homothermal heating zone) in it.

Because of the structure and heat absorptivity differences between base-panels and components and the limitation of equipment's controllable heating rate, base-panels' temperature differences still exist when they are going through the reflow oven. With the help of a balancing zone like a plateau to reduce the temperature gradient, temperature is kept for a period of time when hot-spot temperature reaches below the solder's melting point, so that the cold-spot temperature will catch up with it. After each component reaches the same temperature, another quick temperature rising program will raise component temperature to the peak one, which can avoid partial cold soldering or partial high-temperature carbonization effectively.

On the other hand, after the former obtains a plateau structure, a rapid heating process will appear inevitably in the stage from room temperature to homothermal preheating and the stage from homothermal period to solder melting and this rapid heating process has a close relationship with such defects as solder balls due to splash down and warped components due to imbalanced wettings on components' two sides before solder's melting point. Many quality problems are expected to be prevented and eliminated by using linear heating profile in the stage from room temperature to solder's melting point.

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