SMT reflow soldering four temperature zones’ function

1,352 Published by FASTPCBA 5月 27,2019

SMT reflow soldering four temperature zones function

1, Basic knowledge of reflow soldering

Reflow soldering technology is not strange in the field of electronics manufacturing, the various components on the board which used in computer is welded on the circuit board through this craft, the device’s internal have a heating circuit, heat the air or nitrogen to a high enough temperature to blow onto the circuit board where the components are already mounted. Let the solder on both sides of the component melt and bond with the main board.The advantage of this craft is that the temperature is easy to be controlled, the welding process can also avoid oxidation, manufacturing costs are easier to be controlled.

In the SMT whole line craft, after the SMT machine completes the SMT process, the next craft is the welding process. The reflow craft is the most important one in the entire SMT process. Wave Welding, reflow soldering and other equipment are normal welding devices.

Today I will discuss the functions of four temperature zones in reflow soldering: the preheating zone, the constant temperature zone, the reflow zone and the cooling zone. Each stage of the four temperature zones has its important significance.


2, Reflow solderinginclude which temperature zones?What are the different functions?

SMT reflow preheating zone

The first step of reflow soldering is preheating. Preheating is to activate solder paste and avoid poor welding caused by sharp high temperature heating during soldering. The normal temperature PCB board is uniformly heated to reach the target temperature. To control the heating rate in the heating process, too fast will produce thermal shock, may cause damage to the circuit board and components;too slow solvent volatilization is not sufficient, affecting the quality of welding.


SMT reflow soldering insulation zone

The second stage – insulation stage, the main purpose is to make PCB board in the reflow furnace and each component temperature stability, so that the component temperature remains consistent.Due to the different sizes of components, large components need more heat, heating up slowly, small components heating up fast, in the insulation area to give enough time to make the temperature of the larger components to catch up with the smaller components, so that the flux fully volatile out, avoid welding bubbles.At the end of the insulation section, oxides on the pads, solder balls and component pins are removed under the action of the flux, and the temperature of the entire circuit board is balanced.Reminder: all components should have the same temperature at the end of this section, otherwise there will be a variety of poor welding in the reflow section due to the uneven temperature of each part.


Reflow soldering reflow area

The temperature of the heater in the reflow soldering area rises to the highest, and the temperature of the component rapidly rises to the highest.During reflow step, the peak welding temperature varies with the different paste used, peak temperature generally is 210-230 ℃, reflow time should not be too long, to prevent adverse effects on the components and PCB, may cause circuit boards are scorched etc.


Reflow cooling zone

In the last stage, the temperature is cooled below the solder paste freezing point to make the solder joint solidification.The faster the cooling rate, the better the welding effect.Too slow cooling rate, will lead to excessive eutectic metal compounds, and the welding points easy to have the big grain structure, make the weld strength get low, cooling rate in cooling zone , generally at about 4 ℃ / S, cooled to 75 ℃.


3, Craft process

Reflow soldering processing for the surface mounted board, the process is more complex, can be divided into two kinds: single-side mount, double-side mount.

Single face mount

Pre-coating solder paste →SMT(divided into manual mount and automatic machine mount)→ reflow welding →inspection and electrical testing.

Double-sided mount

Surface A pre-coated solder paste→ mount (divided into manual mount and automatic machine mount) →reflow soldering →surface B pre-coated solder paste  →SMT (divided into manual SMT and automatic machine SMT) →reflow soldering →inspection and electrical testing.

printed circuit board

printed circuit board

4, What are the advantages of reflow solderingtechnology?

(1) During reflow soldering technology welding, do not require immersion of printed circuit board in molten solder, but uses local heating to complete the soldering task;therefore, the soldered components have small thermal shock and will not be damaged by overheating.

(2) Solder is only required to be applied at the welding part in the re-welding technology, and local heating is used to complete the welding, so as to avoid welding defects such as bridge.

(3) solder is used only once, there is no re-use situation in reflow soldering technology, so the solder is very clean, no impurities, which ensure the quality of the solder joint.


5, Reflow welding maintenance precautions

In order to avoid improper ways of cleaning furnace, resulting in combustion or explosion, it is strictly prohibited to use high-volatile solvents to clean the furnace inside and outside. If high-volatile solvents, such as alcohol and isopropanol are inevitable by this method, please make sure that such substances are evaporated completly before using the equipment.All parts of the flux, dust, dirt or other abnormal objects must be cleaned before maintenance then add the oil!Special attention is that when we carry out routine maintenance of reflow soldering, if we find any machine fault, we cannot repair it without authorization, and we must inform the equipment management personnel to deal with it in time.At the same time must pay attention to safe operation in the maintenance process, do not operate out of specification.

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