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Curing, reflow process analysis

Curing, reflow process analysis

  • Categories:Industry News
  • Author:Becky Su
  • Origin:
  • Time of issue:2023-02-13 11:01
  • Views:

(Summary description)Curing, reflow process analysis In the curing and reflow process, the main thing is to control the temperature profile of curing and reflow, that is, the curing and reflow conditions, the correct temperature profile will ensure high quality solder joints. In the reflow oven, its interior is a black box for us, we do not know what is happening inside, so it brings me a lot of difficulties to develop the process. To overcome this difficulty, it is common in the SMT industry to use a temperature tester to derive a temperature profile, which is then referred to in order to change the process. The temperature profile is the temperature applied to the circuit assembly as a function of time. When plotted in the Cartesian plane, the reflow process forms a curve representing the temperature at a specific point on the PCB at any given time. Several parameters affect the shape of the curve, of which the *critical ones are the conveyor belt speed and the temperature setting for each zone. The conveyor speed determines the duration that the board is exposed to the temperature set for each zone, and increasing the duration allows more time for the circuit assembly to approach the temperature setting for that zone. The sum of the durations spent in each zone determines the total processing time. The temperature setting for each zone affects the rate of temperature rise of the PCB, with high temperatures creating a large temperature differential between the PCB and the zone temperature. Increasing the set temperature of a zone allows the board to reach a given temperature faster. Therefore, a graph must be made to determine the temperature profile of the PCB. Next is the outline of this step to generate and optimize the graph. The following equipment and auxiliary tools are required: temperature profiler, thermocouple, tool to attach the thermocouple to the PCB and solder paste parameter table. There are generally two types of temperature measurement instruments: a real-time thermometer that instantly transmits temperature/time data and makes a graph, and another thermometer that samples and stores the data and then uploads it to a computer. The thermocouple will use high-temperature solder such as silver / tin alloy, solder joints as tiny as possible attached to the PCB, or a small amount of thermal compound (also called thermal paste or thermal grease) spots covered with thermocouples, and then sticky attached to the PCB with high-temperature tape (such as Kapton). The location of the attachment should also be chosen, usually good to attach the thermocouple tip between the PCB pads and the corresponding component pins or metal ends. As shown in the figure (Attach the thermocouple tip between the PCB pads and the corresponding component pins or metal ends) A table of paste characteristics is also necessary, which should contain the desired duration of the temperature profile, the paste activity temperature, the alloy melting point and the desired maximum reflow temperature. The ideal temperature profile The theoretically ideal profile consists of four sections or zones, with the first three zones heating and the last zone cooling. The more temperature zones the furnace has, the more accurate and closer to the setting the profile of the temperature curve can be achieved. (Theoretically the ideal reflow profile consists of four zones, with the first three zones heating and the last zone cooling) Pre-heating zone, used to raise the temperature of the PCB from the ambient temperature to the required active temperature. Its temperature rises continuously at a rate of no more than 2 to 5°C per second, the temperature rises too quickly can cause certain defects, such as fine cracks in ceramic capacitors, while the temperature rises too slowly, the solder paste will be over-sensitive to temperature, there is not enough time for the PCB to reach the active temperature. The preheating zone of the furnace generally accounts for the entire heating 25~33% of the length of the heating channel. Active zone, sometimes called dry or wetting zone, this zone generally accounts for 33 to 50% of the heating channel, has two functions, the first is, the PCB will be at a fairly stable temperature sensing temperature, so that different quality components have the same temperature, reducing their considerable temperature difference. The second function is to allow the flux to be active and volatile substances to evaporate from the solder paste. The general prevailing active temperature range is 120 to 150°C. If the temperature in the active zone is set too high, the flux does not have enough time to become active. Therefore the ideal curve requires a fairly smooth temperature so that the temperature of the PCB is equal at the beginning and end of the active zone. Reflow zone, whose role is to raise the temperature of the PCB assembly from the active temperature to the recommended peak temperature. Typical peak temperature range

Curing, reflow process analysis

(Summary description)Curing, reflow process analysis

In the curing and reflow process, the main thing is to control the temperature profile of curing and reflow, that is, the curing and reflow conditions, the correct temperature profile will ensure high quality solder joints. In the reflow oven, its interior is a black box for us, we do not know what is happening inside, so it brings me a lot of difficulties to develop the process. To overcome this difficulty, it is common in the SMT industry to use a temperature tester to derive a temperature profile, which is then referred to in order to change the process.

The temperature profile is the temperature applied to the circuit assembly as a function of time. When plotted in the Cartesian plane, the reflow process forms a curve representing the temperature at a specific point on the PCB at any given time.

Several parameters affect the shape of the curve, of which the *critical ones are the conveyor belt speed and the temperature setting for each zone. The conveyor speed determines the duration that the board is exposed to the temperature set for each zone, and increasing the duration allows more time for the circuit assembly to approach the temperature setting for that zone. The sum of the durations spent in each zone determines the total processing time.

The temperature setting for each zone affects the rate of temperature rise of the PCB, with high temperatures creating a large temperature differential between the PCB and the zone temperature. Increasing the set temperature of a zone allows the board to reach a given temperature faster. Therefore, a graph must be made to determine the temperature profile of the PCB. Next is the outline of this step to generate and optimize the graph.

The following equipment and auxiliary tools are required: temperature profiler, thermocouple, tool to attach the thermocouple to the PCB and solder paste parameter table. There are generally two types of temperature measurement instruments: a real-time thermometer that instantly transmits temperature/time data and makes a graph, and another thermometer that samples and stores the data and then uploads it to a computer.

The thermocouple will use high-temperature solder such as silver / tin alloy, solder joints as tiny as possible attached to the PCB, or a small amount of thermal compound (also called thermal paste or thermal grease) spots covered with thermocouples, and then sticky attached to the PCB with high-temperature tape (such as Kapton).

The location of the attachment should also be chosen, usually good to attach the thermocouple tip between the PCB pads and the corresponding component pins or metal ends. As shown in the figure



(Attach the thermocouple tip between the PCB pads and the corresponding component pins or metal ends)

A table of paste characteristics is also necessary, which should contain the desired duration of the temperature profile, the paste activity temperature, the alloy melting point and the desired maximum reflow temperature.

The ideal temperature profile


The theoretically ideal profile consists of four sections or zones, with the first three zones heating and the last zone cooling. The more temperature zones the furnace has, the more accurate and closer to the setting the profile of the temperature curve can be achieved.



(Theoretically the ideal reflow profile consists of four zones, with the first three zones heating and the last zone cooling)
Pre-heating zone, used to raise the temperature of the PCB from the ambient temperature to the required active temperature. Its temperature rises continuously at a rate of no more than 2 to 5°C per second, the temperature rises too quickly can cause certain defects, such as fine cracks in ceramic capacitors, while the temperature rises too slowly, the solder paste will be over-sensitive to temperature, there is not enough time for the PCB to reach the active temperature. The preheating zone of the furnace generally accounts for the entire heating
25~33% of the length of the heating channel.

Active zone, sometimes called dry or wetting zone, this zone generally accounts for 33 to 50% of the heating channel, has two functions, the first is, the PCB will be at a fairly stable temperature sensing temperature, so that different quality components have the same temperature, reducing their considerable temperature difference. The second function is to allow the flux to be active and volatile substances to evaporate from the solder paste. The general prevailing active temperature range is 120 to 150°C. If the temperature in the active zone is set too high, the flux does not have enough time to become active. Therefore the ideal curve requires a fairly smooth temperature so that the temperature of the PCB is equal at the beginning and end of the active zone.

Reflow zone, whose role is to raise the temperature of the PCB assembly from the active temperature to the recommended peak temperature. Typical peak temperature range

  • Categories:Industry News
  • Author:Becky Su
  • Origin:
  • Time of issue:2023-02-13 11:01
  • Views:
Information

Curing, reflow process analysis

In the curing and reflow process, the main thing is to control the temperature profile of curing and reflow, that is, the curing and reflow conditions, the correct temperature profile will ensure high quality solder joints. In the reflow oven, its interior is a black box for us, we do not know what is happening inside, so it brings me a lot of difficulties to develop the process. To overcome this difficulty, it is common in the SMT industry to use a temperature tester to derive a temperature profile, which is then referred to in order to change the process.

The temperature profile is the temperature applied to the circuit assembly as a function of time. When plotted in the Cartesian plane, the reflow process forms a curve representing the temperature at a specific point on the PCB at any given time.

Several parameters affect the shape of the curve, of which the *critical ones are the conveyor belt speed and the temperature setting for each zone. The conveyor speed determines the duration that the board is exposed to the temperature set for each zone, and increasing the duration allows more time for the circuit assembly to approach the temperature setting for that zone. The sum of the durations spent in each zone determines the total processing time.

The temperature setting for each zone affects the rate of temperature rise of the PCB, with high temperatures creating a large temperature differential between the PCB and the zone temperature. Increasing the set temperature of a zone allows the board to reach a given temperature faster. Therefore, a graph must be made to determine the temperature profile of the PCB. Next is the outline of this step to generate and optimize the graph.

The following equipment and auxiliary tools are required: temperature profiler, thermocouple, tool to attach the thermocouple to the PCB and solder paste parameter table. There are generally two types of temperature measurement instruments: a real-time thermometer that instantly transmits temperature/time data and makes a graph, and another thermometer that samples and stores the data and then uploads it to a computer.

The thermocouple will use high-temperature solder such as silver / tin alloy, solder joints as tiny as possible attached to the PCB, or a small amount of thermal compound (also called thermal paste or thermal grease) spots covered with thermocouples, and then sticky attached to the PCB with high-temperature tape (such as Kapton).

The location of the attachment should also be chosen, usually good to attach the thermocouple tip between the PCB pads and the corresponding component pins or metal ends. As shown in the figure

(Attach the thermocouple tip between the PCB pads and the corresponding component pins or metal ends)

A table of paste characteristics is also necessary, which should contain the desired duration of the temperature profile, the paste activity temperature, the alloy melting point and the desired maximum reflow temperature.

The ideal temperature profile


The theoretically ideal profile consists of four sections or zones, with the first three zones heating and the last zone cooling. The more temperature zones the furnace has, the more accurate and closer to the setting the profile of the temperature curve can be achieved.

(Theoretically the ideal reflow profile consists of four zones, with the first three zones heating and the last zone cooling)
Pre-heating zone, used to raise the temperature of the PCB from the ambient temperature to the required active temperature. Its temperature rises continuously at a rate of no more than 2 to 5°C per second, the temperature rises too quickly can cause certain defects, such as fine cracks in ceramic capacitors, while the temperature rises too slowly, the solder paste will be over-sensitive to temperature, there is not enough time for the PCB to reach the active temperature. The preheating zone of the furnace generally accounts for the entire heating
25~33% of the length of the heating channel.

Active zone, sometimes called dry or wetting zone, this zone generally accounts for 33 to 50% of the heating channel, has two functions, the first is, the PCB will be at a fairly stable temperature sensing temperature, so that different quality components have the same temperature, reducing their considerable temperature difference. The second function is to allow the flux to be active and volatile substances to evaporate from the solder paste. The general prevailing active temperature range is 120 to 150°C. If the temperature in the active zone is set too high, the flux does not have enough time to become active. Therefore the ideal curve requires a fairly smooth temperature so that the temperature of the PCB is equal at the beginning and end of the active zone.

Reflow zone, whose role is to raise the temperature of the PCB assembly from the active temperature to the recommended peak temperature. Typical peak temperature range is 205 to 230°C. Setting the temperature in this zone too high can cause excessive curling, delamination or burnout of the PCB and compromise the integrity of the component.

The ideal cooling zone curve should be a mirror image relationship with the reflow zone curve. The closer to this mirror image relationship, the closer the structure of the solder joint to reach the solid state, the higher the quality of the solder joint obtained, the better the bond integrity.

The actual temperature curve When we set the general PCB reflow temperature, the reflow oven is energized and heated, and when the temperature in the oven reaches a stable level as indicated by the equipment's pro-measurement system, a temperature tester is used to test the temperature curve to see if it matches our predetermined curve. Otherwise, the temperature of each temperature zone is reset and the furnace parameters are adjusted. These parameters include transfer speed, cooling fan speed, forced air impact and inert gas flow until the correct temperature is reached.

Typical PCB reflow interval temperature setting

The following are some of the poor types of reflux curves:

Figure I. Reflux curves for insufficient or excessive preheating.

Figure 2. The temperature of the active zone is too high or too low.

Figure 3. Too much or not enough reflux.

Figure 4, cooling too fast or not enough。

When the final graph matches the desired graph as closely as possible, the parameters of the oven should be recorded or stored for later use. Although this process is slow and laborious at first, it can eventually achieve proficiency and speed, resulting in efficient production of high quality PCBs.

 

 

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