Some friends on the Internet seem to have some misunderstandings about the temperature setting of hotbar's thermodies and the actual measured temperature. Some people think that as long as the temperature of hotbar is set to 400 ° C, the temperature of its hot indenter must reach this temperature before it is normal. However, the actual temperature on the measuring board is only about 280 ° C, so there is a question about the actual temperature of the hot indenter.
I have a good understanding of the misunderstanding caused by the design and principle of hotbar hot press welding machine. Let me use iron as a metaphor to explain. When we set the temperature of the iron at 400 ° C, the temperature actually can be reached. However, when we put the iron on the solder and parts, the temperature of the solder and parts is close to the room temperature, so the iron head actually transmits The temperature of lead to solder and parts is not up to 400 ° C, which may be about 260 ° C in general, while the melting point of sac305 solder is only 217 ° C, so it can be used for soldering. However, if there is a large piece of grounding copper foil on the PCB pad, the temperature loss of soldering iron head will multiply, sometimes it will even cause the problem of false soldering or failure to solder.
Looking back, the same is true for hotbar's hot indenter. Although it is set to 400 ° C, the actual temperature of solder paste will not be as high as 400 ° C after contacting the PCB pad. If there is such a high temperature, many electronic parts and circuit boards should not be able to withstand such a temperature. This is because the conduction of heat decreases with time and distance.
It is recommended to measure the temperature curve of the hotbar hot press welder once according to the reflow temperature curve, and it is better to select at least three pads (grounding pad, power pad, common pad) when measuring the temperature, because grounding and power pad are usually connected to large pieces of copper foil, which is easy to cause heat loss. The purpose of connecting common pads is to avoid temperature The setting is too high to cause coking.
As for the machine setting of hotbar hot press welding machine, there are basically at least two settings, and each stage can set its own temperature and time, which is similar to the reflow oven of two temperature zones, and this is one of the reasons why the welding time of hotbar can be reduced to such a short time. As for the relationship between temperature and time, as soon as the pulse current on the hotbar machine is turned on, it means heating at full speed until the set temperature and time, but the temperature here measures the solder on the hot indenter instead of the circuit board, so the temperature setting must be much higher than the melting temperature (217 ° C), which is similar to reflow's temperature setting, which is basically higher than the actual measurement To the same temperature.
The first stage of heating aims at preheating, which is generally set at about 150 ° c-170 ° C. This temperature is also the temperature that FPC can bear for a long time without damaging its structure. Generally, it takes about 2-3 seconds to heat the temperature of FPC, solder paste and circuit board to a certain extent first, so as to avoid unpredictable problems caused by rapid temperature rise For example, de lamination can also remove water vapor. In addition, it is also necessary to consider whether the pad of hotbar FPC is exposed to the back. If it is only a single-layer plate and there is no pad on the back, it will be very unfavorable for heat conduction to the solder paste. At this time, it may be necessary to increase the preheating temperature.
The second stage is the real solder temperature, so the temperature of lead-free solder is generally set at about 360 ° C ~ 400 ° C, so it is necessary to make the actual temperature of solder fall between 230 ~ 250 ° C, so as to determine the melting tin without scorching the soft plate, and the time is generally set at about 4 ~ 8 seconds. It is recommended to determine the temperature curve based on the characteristics of each plate and the actual measurement Setting of temperature and time in each stage.
Finally, I also stress that the temperature measurement required for hotbar soldering should be the temperature on the solder paste solder joint of PCB, which is also the actual temperature transmitted to PCB after the hot indenter is heated, and it is also the temperature when the solder is actually welded. Therefore, generally, the temperature curve of hotbar hot press welding machine should be measured in thermal mode Couple is connected to the hotbar pad of PCB, and then the PCB is actually heated with hot indenter, so the measured temperature is the temperature that can be used in welding.
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