PCB Technical - Principle and management of PCB circuit board reflow

1. Classification of PCB circuit board reflow curve

Generally speaking, the curve can be roughly divided into (1) RSS type with saddle (2) L type without saddle (RTS type Rampto Spike) (3) long saddle type (LSP type Low Long Spike) now explained below:

(1) With saddle type:

Starting from room temperature, heat up the walking board to the head of the saddle at a rate of 1-1.5°C/sec. Then use a slow rise or constant temperature method to pull up to the end of the saddle at 150-170°C within 60 to 90 seconds. The main function of this section is to allow the PCB circuit board and components to absorb enough heat, and the internal and external temperatures are equal In order to soar the peak temperature. The peak temperature of this Profi1e is about 240±5°C, the TAL (the duration above the melting point) is about 50-80 seconds, and the quenching rate is 3-4°C/sec, totaling 3-4 minutes.

(2) Saddleless type:

The temperature is increased linearly throughout the whole process, and the rate is controlled between 0.8-0.9 degree Celsius/sec, and the peak temperature is increased all the way to 240±5 degree Celsius. The L-shaped curve is preferably straight or slightly concave, and no bulge is allowed to avoid blistering on the surface of the thick plate due to overheating of the plate surface. And the 2/3 length of the heating line should not exceed 150 degree Celsius, and the other parameters are the same as above.

(3) Long saddle type:

When the PCB circuit board needs to be soldered with multiple BGAs, in order to reduce the voiding in the ball and supply sufficient heat to the ball in the abdominal bottom, the saddle with a saddle can be gently extended to drive away Volatile content in inner ball solder paste. The method starts with a heating rate of 1.25°C/sec. When it reaches the first saddle of 120°C, it takes 120-180 seconds to go to the end saddle and then the peak temperature soars. The other parameters are also the same as the above. .

2. The quality and technique of the mobile profiler

The number of thermocouples that this kind of necessary thermometer can pull varies from 4 to 36, and the brand and price are also very different (NT. 100,000 to 300,000). The data that can be recorded by the recorder of a good thermometer should include: the heating rate of the starting section, the temperature difference of the PCB board, the time consumption of the heat absorption section, the soaring rate before the peak temperature, the peak temperature reading, and the liquid state of the molten solder paste Important parameters such as duration (TAL) and the cooling rate of the final course.

In order to achieve the mission, the main box of the thermometer and the internal battery must be heat-resistant, and the shape must be flat enough not to be jammed by the furnace mouth, and the thermal error of the thermocouple wire itself must not exceed ±1 degree Celsius. Temperature measurement sampling The frequency should not exceed 1 time per second, the amount of memory should be large enough, the output data should also have the ability of statistical control (SPC), and the software upgrade should be simple and easy.

Generally, in the reflow of larger PCB circuit boards, the leading edge of the lead-in, of course, must be warmed up and cooled earlier than the middle or trailing edge. Moreover, the heat absorption and temperature rise of the four corners or the edges of the board must be faster and higher than the center, so the attached thermocouple wire should at least include these two areas. In addition, the body of large parts will also absorb heat, causing its pins to heat up more slowly than smaller passive components. Even the heat of the belly bottom of the large BGA is not easy to penetrate. At this time, the PCB of the belly bottom must be drilled separately, and the temperature sensing wire drawn from the bottom surface must be welded from the front of the board, and then measured when the BGA is attached. The temperature at the blind spot. Some components that are sensitive to strong heat should also be deliberately pasted with thermocouple wires as the primary condition for determining the reflow curve.

In order to avoid the heat-sensitive components and the high-level thick board from being damaged by strong heat, a thermometer must be used to find the "hottest spot" and "coldest spot" on the assembly board. The method is to take another test board with printed solder paste, first pass through the reflow oven at high speed (such as 2m/min), and then observe whether the double pads of small passive components (such as capacitors) on the side of the board have been soldered properly? Then reduce the speed again (for example, 1.5m/min) and try welding until the first welding spot appears, which is the hottest spot of the whole board. Then continue to reduce the speed (that is, increase the heat) until the final welding of the large component When the point is also completed, that is the coldest point of the whole board. Therefore, at a predetermined Spike temperature (such as 240°C), it will be possible to try to find the correct conveying speed of the assembled board. At this moment, the liquid state of the solder paste can also be measured by TAL. In this way, heat-sensitive components and high-level thick plates can be safe, so that they will not be burned or surface exploded during the strong heat of reflow.

3. Management of heat absorption in the saddle

In terms of the solder paste specifications of various brands of SAC305 or SAC3807, the time required for the saddle to absorb heat is about 60-120 seconds, and the temperature rises slowly from 110-130°C on the front saddle to 165-190°C. The saddle is hot. When the PCB is a thick multi-layer board (especially a high-level thick board), the components carried are also thick and large, and even if the installed BGA is not a small number, the 4-6 stage heat absorption will be very critical. The inner and outer parts of the thick plate and the thick parts must be fully absorbed by the heat, and the rapid and intense heat of the subsequent soaring peak temperature will not cause the thick plate or thick parts to burst due to the large temperature difference between the inside and the outside. At this time, the profile must be a reflow curve with saddle or hat brim.

However, if it is a small board or a single or double panel, the PCB components carried are mostly small ones, and the temperature difference between inside and outside is not large. In order to strive for production speed, the heat absorption section can be used to shorten the time and quickly heat up <1 degree Celsius/sec above), the saddle will disappear at this time, and the L-shaped profile will rise up and down like a roof along the road. However, the quality of the reflow furnace itself will be greatly affected at this time. The heat transfer efficiency of each section must be efficient and uniform, and when the board enters and loses temperature instantaneously, its rapid and accurate compensation capability must be fast and efficient. Do not cause the regional drop to be too large (the partial plate surface should not exceed 4 degree Celsius).

Fourth, the fit of solder paste and curve

In general solder paste formulations, 90% by weight is powdered (small ball) metal solder, and 10% is organic auxiliary materials. Excluding a small amount of supporting angle metals (such as antimony, indium, germanium, etc.), even if the main components are exactly the same (such as SAC305), the size and quantity of tin particles are in different proportions, and their fluidity and healing properties are also present. Quite a big difference.

As for the liquid duration (TAL) of the molten solder paste, it is also related to the type and thickness of the surface treatment of the PCB board to be soldered. Generally, the TAL commonly used in boards is 60 ah 100 seconds to reduce the strong heat treatment of the soldering board and the flux. However, it is still necessary to complete the healing of solder paste and the goodness of soldering or IMC. If the TAL is too long, it will cause damage to the parts and the board. Even the flux is also carbonized (Charring). However, if TAL is too short, it will obviously lead to smelting welding, poor tin eating or insufficient solder paste healing, resulting in loss of granular appearance. For the highly heat-sensitive board to be welded, it seems that the shortest TAL can be started from the test welding, and the most suitable welding conditions can be found through the more convenient fine-tuning of the travel speed without changing the hot air conditions of each section.