Some methods and summary of BGA welding process

May 15, 2023

according to the type of solder ball selected and the size of PCB.Parameter 3: The holding time at this temperature after the heating reaches the higher temperature of this section, which is usually set to 30 seconds.The general method of adjustment is to find a motherboard with a flat and invisible PCB, weld it with the curve of the soldering station, and insert the temperature measuring line of the soldering station between the chip and the PCB at the end of the fourth curve to obtain the temperature at this moment.The negative value can reach 217 degrees without lead and 183 degrees with lead.These two temperatures are the theoretical melting points of the above two kinds of tin balls!But at the moment, the solder ball under the chip is not completely melted. From the point of view of repair, the temperature is at the mercy of 235 degrees for lead-free and 200 degrees for lead.At this moment, the chip solder ball will reach the maximum strength after melting and then cooling. Take lead-free as an example: after the end of the four curves, if the temperature does not reach 217 degrees, the temperature of the third and fourth curves will be moderately increased according to the size of the difference.For example, if the measured temperature is 205 degrees, the upper and lower air outlet temperatures are increased by 10 degrees respectively. If the distance is large, for example, if the measured temperature is 195 degrees, the lower air outlet temperature can be increased by 30 degrees and the upper air outlet temperature can be increased by 20 degrees. Note that the upper temperature should not be increased too much to avoid harm to the chip!After heating, the measured value is 217 degrees, which is the desired state. If it exceeds 220 degrees, the higher temperature of the chip before the end of the fifth curve should be investigated. Usually, it should be avoided to exceed 245 degrees. If it exceeds too much, the temperature set by the fifth curve can be moderately reduced.

4. Pay attention to the fourth point of welding: proper use of flux!BGA flux has a special meaning in the soldering process!Whether welding from the beginning or direct repair welding, we need to apply flux first.When welding the chip, you can use a small brush to apply a thin layer on the clean pad, try to wipe it evenly, do not brush too much, otherwise it will also affect the welding.During repair welding, a brush can be used to dip a small amount of flux near the chip.Flux, please choose BGA welding special flux!5. Pay attention to the fifth point of welding: the alignment must be accurate during chip welding.Since our rework stations are equipped with infrared scanning imaging to assist in alignment, there should be no doubt about this.If there is no infrared assistance, we can also refer to the frame line near the chip for alignment.Be careful to place the chip in the center of the square line as far as possible. A slight offset is too big. Because the solder ball will have an active return process when it melts, a slight offset will be active return!In the process of soldering, we will inevitably come into contact with the operation of planting tin balls.Generally speaking, the following things are required for ball planting: 1. Tin ball.Now the diameter of the tin ball we commonly use is usually 0.2mm-0.76mm.During this period, 0.6MM standard solder balls are only found on the main chip of MS99 movement, and 0.25 MM standard solder balls are only used on EMMC program IC.Other chips, whether DDR or the main chip, all use the 0.45 MM standard (of course, it is also possible to use 0.4MM).Advocate the use of lead and tin balls together, so as to contrast with simple soldering.2. Steel mesh.Due to the limitation of the universality of our chips, there are few matching steel meshes on the market that are completely consistent with our chips. I personally choose the etched steel mesh of Dataifeng Company. This steel mesh has an advantage that it is relatively thin, without burrs, and relatively smooth. Whether it is scraping solder paste or placing solder balls, the effect of planting balls is very good.3. Planting table.There are many kinds of ball planting tables, which are used more and have a high success rate. I recommend the customized ball planting table of Dataifeng, which is easy to operate, accurate in positioning, fast and accurate in ball planting, time-saving and labor-saving, and has a high yield rate.Moreover, for different BGA ball planting, only the mold core and the steel mesh can be replaced, and the base and the ball frame under the scraping tin frame are universal, which is more economical.Detailed operation method 1. The removed chip pad shall be treated flat with tin suction wire and rosin solder paste, and shall be cleaned with board washing water and then dried with an air gun.2. Apply a layer of flux evenly on the chip, not too much, but a thin layer.3. Place the chip under the steel mesh and place it on the ball placement table, and then carefully adjust the placement position of the steel mesh hole and the chip pad to ensure accurate correspondence.4. After alignment, scatter the solder balls onto the steel mesh, and shake the ball planting table back and forth and left and right, so that each BGA pad is stuck with the solder balls.5. Miss the target. Check that each BGA is stuck with a solder ball, and then the ball planting is completed.6. After the above steps are ready, put the BGA on the heating platform for heating, remove the air gathering port in front of the air gun, and adjust the temperature of the air gun to a suitable temperature (with lead, use the temperature melting ball with lead tin ball, and without lead, adjust to the temperature melting ball without lead tin ball), and adjust the wind speed to a very low level.To avoid too high temperature or too rough wind speed to cause the failure of ball planting, after adjustment, the tin ball can be heated evenly.When heating, pay attention to the color change of the tin balls. When the tin balls are heated to a significant brightness and the tin balls are arranged in an orderly manner, they can be stopped. The whole process takes about 20-30 seconds.Details are related to the air gun and the solder ball used.7. After stopping heating, put the BGA on the cooling surface for natural cooling.The solder balls have been basically implanted. The rest of the work is to clean the chip pads with board washing water and remove the solder balls on some vacant positions. Let's see if there is a single pad that is not implanted or not implanted well.In this case, you can paint a small amount of flux on the pad and put the solder ball in place with a camera, and then blow it with a small air gun.This is the end of the ball planting.

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