BGA(Ball Grid Array), a surface-bonding encapsulation technique used in integrated circuits. Compared with other packaging technologies, BGA can accommodate more pins. (More at Wikipedia)
Advantages are high density, low thermal impedance and low inductance pin. Disadvantages are non-ductility, difficulty in inspection, difficulty in developing circuit and high cost.
According to different packaging materials, BGA components mainly include the following: plastic BGA, ceramic BGA, ceramic column BGA, tape BGA and chip scale package.
(BGA Type at Introduce About BGA)
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In BGA encapsulation, the pins at the bottom of the encapsulation are replaced by solder balls, each of which is initially secured with a small solder ball. Configure these balls manually or by automated machines and position them by flux. When the surface welding device fix to the PCB, the arrangement of the bottom welding balls corresponds to the position of the copper foil on the board.The line is then heated in a reflow or infrared furnace to dissolve the weld ball.Surface tension causes the molten ball to support the packing and align with the circuit board.At the correct separation distance, when the ball is cooled and fixed, the resulting solder joint can be connected to the device and PCB. The composition of the solder alloy and the soldering temperature are carefully chosen to ensure the solder does not completely melt, but stays semi-liquid, allowing each ball to stay separate from its neighbours.
Before soldering BGA, first we need to thoroughly clean PCBs and BGA. For BGA, the requirement is to make its surface smooth without any burr.
BGA Cleaning Method:
1. Place the BGA on a conductive pad and apply a small amount of solder paste on the surface of the BGA.
2. Remove the ball from the BGA with a solder wire and soldering iron.Place the solder cord on the solder aid and place the soldering iron on the solder cord. Allow the soldering iron to heat the absorbent wire and melt the tin ball before you swipe the wash line over the BGA surface. Note: do not let the soldering iron press on the surface.Too much pressure will cause cracks on the surface to scrape off the pad.
3. Clean the BGA surface immediately with industrial alcohol. Use friction motion to remove solder aid on BGA surface.Keep moving and cleaning.Always start with the edges and don’t forget the corners. Clean each BGA with a clean solvent
4. Examination under a microscope, observe clean pads, damaged pads, and unremoved tin balls.
5. Scrub the BGA surface with deionized water and a brush.
6. Rinse BGA surface with deionized water and brush.This helps remove residual solder paste from the BGA surface. Let BGA dry in the air.Double-check the BGA surface.
Then, bake the PCB and BGA in a constant temperature oven at 80℃~90℃ for 10~20 hours, in order to remove moisture and adjust the baking temperature and time appropriately with different degrees of moisture. In particular, wear static rings or antistatic gloves during all of the following operations to avoid possible damage to the chip caused by static electricity.
Before welding the BGA, align the BGA exactly on the pad on the PCB. There are two methods: optical alignment and manual alignment. At present, the main use of manual alignment, that is, the BGA around the PCB pad around the silk screen line alignment. Here is a trick: in the process of aligning the BGA and the silk screen line, not fully aligned in time, even if the tin ball and solder pad deviate from about 30%, still can be welded. Because during the melting process, the ball will automatically align with the pad due to the tension between it and the pad. After the alignment operation is completed, place the PCB on the BGA workstation and fix it so that it is level with the BGA workstation. Select the appropriate hot air nozzle, then select the corresponding temperature curve, start welding, when the temperature curve is finished, cooling, and then complete the welding of BGA.
Since the pads are below the package it is not possible to do a visual inspection of the soldering. so electronics manufacturers commonly use Automated X-ray Inspection to inspect the BGA soldering quality. This form of BGA inspection is able to look through the device at the soldered joint beneath.
With the X-ray device, the image of the solder on the pad layer is “Shadowed” because the solder on the spot is above it. Cross-sectional X-ray detection technology has the ability to detect hidden weld defects.
It can reveal the connection of BGA welding points by focusing on the soldering points of the disc layer. Cross section images produced by direct measurement of two or more different heights of the BGA connection and combined with multiple slice measurements of similar BGA connection points, it can effectively provide three-dimensional testing, which can be conducted without physical cross section of BGA connection points .
According to the conventional structure of the BGA junction, the characteristics of the junction are separated and measured in each cross-sectional X-ray image to provide quantitative statistical process control (SPC) measurements. SPC measurements can be used to track process offsets and classify their characteristics into the corresponding defect category. More than three images can be used to clearly observe the BGA welding point, thus detecting the BGA.
When the soldering has not correctly performed, we need to rework.
If the rework need no removing the component, you have to re-heat the PCB around the component in order to try to melt again the solder and to improve the soldering.
If it need to remove the component and re-solder it again. It is more complicated than previous one, rework and hand soldering of BGA Packages is the toughest part.
1. Remove the BGA
Clean the residual solder of PCB pad. Do not damage the pad and solder mask during operation.
2. Remove the moisture
Because BGA is sensitive to moisture, check the device for moisture and moisture before assembly.
3. Printing solder paste
Since other components are already installed on the surface, must choose small template for BGA. The thickness and opening size of the template should be determined according to ball diameter and ball distance. After printing, we must check the printing quality. For CSP with ball spacing below 0.4mm, can not print solder paste, apply it directly on the PCB pad.
4. Clean the weld pad.
Clean up the residual solder of PCB pad with a soldering iron and level it off. Use the disassembly belt and the flat flat soldering tip to clean it up. Do not damage the pad and solder mask during operation.
5. Remove the moisture again and print the solder paste again
6. Mount BGA
Put the surface assembly plate printed with solder paste on the worktable, then select the proper suction nozzle and turn on the vacuum pump. Suck up the BGA device, move the nozzle down after the bottom of the BGA device and the PCB pad completely coincide, attach the BGA device to the PCB, and then close the vacuum pump.
7. Reflow soldering.
Set the soldering temperature according to the device size, PCB thickness and other specific conditions. Compared with traditional SMD, the soldering temperature of BGA is about 15 degrees higher.
Hold up pcb assembly board of the soldered BGA, look at the light around the BGA, observe whether the light is permeable, whether the distance between the BGA and PCB is consistent, whether the solder paste is completely melted, whether the shape of the welding ball is correct, the degree of collapse of the welding ball etc..
BGA is important for making electronic devises and gadgets smaller and slimmer. BGA technology and soldering is very reliable if done using the correct procedure.
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