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PCBA quality inspection
1.X-ray inspection
After PCBA assembly, the use of X-ray can see the hidden solder joints on the bottom of the BGA belly of the traction, open circuit, insufficient solder, excessive solder, ball drop, loss of Huaihua, popcorn, as well as the most commonly occurring voids and other deficiencies.
2.Scanning Ultrasonic Microscopy
Completed assembly boards can use SAM scanning to check a variety of internal hidden conditions, the packaging industry is used to detect a variety of hidden voids and delamination. The SAM method can be divided into A (point), B (line), C (surface) and other three types of scanning imaging pipeline, to C-SAM surface scanners are most commonly used.
3.Side-view sample sharpening method for PCBA inspection
This method can focus on the confined dead zone of the tiny things, optical magnification of the lateral visual inspection. BGA's ball and socket soldering can be used to check the outer ring. This method uses a prism that rotates 90° to focus the lens and a high resolution CCD to transmit the image. The magnification ranges from 50X to 200X, and it is possible to observe both front and backlight. The visible conditions of the solder joints include: general appearance, solder consumption, shape of the solder joints, patterns on the surface of the solder joints, flux residues, and other defects. However, the inner ball of the BGA cannot be seen by this method, and it is necessary to use a very small fibre optic tube to reach into the belly to observe it directly. However, although the concept is good but not practical, not only expensive and easy to break.
4.Screwdriver Intensity Measurement Method
The use of special screwdriver rotating torsional moment that occurs, to top up and tear the welded joints, in order to observe the strength of how. This method can find out the solder joints floating away, interface splitting, or welding body cracking and other defects, but the effect on the PCB thin board is not good.
5.Micro slicing method
This method not only requires a variety of facilities for sample preparation, but also requires sophisticated skills and a wealth of knowledge of interpretation, in order to destroy the practice, to find out the root of the real problem.
6.Infiltration dyeing method (commonly known as the red ink method)
The sample is immersed in a diluted solution of the special red dye, so that the cracks and holes of the various solder joints are penetrated by its capillary, and then baked to dry. When the test ball and pin is pulled or pried apart, the integrity of the solder joint can be seen by checking for red spots on the cross-section. This method is also known as Dye and Pry, and the dye solution can also be prepared with fluorescent dyes, making it easier to see the truth in a UV environment.
BGA Ball-Grid Void and Other Defects
Causes of Solder Joint Voiding
All kinds of SMT solder paste formed by the solder joints, will inevitably appear in the size of the hole, especially in the BGA/CSP ball foot type of solder joints of the hole more,and into the high heat of the lead-free soldering,the tendency of the hole is even more fuel, the degree of deterioration is bound to be far worse than before.The causes can be categorised as follows:
Organic materials: solder paste contains organic matter about 10-12% by wt,which is more flux has the greatest impact, a variety of flux cracking gas degree is different, should be selected less gas rate for the best policy. Secondly,the flux in the high heat will turn on the oxide on the surface of the solder, so it can quickly remove the oxide can reduce the generation of voids. Since lead-free solder is not good, it will also worsen the voiding.
PCBA solder: When molten solder comes into contact with a clean surface to be soldered, IMC will be generated immediately and the solder will hold. However, this reaction will be affected by the size of the surface tension of the solder, the surface tension is larger the cohesion is also large, so the adhesion or mobility required to expand outward will be worse. As a result, the SAC305 with higher surface tension will not allow organic matter or air bubbles in the solder joints to escape from the solder body, but can only be detained inside the body to become a cavity. Once the melting point of the solder ball is lower than that of the solder paste, the voids will continue to float into the ball and gather more and more, the following two diagrams are illustrative of its ideas
Circuit board surface treatment: Where the pad surface treatment film is easy to stain the tin, the cavity will be reduced, otherwise the shrinkage of tin or refused to solder, etc., will lead to the aggregation of gas bubbles into a large hole. As for the interface micro-hole which is easy to cause solder joint cracking, it occurs more often in the case of silver dipping. There is a transparent organic film on the surface of immersion silver, which can be used to prevent silver from discolouring; since the silver layer will dissolve rapidly in liquid tin during soldering to form the IMC of Ag3Sn5, the remaining organic film will inevitably crack and become a micro-hole in strong heat, which is specially known as ‘champagne bubbles’, so it is known that the silver layer should not be too thick, and a layer of 0.2 μm or less is preferable. If the OSP is too thick, it will also produce micro-holes in the interface, and its film should not exceed 0.4 μm.
Immersion silver plating is used to replace the film produced by the pipeline on the copper surface of the acidic bath. In order to fulfil the function of anti-colouring, a film of organic molecules is applied to the outer surface so that the silver surface does not deteriorate to the extent that the solderability is affected. During the subsequent soldering reaction, the silver is rapidly dissolved into the liquid tin to form Ag3Sn5 flake IMC. Once the organic film left on the interface is subjected to intense heat, it is likely to crack and become a multitude of small voids,which is specially known as ‘champagne bubbles’.
Sometimes the solder pad area is larger is also more prone to cavities or micro-holes,this time the split method can be added to a number of gas out of the groove, or printed on the green paint crosshairs, to facilitate the escape of gas and avoid cavities. As for micro blind holes, electroplated copper reclamation is the best choice.Avoiding water absorption of solder paste, eliminating excessive roughness of copper surface or organic film residue, etc. are also effective practices to reduce voids.
PCBA BGA Voiding Allowance Specification
Too many holes in the ball foot will affect the conductivity and heat transfer, and the reliability of the solder joint is not good.The table below shows the maximum allowance of 25% of the diameter of the hole in the top-view profile of the ball diameter, this 25% of the diameter is equal to 6% of the total contact area, and the size of the hole should be calculated together.The main cause of cracking is the hole at the interface between the ball and the carrier board or the pads on the top and bottom of the board.
The diameter of the hole must not exceed 25% of the top viewed ball diameter. The middle figure illustrates that if the diameter of the hole is 35% of the diameter of the ball, it is equivalent to 12% of the area of the pad. The right figure shows the new X-Ray instrument with special software, which can measure the ball diameter, void area and a single large hole.
PCBA BGA Void Classification
BGA voids can be classified into 5 categories according to their location and source. In all conscience, the above list is very rough in classifying the voids, and will certainly be revised again in the future.
BGA Leads
The causes of BGA wire tracing may include: poor solder paste printing, incorrect placement of components, manual adjustment after placement, or spattering of solder during soldering. Open can be caused by poor solder paste printing, manual adjustment after placement, poor coplanarity, or poor solderability of the board pads.
BGA Cold Solder
Cold Solder is mainly caused by insufficient heat, the IMC is not formed between the solder and the soldered surface, or the amount and thickness of IMC are insufficient to show strong strength. Such defects can only be examined by optical microscope and microtome.