PCB Technical - Circuit board quality inspection and lack of SMT technology

Circuit board quality inspection and lack of SMT technology

1. Quality inspection
(1) X-ray inspection
After assembling, use X-ray to see the defects such as bridging, open circuit, insufficient solder, excessive solder, ball drop, missing line, popcorn, and the most common voids in the hidden solder joints of the bottom of the BGA. The following table shows the occasions and effects where various inspection methods can be implemented.

(2) Scanning ultrasonic microscopy
The finished assembly board can be scanned by SAM to check various hidden conditions. The packaging industry is used to detect various hidden voids and delaminations. This SAM method can be further divided into three scanning imaging methods: A (dotted), B (linear), and C (surface). The C-SAM surface scanner is the most commonly used.

(3) Side-view Yan-like sharp method
The method can be used for lateral visual inspection with optical magnification for tiny things in the restricted blind area. The welding condition of the BGA ball can be used to check the condition of the outer ring. This method uses a prism to rotate a 90° lens to focus, and then pair it with a high-resolution CCD to transmit the image. The magnification is between 50X and 200X, and positive light and backlight observation can also be implemented. It can be seen that the solder joints are: overall appearance, tin consumption, solder joint shape, solder joint surface pattern, flux residue and other shortcomings. However, this method cannot see the inner ball of the BGA, and it is necessary to use a very thin fiber tube endoscope to extend into the abdomen for direct observation. However, although the concept is good, it is not pragmatic. It is not only expensive but also easy to break.

(4) Screwdriver strength measurement method
Use the torsion moment that occurs when the special screwdriver rotates to lift and tear the solder joints to observe how strong it is. Although this method can find defects such as floating of solder joints, interface split, or weld body cracking, it is not effective for thin plates.

(5) Microsection method
This method not only requires various facilities for sample preparation, but also requires sophisticated skills and rich interpretation knowledge in order to use a destructive approach to find out the true problem.

(6) Infiltration dyeing method (commonly known as red ink method)
The sample is immersed in the diluted special red dye solution, so the cracks and small holes in the various solder joints are capillary infiltrated, and then they are dried. After each test ball is pulled or pried off by force, you can check whether there is erythema on the cross section, and see how the integrity of the solder joint is? This method is also known as Dye and Pry. The dye solution can also be separately prepared with fluorescent dyes, which will make it easier to see the phase in the ultraviolet light environment.

2. Hollow feet and other shortcomings

(1) Causes of solder joint voids
The solder joints formed by various SMT solder pastes will inevitably have cavities of varying sizes, especially the BGA/CSP ball pin solder joints have more cavities, and after entering the high-heat lead-free soldering, their cavities are The trend is adding fuel to the fire, and the severity is bound to be far greater than before. Investigating its causes can be roughly classified into the following categories:

1) Organic materials: The solder paste contains about 10-12% organic matter by wt. Among them, more fluxes have the greatest influence. The degree of cracking and gassing of various fluxes is different, and the one with less gassing rate should be selected. The best policy. Secondly, the flux in the high heat will adhere to the oxide on the solder surface, so the oxide can be quickly removed to reduce the formation of voids. Since the lead-free solder is not good, it will make the void worse.

2) Solder: When the molten solder comes into contact with the clean surface to be soldered, it will immediately generate IMC and be welded firmly. However, this reaction will be affected by the surface tension of the solder. The greater the surface tension, the greater the cohesion, so the adhesion or fluidity required for outward expansion will become worse. As a result, the organic matter or bubbles in the solder paste solder joint of SAC305, which has a large surface tension, cannot escape from the solder body, but can only be detained in the body and 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 accumulate more.
3) Surface treatment: Where the surface treatment film is prone to staining, the voids will be reduced, otherwise the shrinkage or solder rejection will cause bubbles to gather and form large holes. As for the interface micro-holes that are prone to cracking of the solder joints, the two types of silver immersion are more common. There is a transparent organic film on the surface of the immersion silver, which can be used to prevent the silver from discoloration; because the silver layer will quickly dissolve in the liquid tin during soldering to form Ag3Sn5 IMC. The remaining organic film will inevitably crack and become micro-holes in strong heat, especially called "champagne bubble wipe". Therefore, it is known that the silver layer should not be too thick and should be less than 0.2μm. If the OSP is too thick, it will also produce interface micro-holes, and the film should not exceed 0.4 μm.

4) Sometimes those with larger pad area are more likely to have voids or micro-holes. In this case, splitting can be used to add several gas-out ditches, or green paint crosses can be printed to facilitate gas escape and avoid voids. As for the voids caused by the micro-blind holes, of course, the best choice is the electroplated copper hole. Other effective methods to avoid solder paste absorption, to prevent excessive roughness or organic residual film on the copper surface, are also effective methods to reduce voids.

(2) Hollow acceptance specifications
Too many holes in the ball will affect its electrical conductivity and heat transfer, and the reliability of the solder joints is not good. In the table below, the allowable upper limit of the diameter of the hole in the top view section of the ball diameter is 25%. The diameter of this 25% is approximately equal to 6% of the total contact area, and the large and small holes must be calculated together. Holes in the interface between the ball pin and the carrier board or the upper and lower solder pads on the circuit board are actually the main cause of cracking.

(3) Void classification
BGA voids can be divided into 5 categories according to their location and origin. The classification of voids in the above list chart can be said to be very rough according to conscience, and it will inevitably be revised in the future.

(4) Build a bridge
The reasons for the bridge and short circuit between the balls may include: poor solder paste printing, incorrect placement of components, manual adjustment after placement, or tin splashing during welding. The reasons for Open include poor solder paste printing, mobilization after placement, poor coplanarity, or poor solderability of the PCB board surface pad.

(5) Cold bomb
The main reason for Cold Solder is: insufficient heat, no IMC is formed between the solder and the soldered surface, or the number and thickness of IMC are insufficient, so that it fails to exhibit strong strength. This kind of shortcoming can only be carefully checked with an optical microscope and microsections.