1) X-ray testing
After assembly, X-ray inspection can be used to inspect the hidden solder joints at the bottom of BGA for short circuits, open circuits, insufficient solder, excessive solder, ball drop, loss of bad points, popcorn, and the most common holes and other defects.
2) Scanning ultrasound microscope
The assembled circuit board can be scanned using SAM to check for various hidden situations, and the packaging industry can detect various hidden holes and layers. This SAM method can be divided into three scanning and imaging methods: A (point), B (line), and C (plane), among which C-SAM plane scanning is the most commonly used method< Img src=“ https://www.wellcircuits.com/wp-content/uploads/20230109/3e413d8a-b41d-11ee-89cc-705ab6a7e753/fb5ef3fd9d4309283aefd2d17d5cf590.jpg 3) Side view method
The side visual inspection of optical amplification can be used to examine small objects in the restricted dead angle area, such as checking the outer ring of BGA’s ball foot welding. In this method, the prism is rotated 90° to focus the lens, and a CCD with high resolution is used to transmit the picture. The magnification is between 50X and 200X, and positive light and backlight observation can also be carried out. This method allows for the examination of solder joint conditions, including overall appearance, tin eating, solder joint shape, solder joint surface pattern, flux residue, and other defects. However, the inner sphere of BGA cannot be seen using this method and requires direct observation using a very thin fiber tube endoscope. Despite the potential, this method is not practical due to its high cost and susceptibility to breakage.
4) Screwdriver strength measurement
This method involves using the torsion moment generated by a special screwdriver to jack up and tear the solder joint in order to observe its strength. While this method can identify defects such as floating solder joints, interface splitting, or weld body cracking, it is not effective for thin plates.
5) Microsection method
This method requires various facilities for sample cutting and preparation, as well as sophisticated skills and rich interpretation knowledge to find the root cause of the problem in a destructive manner.
6) Infiltration dyeing method (commonly known as red ink method)
This method involves immersing the sample in a diluted special red dye solution to capillary infiltrate the cracks and small holes of various solder joints, followed by drying. This allows for the identification of red spots on the section when each test ball foot is pulled or pried off by force, providing insight into the completeness of the solder joint. The dye solution can also be prepared with fluorescent dyes for easier visualization in ultraviolet environments.
2) Hollow ball foot and other defects
2.1 Causes of solder joint voids
Solder joints formed by various SMT solder pastes inevitably have holes of different sizes, especially BGA/CSP ball pin solder joints, which worsen after entering high-heat lead-free soldering. The causes can be classified into several categories:
2.1.1 Organic materials: The solder paste contains about 10-12% by wt., with fluxes having the greatest impact. The degree of cracking and gas generation varies among fluxes, so selecting one with a lower gas generation rate is the best strategy. Additionally, flux adhering to the oxide on the solder surface can be reduced to minimize void formation. Lead-free soldering exacerbates the cavity formation.
2.1.2 Solder: The reaction of molten solder with the clean surface to be welded is affected by the size of the solder’s surface tension. A larger surface tension results in greater cohesion, making outward expansion more challenging. This can trap organic matter or bubbles in the solder paste, forming a cavity. Lower melting point solder balls can allow holes to float into the ball and accumulate more.
2.1.3 Surface treatment: The film of the surface treatment can impact hole formation, particularly those caused by solder joint cracking. Silver immersion can lead to interface micro-holes due to the rapid dissolution of the silver layer in liquid tin during welding, forming IMC of Ag3Sn5 which inevitably results in “champagne bubbles.” Excessive thickness of OSP can also generate interface micro-holes.
2.1.4 Large welding pads and micro-blind holes can be prone to cavities or microholes. Adding outgoing ditches or printing a green paint cross line can facilitate gas escape and prevent cavities. Filling the holes with electroplated copper is also effective in reducing cavities.
2.2 Hole acceptance specification
Excessive holes in the ball foot can affect conductivity and heat transfer, leading to poor solder joint reliability. The upper limit of allowable hole diameter in the top section is 25%, with the diameter of this 25% being about 6% of the total contact area. Void size must also be considered. Voids at the interface between the ball foot and the carrier plate or upper and lower welding pads of the fr4 pcb are a main cause of cracking.
2.3 Cavity classification
BGA holes can be categorized into five types based on their location and source, although this classification may need future revision.
2.4 Bridging
Bridging and short circuits between ball pins may result from poor solder paste printing, incorrect component placement, manual adjustment after placement, or tin splashing during fusion welding. Open circuits may occur due to poor solder paste printing, adjustment after placement, poor coplanarity, or poor soldering of the solder pad on the board.
2.5 Cold elasticity
Cold Solder results from insufficient heat, causing a lack of IMC formation between the solder and the surface to be welded. This can only be examined carefully by optical microscope and microsection.
