PCB Technical - Simple Failure Analysis Method of Solder Joint Strength of Circuit Board

Simple Failure Analysis Method of Solder Joint Strength of Circuit Board

I. Introduction
There are many failure analysis methods for the strength of SMT solder joints, but for BGA ball-foot soldering feet that are deep in the bottom of the stomach and do not see the sun, most of them are destructive post-mortem judgments. Common ones include the push-ball shear test, the three-point bending or four-point bending test of the finished assembled panel, the tear test of the frontal direct tension, etc.; even if the root cause such as the fracture point is found afterwards, However, often due to the excessively violent actions of the experiment, it is inevitable that the evidence will be destroyed by obscure clues.

This is a schematic diagram of the so-called three-point bending destructive test by applying pressure at a single point in the opposite direction of the BGA obliquely. This is a schematic diagram of the so-called three-point bending destructive test by applying a single point diagonally to the opposite direction of the BGA; the other four-point bending test refers to a two-point diagonal pressing down and a twisting deformation with two diagonal supports test. If the sample can be dyed before the various damage tests, that is, the red dye exploration method is used to make the details of the cracks penetrated into the micro-slit failure, and then to perform the forceful damage to both sides, the original failure There is still a surviving experience in the original situation, so when the failure analysis is carried out under such conclusive evidence, of course, there will be no scenes of guessing and shooting birds.

For BGA's many ball-and-foot area mountings, the peripheral solder joints may still be able to be visually inspected by oblique peeking, but most of the ventral solder joints that are hidden and isolated from the world can only rely on X-Ray The fuzzy image of perspective is a guess. At this time, if you can dye the paste first and then destroy it, you will be very clear, and the original shape will show up, how can it be compared with X-ray?

Second, the implementation of the dyeing method
A major advantage of this Dry-n-Pry (dyeing and reconnaissance) dyeing method is that it does not require expensive and complicated machinery. The basic condition of these special dyes is that they can be added with a solvent to reduce their viscosity, so that micro cracks in some solder joints can be used to sneak in. Another skill that must be possessed is "quick-drying and easy-drying". If necessary, the temperature can be increased to accelerate the drying to facilitate subsequent observations and judgments. Some mechanical workshops often use marking inks on metal materials, and Chu Caijin can use it for this occasion.

The operation method can use a straw to draw a little dye, and then aim at the BGA to be inspected, so that the dye can be carefully scattered into the target area, and the angle of the board can be changed repeatedly if necessary to assist the penetration and dispersion of the dye. It is best to put the board to be inspected into the vacuum box, and with the assistance of vacuuming, let the dye smoothly enter the cracks of the solder joint. This method can also accelerate the escape of the solvent and the drying of the dye.

After the above-mentioned dye treatment and drying, the failed BGA or CSP can be rigidly torn apart under the force of external force. Then you can directly observe the cracks of each solder joint. Of course, the new shiny section is not the previous failure, and the crack or section that has been dyed must be the original failure location.

This is the appearance of densely mounted multiple small BGAs or CSPs on the surface of the circuit board. This is the appearance of densely mounting multiple small BGAs or CSPs on the board; you can apply dye (the red dot next to the arrow) from the outer edge of the package (that is, where the arrow points), and make it gradually infiltrate the BGA or CSP Color the failure position in various places on the bottom of the abdomen. There are many ways to remove the BGA. For example, a flat-head screwdriver can be used to pry it open, or it can be rotated left and right to facilitate its separation and ejection. For thin plates, you can also bend the surface carefully in the opposite direction to tear the solder joints; but if the surface is too small or too thick, if it is not easy to bend, you have to remove the rubber cover on the upper layer of the BGA to reduce it. tenacious. At this time, you can use a flat wide-mouth spatula to forcefully insert or squeeze into the interface between the rubber cover and the carrier board. Once the rubber cover is pried off, the remaining carrier board will naturally be easier to remove with needle-nose pliers. It was carefully removed from the PCB.

3. Examples of failure analysis
First, use a few examples to illustrate the application of the above-mentioned dyeing fault detection method, which is aimed at the failure analysis of a certain BGA solder pad, and the many ball-foot solder pads of a certain BGA have cracked display on the assembly board. It can be seen from observation that the floating crack is due to the separation of the black pad in ENIG. This kind of method of preventing the evidence from disappearing due to the dye sneaking in first, the creativity is not great, but it is brilliant! Of course, those without traces of dye are good solder joints. Under the pre-layout of the dye, as long as the BGA on the assembly board is smoothly pried, the quality of the many solder pads at the bottom of the belly will naturally be completely clear.

The second case is the "fatigue cracking" that often occurs in the ball-foot solder joints of a CBGA after temperature cycling. The rest of the shiny and round cushion surface, the outer periphery is surrounded by red, of course, the interconnection points with good strength have not failed. The BGA ball pad on the board has a black pad due to the poor surface treatment of ENIG, and it can also be penetrated by the dye first, making the original shape uncovered afterwards.

The BGA ball pad on the board has a black pad due to the poor surface treatment of ENIG, and it can also be penetrated by the dye first, making the original shape uncovered afterwards. This is the appearance of the large multi-leg CBGA board surface solder pad after tearing. The center is less applied, and the periphery and even the four corners are pulled the most strongly by external forces, and fatigue often occurs after the temperature cycle test. Rift.

After the large-scale multi-leg CBGA is torn apart, the surface of the board surface solder pad has less application at the center, and the periphery and even the four corners are pulled the most strongly by external forces, and fatigue cracks often appear after the temperature cycle test. .

The "cold solder joints" revealed with the assistance of dyes. Before the large BGA is mounted on the PCB, of course, solder paste should be printed on the ball pads on the board, and then the ball pins of the BGA should be seated in pairs, and the subsequent hot air Fusion welding. It may be due to the lack of detail in the finalization of the heating curve, which caused the twists and turns of the curve to take too long for the constant temperature absorption period, so that the flux in the solder paste was dried or even cracked, not only failed to solder the ball foot and the pad. At the critical moment, he reached out to help, but the corpse position vegetarian meal formed an obstacle to welding. As long as such cold welding or pseudo welding is thoroughly exposed by the dye and then tested by external forces, the facts about Dried lux will of course become clear. Because the fusion welding heating section is too long, the flux in the solder paste is dried to form a cold solder joint. Once the board is bent in the opposite direction, each dummy solder joint will split in response.