Reasons For PCB Resin Material Cracking Under BGA Pads During SMTP Processing
Cracking of PCB resin material occurs due to the presence of entrapped moisture. The reason for this is a high soldering temperature that results in an increase in vapor pressure. The cracks can also occur because the board’s thermal expansion causes the spacing between the BGA pads to change. To mitigate the risk of this type of fault, alternative pad finishes can be used, which reduces the thermal impact on adjacent packages.
Entrapped moisture causes pcb resin material cracks
Entrapped moisture can cause a wide range of PCB failures, including delamination, blistering, and metal migration. It can also change the dielectric constant and dissipation factor, reducing circuit switching speed. Moisture also increases stress levels in various PCB features, including copper and bga pads. It can also lead to oxidation on copper surfaces, which reduces the wettability of finishes. Additionally, it can increase the occurrence of electrical shorts and opens. This is especially problematic because PCB fabrication involves many steps that involve the use of water.
During smt processing, entrapped moisture can result in cracks in the PCB resin material. Because of this, PCB manufacturers should pay attention to the size of the solder mask opening. The size should be smaller than the desired land area. If the pad area of the SMD is too big, it will become difficult to route the solder ball.
Reflow soldering temperatures increase vapor pressure
Various factors can influence package warpage during BGA soldering. These include preferential heating, shadow effects, and highly reflective surfaces. Fortunately, forced convection reflow processes can reduce these effects.
A high reflow temperature can lead to a deterioration of the solder bump. The rise in temperature can lead to a reduction in solder joint height, resulting in a solder standoff that is smaller than the original height of the solder bump.
The shape of the attachment pad is also an important factor in determining the robustness of the solder joint. It is recommended to use larger, wider pads than smaller ones. The increased area increases the chance of cracking.
Tacky flux reduces thermal impact on adjacent packages
Tacky flux is a thermosettable material used during chip scale and flip chip package assembly. Its composition consists of reactive chemicals, which are solubilized in the underfill material during reflow heating. Once cured, the tacky flux becomes part of the net work structure of the final package.
A chemical wetting agent, fluxes facilitate the soldering process by reducing the surface tension of molten solder, allowing it to flow more freely. They can be applied by dipping, printing, or pin-transferring. In many cases, they are compatible with epoxy underfill. This enables them to reduce the thermal impact of adjacent packages during smt processing.
Using tacky flux reduces thermal impact on adjacent packages during soldering. However, this method has limitations. Several factors can cause flux to fail. Impurities in the flux can interfere with the soldering process, making the solder joint weak. Additionally, it requires costly equipment to properly clean the solder paste before soldering.
Alternative pad finishes
The crack propagation behavior of a PCB can be affected by the pad finishes used. Various methods have been developed to solve this problem. One of these methods is the use of an organic solderability preservative. This preservative is effective against pad oxidation. In addition, it helps maintain the solder joint quality.
The pad geometry defines the stiffness of the board. It also defines the opening of the solder mask. The thickness of the board and the materials used to create each layer influence the stiffness of the board. Generally, a 1:1 pad-to-device ratio is optimal.
Test methods to characterize pcb resin material cracking
Various test methods are available to characterize the performance of PCB resin materials during SMTP processing. These include electrical characterization, nondestructive methods, and physical properties tests. In some cases, a combination of these tests may be used to detect pad cratering.
One test method to identify cracking is to measure the distance between pins. Typically, 0.004 inch is acceptable for peripheral packages, and 0.008 inch is acceptable for BGA Packages. Another test method to characterize PCB resin material is to measure the coefficient of thermal expansion. This coefficient is expressed as ppm/degree Celsius.
Another method is the flip chip technique. This process enables fabrication of high-density flip chip BGA substrates. It is widely used in advanced IC packaging. The flip chip process requires high-quality finishes that are uniform and free of impurities for solderability. These are typically achieved by electroless nickel plating over the copper pad and a thin layer of immersion gold. The thickness of the ENIG layer depends on the lifetime of the PCB assembly, but it is usually about 5 um for nickel and 0.05 um for gold.