User:Kandi111777/Head in pillow

Overview
"A head-in-pillow defect is the incomplete wetting of the entire solder joint of a ball-grid array (BGA), chip-scale package (CSP), or even a package-on-package (PoP), and is characterized as a process anomaly, where the solder paste and BGA ball both reflow but do not coalesce. When looking at a cross-section, it actually looks like a head has pressed into a soft pillow." This type of defect is also known as hidden pillow, head-on-pillow, foot in mud, or ball-in-cup (BIC) defects.

Causes
The two causes of head-in-pillow defects are poor wetting and warping of the component. These two causes can be identified because arbitrary head-in-pillow defects are caused by inadequate wetting, whereas warping causes edge or center defects. Causes of poor wetting or warping can be sorted into three possible issues; supply issues, process issues, and material issues. They can be defined like this: "Supply issues include problems that occur before you put the component on the line. This includes oxidation, or any other oxidation/hydroxide effects. Process issues are everything that is on-line, including printing, placement, and reflow. Material issues have to do with soldering itself, such as wetting capacity or flux exhaustion."

Supply Issues
Head-in-pillow defects are formed before the BGA, CSP or PoP components are placed on the assembly line for production. This includes sphere oxidation in the course of manufacturing, packaging of the semiconductor, or shipping and storage. The manufacturer or product assembler cannot easily control these issues, but they must understand them and take preventative steps in place to reduce the problems.

Silver segregation is one manufacturing defect that has emerged as a possible, but not well-recognized contributor to head-in-pillow. Silver segregation is the movement of silver-laden intermetallics inside the solder to the exterior surface of the sphere upon cooling. Some test cases have measured silver levels as high as 35% at the sphere surface. The entire dynamic of the wetting and reflow phase of the component attach process is changed due to the high silver content at the surface. Silver segregation will result in an "improper melting of the sphere such that it will not wet to the bulk solder of the paste". The cause of the silver segregation is not well known, however it seems to stem from the supplier's lack of control within the cooling process.

The oxidation of the package's spheres is another major contributor to supply issues. "Sources of sphere oxidation are component storage, which includes moisture sensitivity level (MSL) and inert gas dry-box storage, baking processes and the component's on-line time." The main controllable storage factors are inert atmosphere and humidity levels, which inhibit oxidation and/or hydroxide effects. The baking process of the component can, and will, add to the increased oxidation layer of solder spheres.

Process Issues
Process issues that result in head-in-pillow defects are caused by assembly line setup. "These include printer setup, placement setup and reflow. If the solder printer is not set up properly then the past printing process will not work effectively." Printing issues not related to solder paste properties are poor registration, imperfect or improper printer setup and poor stencil design. An incorrect board setup in the printing process can also add to poor and inconsistent transfer efficiency.

"Poor registration leads to printing off-pad or pump-out and is an additional part of the printer setup. Improper or poor setup may not be easily recognized for large or standard size component assembly, but when the solder paste deposits decrease below an area ratio of 0.66, the board setup is very critical." The last significant area of focus for the printer setup is with board support and gasketing of the board to the stencil. To avoid this, it is important to ensure that there is no standoff between the PWB and the stencil. In some instances, dedicated vacuum board support might be necessary to obtain the best possible gasketing and registration.

Stencil design is perhaps the most significant of the process issues. "Poor stencil design can lead to insufficient solder deposits", which can inhibit the component from making contact with the paste or not having enough flux to overcome the oxide on the sphere or in the paste. Transfer efficiency, as well as area ratio, plays a very large role here. Even though stencils may present a small increase in the amount of paste applied, it is the paste measurement system at onset, the system can compute the theoretical quantity of paste that should be deposited. It will then generate a percentage (efficiency) by measuring the amount of paste that was in fact deposited.

Material Issues
Head-in-pillow defects related to solder paste or flux performance are classified as material issues. These include poor transfer efficiency on standard apertures, insufficient wetting (fluxing) capacity, low oxidation barrier and low activity. The key to overcoming head-in-pillow defects is to get each component sphere to contact, and stay in contact, with the soldering material, mainly the solder paste. If the solder paste itself has poor or inconsistent transfer efficiency, then how do you know that there is even going to be contact between the sphere and the paste? Low area ratios can account for a lot of the transfer issues, especially if the stencils are not electro-polished or electro-formed (e-fab); you must match the material set to the process and stencil design.

The second half of the solder paste equation is the fluxing action. There are three parts to this; activation, oxidation barrier and stencil/tack life. High activation is an obvious choice because this is the working part of the flux, which removes the oxides from the solder and the spheres. Oxidation barriers, such as a higher rosin content of the paste's flux, are useful because it will protect the alloy from forming new oxide, which means there's more activation for the component's oxide. Also, it usually adds tack, which is a huge benefit for overcoming head-in-pillow.

If the paste stays tacky and the package does warp, the paste will stretch to provide a continuum, so the solder component will become a single allow mass upon reflow. There are artificial ways to add an oxidation barrier and additional activation, such as nitrogen reflow or a flux/paste dipping process. Nitrogen reflow prevents the formation of additional oxides during the reflow process, but down not remove oxides and hydroxides that have already formed on the components. Flux or paste dipping are viable options because this adds activation directly on the component, rather than leaving it to chance on the board. In addition, this flux or paste can be used for rework on the back-end. Material solutions such as matching the solder paste to the process can overcome both supply and process issues.

Appearance
A head-in-pillow defect is the incomplete coalescence of the solder joint between a ball-grid array (BGA), chip-scale package (CSP), or package-on-package (PoP) and the printed solder paste. For some reason, the printed wired board's (PWB's) printed solder and the package's solder spheres do not come together to form a single mass. At first glance, it looks as if a film has formed on the surface of the molten solder, preventing the merging of the printing and package solders. In fact, this may be true, as in some instances there seems to be an oxide film on the surface of the molten solders. In other instances, it appears that upon cooling, the exterior has already cooled enough to prevent the coalescence of the printed paste and the sphere at re-connect when the warpage subsides. From the cross-sections, it actually looks like a head has pressed into a soft pillow.

Prevention
The most difficult issue for the user to control is the supply. The BGA or CSP manufacturer may provide a component that will always have the tendency to warp or not have controls in place to reduce the oxidation level on the spheres. Therefore, the user must then make sure that the manufacturing processes and controls within the product assembly are optimized. Viewing and adjusting this process through the use of statistics yields two important objectives. First, an outside perspective on each part of the process arises by focusing on the details of each segment step, sharply increasing the understanding of the process.

Secondly, using this data to eliminate problems from the process itself while streamlining each step of the process and discarding surplus, increases process flow and cost savings, while defect minimized and yields increased. Polishing the printing process, where the majority of all solder issues can be traced, sets the foundation for success. Once consistent printing is assured, then other issues such as graping and head-in-pillow defects may be eliminated through optimization of the reflow parameters or evaluating a solder paste with an enhanced oxidation barrier, longer tack life, or better wetting performance. In short, head-in-pillow defects can be eliminated through tight process controls and robust materials.