MicroBGA qualification process

Reg: This is copy / paste from draft version of IPC 7095 ( issued May 1999 ) ... 7.3 Assembly accept/reject criteria 7.3.1 Voids in solder joint a. Sources of Voids There can be voids in solder balls, or at the solder joints to the BGA, or at the solder joints to the PCB. Various sources or reasons can be responsible for these voids. � Voids can be carried over from original voids in solder balls. These voids could be the result of ball manufacture process or materials used. � Voids can be induced into the reflowed solder joint by either the voids in the original component solder ball, or during the reflow attachment process. These voids could be the result of ball manufacture process or attach process parameters, or board design (e.g., via in pad), or materials used. � Voids can also form near the PCB-Ball interface during BGA attach to PCB. These voids are typically induced during the assembly process, which is typically flux that has been vaporized during the reflow process, and entrapped during the solidification of the molten solder. The source of vaporized flux can be either from flux itself (typically rework), or flux which is one of the constituents of the solder paste used in the reflow assembly process. � Voids can also be formed via expanded air from plugged vias (via in pad consideration) in the PCB. Expanding air from plugged via under pad may also create a void in molten ball. Typically most voids are detected in the middle to top (ball / BGA interface) of the reflowed solder joint. This is expected because the entrapped air bubble and the vaporized flux, which is applied to the PCB BGA pads, rises during the reflow profile. This occurs when the applied solder paste and the BGA's collapsible eutectic solder ball(s) melt together during the reflow profile (typically 235 to 240 *C peak temperature). If the reflow profile cycle doesn't allow sufficient time for either the entrapped air or vaporized flux to escape, a void is formed as the molten solder solidifies in the cool down area of the reflow profile. Therefore, the development of the reflow profile is extremely important as a contributor to the formation of voids. BGA components having non-collapsible balls (high temperature solder (90% Pb 10% Sn, with a melting point of 302 *C) typically will have little or none induced voids because the ball solder metallurgy never melts during the reflow profile. Voids in solder joints are not new because of the use of collapsible BGAs. Voids can be detected under leaded components when using x-ray equipment. However leaded component solder joints are usually visually inspected, not x-rayed, and therefore never detected.

b. Impact of Voids

The questions are how many and how large are these voids allowable in the product that will impact the product's required reliability. Another question is where in the "Z-axis" in conjunction with the void size is also allowable without impacting the required reliability. Voids at either interface, solder ball to BGA or BGA to PCB, can have quality and reliability implications depending on their size, number, and location. Gases entrapped in the voids may give rise to stress while expanding and contracting during heat excursions. They can serve as stress initiation (and in some cases, stress absorption points). They can start (and in some cases, terminate) a stress crack. Their elimination or, at least, substantial reduction is preferred.

Large voids are more detrimental, but small pre-existing voids can merge during reflow to create large voids. Voids reduce the mechanical strength of the interface by reducing the interface area. The impact of their presence is a function of the properties (such as CTEs) of the materials surrounding the solder and their dimensional location, shapes and relationships.

c. Inspection of Assemblies for Voids

Inspection of BGA joints using an x-ray can detect these voids. A sample in-coming x-ray inspection should bring the issue to light. X-ray analysis is therefore required for the inspection and the detection of voids in BGA solder joints. Unlike a leaded component, BGAs have solder joints that are not only on the component's periphery, but have internal solder joints that are not inspectable by normal visual techniques.

X-ray equipment is required for inspection or detection of voids in BGA's solder joints. This is probably one of the two new pieces of equipment (the other could be hot gas rework equipment) that's required for the qualification of a SMT BGA assembly line. X-ray equipment can range from $30K to $500K. The lower cost equipment is transmission x-ray, whereas the higher cost equipment is x-ray laminography. The difference between the two pieces of analytical equipment is that the transmission x-ray can only detect a void, but can not determine where the void exist in the "Z-axis" (bottom, middle, or top of the solder joint). The x-ray laminography via programming can take slices of the solder joint in the "Z-axis", and determine the location of the void(s).

Typically because of the high cost of x-ray laminography equipment, only larger or high volume assembly shops can afford this equipment. One lower cost scenario for achieving either sampling or 100% x-ray inspection of BGAs would be to have a transmission x-ray in each SMT line. If a suspicious void is detected, that the suspicious assembly is taken off line for finer analysis and defect determination on off line x-ray laminography equipment.

d. Elimination of Voids

The assemblers can work with their suppliers to eliminate voids in the incoming BGA solder balls. The manufacturers can adjust their process and/or materials to eliminate these voids. The user can work with the supplier to eliminate voids in incoming BGA.

Recently and typically, little or no voids are detected in the incoming BGA solder joints. Reflow time-temperature profile, flux amount, type and properties should be investigated for improvement. Such voids can again be eliminated through material and/or process adjustment and optimization.

e. Accept/reject criteria for voids in solder balls:

With constantly decreasing BGA pad sizes, solder ball sizes and pad pitches, the product dimensional parameters will be continually changing on the production floor. Materials and processes will be changing to attain pricing, throughput, quality and reliability goals. There will be a continuous need for process development for changing configurations.

Voids and other defects will be encountered in various stages of product's developmental and manufacturing ramp up life. It will be necessary to maintain a minimum acceptable standard so that the product is manufacturable, meets customer expectations, has a useful working life and meets product reliability requirements. The manufacturers need to be encouraged to use process control for continuous product improvement.

Voids are an anomaly. Manufacturer should work towards their elimination using available statistical process control and process improvement tools. A general presence of voids should indicate a need to control and/or to improve process and materials. An accept/reject limit can be used to help the manufacturer realize the need to eliminate voids.

As an example, a reject limit of no more than 5% solder balls with voids set by a customer can alert the manufacturer that presence of voids is not acceptable. A profusion of voids all over the part or a general presence of voids should not be considered an acceptable norm.

Voids, when present, will normally have a distribution of sizes. The size of the largest probable void can be estimated from measurements performed on a sample. A largest probable size detrimental to the life of the contact should not be acceptable. The unacceptable void size may vary from one BGA design to another. A void larger than 25% of solder ball contact area diameter (~6% of contact area) can be rendered unacceptable by a customer, again to encourage elimination of voids. (see Figure 36) Any such criterion will be used in conjunction with the reject criteria for the proportion of voided acceptable solder balls. When there are more than one voids per solder ball, then the dimensions of the voids will be added to calculate the total voiding in that solder ball. Solder outline Void outline d 0.25 d Figure 36 Example of voided area at land and board interface

Defective determination is made by the product's reliability requirements. Typically the maximum allowable size void is 30% of the solder ball diameter (equivalent to 11% of area). This can be either one void, or the summation of many voids. Unfortunately the currently algorithms for x-ray laminography doesn't perform the summation of the voids. X-ray laminography via programming can determine a defect greater than the pre-determined size. Therefore if the diameter of a reflowed solder joint is .030", the maximum size of the void in the middle of the joint would be .009". If the .009" void was detected at either the top (BGA interface) or bottom (PCB interface) that joint would be rejected because the diameter of that joint at that slice would be less than .030", and the void would be greater than 30% of the diameter.

The theory is believed that the voids at either interface can be more of a reliability problem. Greater stress is achieved at the interfaces as the distance from the neutral point grows, and the component substrate and PCB are of different materials. This causes greater stress because of the possible mismatch of CTEs. However stress testing / thermal cycling with voids at 30% have not failed medical class III criteria.

Smaller voids are believed by some to be more favorable than no voids at all. The reason being as a crack is propagated, upon reaching a small void, the void stops the crack propagation. Voids have been either reduced or eliminated with the use of a nitrogen blanket during the reflow process. Accept/reject criteria for the number and size of voids, on the one hand discourages a general presence of voids which indicates an out of control process and on the other hand encourages the manufacturer to use necessary tools for process and material improvement.

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