Voids in BGAs in SMT Assemblies - SMT Article

Ball grid array (BGA) is no longer considered a technology of tomorrow but a technology used today. If you are still new to BGA technology, a good place to start is IPC-7095 “Design and Assembly Process Implementation for BGAs.” IPC-7095 describes design and assembly challenges for implementing BGA and fine-pitch BGA (FBGA) technology. BGA and FBGA effects on current technology and component types also are addressed. This document focuses on critical inspection, repair and reliability issues associated with BGAs.

The target audience for this document is managers, design and process engineers, and operators and technicians who deal with the electronics assembly, inspection and repair processes. The intent is to provide useful and practical information to those using BGAs and those who considering BGA implementation.

One issue that has been left unresolved by the industry has been acceptance criteria for voids in BGAs. I will discuss these criteria in this column. But first, I would like to point out that there were many contributors to IPC-7095 development but, as the chairman of this document, I want to acknowledge Syed Sajid Ahmad of Micron Technology for his contribution in collating the information on void acceptance criteria.

Before I proceed with the discussion of voids, I want to point out that voids are not new to BGAs. Voids are seen in through-hole and other surface mount joints. However, leaded surface mount and through-hole component solder joints typically are inspected visually, not X-rayed and, therefore, voids generally go undetected. In BGAs, however, because joints are hidden under the package and cannot be seen visually, X-ray has become a common inspection method. While looking for other defects during X-ray, one invariably sees some voids and may become alarmed. If similar inspections were done for other types of components, voids definitely would be seen. In any event, voids were addressed in the IPC document because they have become a concern in BGAs. These requirements are not cast in concrete. If there is evidence to support some different requirements, please contact the IPC or me and we will take into account your input during the next revision of this document.

Are voids bad for reliability? Not necessarily. Some have even advocated that voids are good for reliability. The IPC-7095 committee decided that because it is possible to minimize voids but not eliminate them entirely, it was reasonable to set a limit on acceptance criteria that can be easily met by using sensible process parameters.

Void Sources and Locations
Where can we expect voids in BGA solder joints? There can be voids in solder balls, at the solder ball and BGA substrate interface (top of the ball), or at the solder ball and PCB land interface (bottom of the ball). There can be various sources for these voids. For example, voids can be carried over from original voids in solder balls that may have been created during ball manufacture processes.

Additionally, voids can be induced into the reflowed solder joint during the reflow attachment process. These voids may be the result of ball manufacture processes or attach process parameters or materials. Board design also plays a major role in void formation. For example, via-in-pad design is sure to cause some voids in the solder joint. Expanding air from a plugged via under a pad creates voids in a molten ball.
Voids near the printed circuit board (PCB)/ball interface during BGA attach typically are caused by improper reflow profile in which the flux is vaporized during the reflow process and entrapped during molten solder solidification. Poor plating or under pad contamination also can cause voids at the interface.
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 reflow. If the reflow profile cycle does not 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, reflow profile development contributes to void formations. BGA components with non-collapsible balls (high-temperature solder — 10Sn/90Pb — with a melting point of 302°C) typically will have little or no induced voids because the ball never melts during the reflow profile.

Void Acceptance Criteria
Gases entrapped in the voids may give rise to stress while contracting and expanding 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.

Voids reduce the mechanical strength of the interface by reducing the interface area. The impact of their presence is a function of material properties surrounding the solder and their dimensional location, shapes and relationships.
The accept/reject criteria for voids as established in IPC-7095 is shown in the table. There are two noteworthy points — location and size of voids. Voids at either interface, solder ball to BGA or BGA to PCB, can have quality and reliability implications depending on size and number. This is why the committee decided to allow smaller voids at the interface rather than voids inside the ball itself. Also, the requirements for high reliability applications of Class III are more stringent than the requirements for Class I. The relationship between the void diameter and its area are achieved by dividing the void area by the ball area. For example, if the void diameter is 60 percent of the ball’s diameter, then:
Void area = P(0.6d)2/4 where d is the ball diameter and the void diameter is 60 percent of the ball. Dividing this by the ball area (P(d)2/4) results in 36 percent.

Assembly Inspection for Voids
X-ray inspection is required for inspecting voids in BGA solder joints. X-ray equipment can range from $50 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 transmission X-ray can detect a void, but cannot determine where the void exists in the “Z-axis” (bottom, middle or top of the solder joint). X-ray laminography via programming takes slices of the solder joint in the Z-axis and determines the void(s) location. Because of the high cost of X-ray laminography equipment, only large or high-volume assembly shops can afford this equipment.
One lower cost scenario for achieving either sampling or 100 percent X-ray inspection of BGAs is to have a transmission X-ray in each SMT line. If a suspicious void is detected, the assembly is taken off-line for finer analysis and defect determination on off-line X-ray laminography equipment.
Manufacturers can adjust their process and materials to reduce void occurrence. Users can work with suppliers to eliminate voids in incoming BGAs. 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 be eliminated through optimization and material and process adjustment.

RAY P. PRASAD is an SMT Editorial Advisory Board member and author of the textbook Surface Mount Technology: Principles and Practice. Additionally, he is president of BeamWorks Inc. (www.beamworks.com), a supplier of selective automated assembly systems, located in Portland, OR and founder of the Ray Prasad Consultancy Group, which specializes in helping companies establish strong internal SMT infrastructure. Contact him from his web site: www.rayprasad.com.

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