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Are Your BGA Operators Competent?
Production Floor |
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Authored By:Andy Price, National Sales Manager Peter Vigneau, Vice President Circuit Technology Center, Inc. Haverhill, MA USA TranscriptCan every operator handle your BGA rework equally well? In all but just a few situations, the answer is no. Why is this so? Many factors come into play, including experience, manual dexterity, and sometimes just plain talent. It's critical to have your most experienced operators handle your most demanding BGA rework. Certainly, there is delicacy involved in making such assignments, to make sure that all operators understand the reasons and know that they are being treated fairly, but that level of personnel management is not the focus of this article; here we're dealing with the realities and challenges of BGA rework. Since BGA rework arrived on the scene many years ago, a multitude of issues have arisen. Choosing the right operator to handle your BGA rework is one of the most critical steps when beginning the rework cycle. It's also the most misunderstood. In our experience, we find that BGA operators are a breed apart from other technicians. It may not sound fair, but it's true. BGA rework presents new challenges for managers, planners, engineers and rework technicians. Gone are the days when someone with a simple de-soldering tool and a magnifier light could perform all the rework and inspection needed. Of course, those rework personnel had to be skilled, but it was easy enough to train them on a select set of skills, and equally easy to inspect their work. With BGA components, the rework model has changed forever. Instead of a simple rework station, you might require an $80,000 BGA rework machine. Instead of a simple microscope or magnifier light for inspection, you may need a $150,000 X-ray system. Instead of an upper level assembler, you need a skilled operator, one with computer skills, an understanding of solder paste, good dexterity, an understanding of x-ray equipment, and the knowledge to interpret x-ray images. An operator is, in some ways, like a doctor. Operators that must perform BGA rework need a broad range of skills, part operator, part technician, part artist. When choosing a BGA rework operator, you must begin by knowing what these people do in detail in order to make the right choice. The basic machine operator is the person who hopefully operates under and follows an engineer's guidance. The engineer may develop profiles, and set up tooling and procedures, and the machine operator follows them. That's the ideal situation and it sounds simple enough, but it's usually more complicated than you may think. There are a multitude of small details that can swamp the BGA rework process. For example, today's topflight BGA rework machines are run by computers. These computers may look like a familiar desk top unit, but the software is customized and often a little cranky. Many of today's machine operators, particularly in electronics manufacturing, are just not that computer savvy. Unless your engineer enjoys holding hands, the machine operator must have a firm grasp of the computer system and its custom software. The machine operator must also be mechanically inclined, able to adjust machine settings, and select and place the proper tooling and nozzles. The machine operator must be skilled at proper placement of circuit boards into rework system fixtures. One of the most critical aspects of proper BGA rework is circuit board stability during the reflow process. Typically, the site will see reflow temperature for 30 to 90 seconds. Due to the nature of most circuit boards, the whole board must be significantly heated to prevent bowing in the rework area. As the board is heated it will often approach the glass transition temperature and begin to move. Being able to fixture and support the board is a critical skill. Many circuit boards have components placed under a BGA component rework site and around the edges of circuit board. Some components may be so large that they prevent the circuit board from sitting at the proper height for optical alignment. Particularly vexing are the adjacent components that get in the way of the alignment system, therein blocking the ability to properly align the BGA component for rework. Some boards are longer than the fixturing apparatus that the rework machine is supplied with, and may have a heavy component, such as a transformer, hanging over the edge. As the board becomes heated and flexible, the transformer may cause bending of the circuit board. The operator must know how to properly support the circuit board in all cases. You may have the luxury of a second operator who will prepare the circuit board for the machine operator, but when that is not the case, the machine operator must have this skill. Once the BGA component is removed from a circuit board, the component location must be cleaned and prepared for placement and attachment of the new component. Your machine operator must be knowledgeable and skillful enough to properly inspect the site after the initial component removal. At this point and during the next step, the operator is determining if there is any damage to the BGA pads or to the sensitive solder mask. The solder mask prevents bridges and maintains the proper solder volume during placement. Evaluating mask condition requires experience. Next, someone will have to remove the excess solder from the location. After that is completed, hopefully without doing any damage to the sensitive solder mask, the BGA pads have to be prepared to receive the new BGA component. Some people swear that solder paste must be used. Others believe that bumping, or filling the pads with solder by hand soldering, is the only way to go, and still others believe bumping is not necessary, and simple tinning of the pads is all that is required. Whatever your methods of choice, you'll require someone who can perform the task properly. During the component replacement phase, the component must be precisely aligned with the circuit board pads using a microscope-enhanced optical feature. This step requires confidence, nerve, and good hand/eye coordination. Often, these optical alignment systems are precisely calibrated, assuming that the component location will be at a set height. Unfortunately, due to fixturing issues, the board is frequently not at that height. The operator will need to make the proper adjustments. If an operator can get this far and stay on track, he or she is doing very well. You would think that the next step, placing the component on the prepared site would be a given. Unfortunately, more errors are encountered at this step than at any other. Often, problems are due to improper fixturing, nozzle selection, poor site preparation, poorly timed vacuum release or aggressive placement. Any one of these issues might cause the part to slide off its precise location. Proper placement depends on this step. A component being placed needs to be brought into the ready position prior to reflow. That ready position is either a few thousandths of an inch above the location, or barely touching the location. Finally, the machine operator must also be skilled at optical and X-ray inspection. Since BGA solder joints are under the component, the ability to view the joints is severely limited. X-ray images are essential. Machine operators unfamiliar with the nuances of X-ray imaging can easily be fooled by what they see. Selecting the proper operator for this complex and demanding task may be the most important aspect of setting the stage for successful BGA rework. For BGA rework, it's the operator, more than the equipment, that's key. Assigning the right operator to the right job may take some diplomacy, but in the end you have little choice in the matter when product quality hangs in the balance. For more information visit: http://www.circuitrework.com/features/615.html |
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