Can every operator handle your BGA rework equally well? In real life, 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 is critical to have your most skilled 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; dealing with the realities of BGA rework challenges is.
Since BGA rework arrived on the scene many years ago, a multitude of issues have arisen to confront companies needing to work with BGA components. Choosing the right operator to handle BGA rework is one of the most critical prerequisites to beginning the rework cycle. It is also the most misunderstood; in our own experience, we find that BGA operators are a breed apart from other technicians. It may not sound fair, but it is true.
Figure 1: It is critical to have your most "BGA-inclined" operators handle your most demanding BGA rework.
BGA rework presents new challenges to managers, planners, engineers and rework technicians. Gone are the days when someone with a simple desoldering tool and a magnifier light could perform all the rework and inspection ever 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. Now that BGA components have entered the scene, the rework model has changed forever.
Instead of a simple rework station, one might require an $80,000 BGA rework machine. Instead of a simple microscope or magnifier light for inspection, one may need a $120,000 X-ray system. Instead of an upper level assembler, one needs a skilled operator, one with computer skills, an understanding of solder paste, good dexterity, an understanding of x-ray equipment operation, and the knowledge to interpret x-ray images!
An operator is, in some ways, like a medical surgeon. Operators that must perform BGA rework need a broad range of skills, part operator, part technician, part magician. When choosing a BGA rework operator, one must begin by knowing what these people do in detail in order to make the right choice.
Figure 2: BGA operators must also be skilled at optical and X-ray inspection.
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 is 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 board stability during the reflow process. Typically, the site will see reflow temperature at the BGA ball interface for 30 to 90 seconds. Additionally, that temperature will clearly be exceeded throughout the array to ensure proper flow across the entire array. Due to the nature of most circuit boards, the whole board must be significantly heated to prevent bow in the rework area. As the board is heated it will often approach its 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 (on the other side of the board) and around the edges of boards. There are other components that may be so large or high that they prevent the board from sitting at the proper height for optical prism alignment.
Particularly vexing are the parts that get in the way of the prism altogether, therein blocking one's ability to properly align the component for rework. Some boards are longer than the fixturing apparatus that the Rework machine is supplied with, and may have a heavy part (like a transformer) hanging over the edge. As the board becomes heated and flexible, the transformer may cause bending of the board.
The operator must know how to properly support the board in all cases. You may have the luxury of a second operator who will prepare the board for the machine operator, but when that is not the case, the machine operator must have this skill also.
Once a component is removed from a board, the part's location must be cleaned and prepared for placement and attachment of a new component. Your machine operator must be knowledgeable and skillful enough to properly inspect the site after part 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 site mask. The site mask is responsible for preventing 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 site mask), the BGA pads have to be prepared to receive the new part. Some people swear that the only way to have the site ready for solder is to have it pasted.
Others have sworn that 'bumping' (filling the pads with solder via hand soldering) is the only way, and still others have said that bumping, is not necessary, but only site tinning is required. We have seen a need for all three. Whatever your method(s) of choice, you'll require someone who can perform the task properly and economically.
During the component replacement phase, the component must be precisely aligned with the circuit board pads using a microscope-enhanced optical prism or similar machine 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' success. 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 just 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 above the site or barely touching the location (with about 50 grams of force - less than 2 ounces).
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, and the new fiber-optic vision inspection machines now available are very helpful but require a skilled and trained operator to use effectively. In X-ray inspection, machine operators unfamiliar with the nuances of X-ray details 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. In BGA rework, it's the human resource, more than the equipment resource that's key. Assigning the right operator to the right job may take some diplomacy, but in the end the employer has very little choice in the matter when product quality hangs in the balance.
Several members of the Circuit Technology Center team contributed to this feature story.