Zulki's PCB Nuggets: FAI - An Ounce of Prevention, Worth a Pound of Cure
April 10, 2013 |Estimated reading time: 6 minutes
Editor's Note: This column originally appeared in the March 2013 issue of SMT Magazine.Many years ago, Benjamin Franklin said, “An ounce of prevention is worth a pound of cure.” Today, those famous words speak volumes to those of us in the PCB design and assembly business. You can interpret that quote several ways, but, in modern terms, what Franklin’s ageless universal truth means is that the best approach to any project is to scope out, analyze, and avoid problems from the beginning--not waiting until later to fix them once they arise. By doing so, costs to all parties involved will be reduced and the OEM customer can meet time-to-market requirements.
The reason I call Franklin’s quote an “ageless universal truth” is because it is highly applicable to our industry and particularly to first article inspection (FAI) on the PCB assembly floor. In this day and age, FAI takes on a more definitive meaning and an even greater importance. We’re at the threshold of a changing technology. Large numbers of products are becoming smaller, wireless, and portable. This means shrinking PCB real estate, ultra-fine-pitch devices, package-on-package (PoP) technologies, in some cases thinner boards, and the recent adoption of challenging device packaging.
The land grid array (LGA) is one such package (Figure 1). Although it is not exactly a new packaging concept, it is gaining popularity and creating mayhem on the manufacturing front. The LGA has flat bumps and, due to that geometry, is difficult to rework. Whether or not PoP, thinner boards, advanced packaging, or other PCB technology is involved, the point to be made here is that FAI is critical to detect issues newer technologies introduce. Figure 1: Land grid array (LGA).
Let’s now look at a more basic example. Let’s take a batch of 500 boards and say that one particular component was incorrectly placed or not placed at all. But FAI isn’t used on the assembly floor. That means those 500 locations must be reworked. Not only does that incur extra cost, but also subjects the boards to an unnecessary thermal cycle, which could have been avoided if the FAI oversight had not occurred.
Speaking of reworking, BGAs, QFNs, CSPs, and similar package types incur an inordinately large amount of time and money. Sometimes in the course of the rework those expensive BGA or QFN components are damaged due to excessive rework, adding considerable expense for replacements. In particular, military and aerospace OEMs and subcontractors limit the amount of PCB rework to just one instance.
Golden Board
A first article is also known as the “golden board” and FAI can best be described as a process validator. It provides a way to investigate issues at the root causes. FAI defines how the manufacturing process instructions (MPI) are set up in the process engineering department. Once quality control checks the first article, that golden board tells the process engineer and manufacturing personnel how efficiently thermal profile creation, solder paste printing, and placement have been performed.
At this stage, the first article auditor checks the golden board after printing. Here, the amount of paste on the pads is measured and verified using a paste height inspection machine. With today’s 0.4 mm ultra-fine-pitch BGAs, QFNs, LGAs, and flip chips becoming more common, the amount of paste deposition on these pads is extremely critical. An assembly issue may arise due to improper printing, if the stencil design is flawed, or if the fabrication house has manipulated the data for ease of fabrication.
For example, let’s say the fabrication house is working on a board using ultra-fine-pitch devices. Those devices have individual solder masks separating these leads or slots. If the fabrication house decides on making a change and creating a “gang relief” on the solder mask, meaning combining the solder mask, that change will not only create an assembly issue, but will also create shorts between those pins or pads.
FAI then moves to the next step for post-placement. The auditor checks to determine that SMT components have been correctly placed, SMT programming is properly performed, and that those components are picked and placed in the right manner. Or, to put it another way, the auditor discovers whether or not components are skewed, wrong components were used, components are missing, or wrong values added.
For example, let’s say we’re dealing with a 10-board batch and each board has 10 tantalum capacitors with the same value and same footprint. However, due to a small error in the pick-and-place machine, these caps were placed incorrectly. In this case, due diligence at FAI was not performed after pick and place. Consequently, 100 locations need to be reworked after assembly is complete.
At the third stage, FAI looks into the thermal profile to determine if any component was skewed, tilted, or burned during reflow. Perhaps there’s a capacitor or resistor tombstoned as a result of an improper profile or paste deposition. If the thermal profile isn’t properly created, FAI will reveal issues or problems at this stage.
The beauty of a thorough FAI is that it allows sufficient time to make corrections. On the other hand, sometimes the substitute for FAI in a zero-defect manufacturing organization is the mantra to “do it right the first time.” However, there are times when operator error creeps in and the result isn’t caught, or a fixture is omitted and again the result goes undetected. Or when using a big panel, printing may shift in one direction or another and the problem goes unnoticed. But with proper FAI those issues and many others are highlighted and corrected.
The Right Mindset
By its very nature, FAI is cumbersome and demands a correct, positive, and disciplined mindset. The contract manufacturer or EMS provider must invest and devote time and resources for the process to be validated. Generally speaking, contract manufacturers not in the top tier tend to take shortcuts by bypassing FAI.
Shortcuts are often taken due to quickly approaching shipping deadlines, for instance, and there simply isn’t time to perform FAI. But let’s use that 500-board batch again as an example. That extra day or so wasn’t allocated to applying FAI to assure no issues exist. Consequently, there’s a high probability this batch incurred defective assembly.
Plus, a number of other reasons exist as to why FAI is omitted in the assembly process. Sometimes, assembly engineering gets cocky and believes a certain board is so simple, so easy to correctly manufacture, there’s no reason to spend time and money on FAI. Other times, avoiding FAI is due to pure callousness and laziness. In these cases, assembly personnel don’t want machine downtime. A machine might be down for three to five hours while FAI is performed and this means another project is left idle and waiting.
Reputable CMs and EMS providers, on the other hand, implement FAI as standard practice with a positive mindset of creating whatever is needed at the end of the FAI process. FAI is imperative because, as Franklin said, “An ounce of prevention is worth a pound of cure.” Zulki Khan is the founder and president of NexLogic Technologies, Inc., in San Jose, California, an ISO 9001:2008-certified company, ISO 13485-certified for manufacturing medical devices and a RoHS-compliant EMS provider. Prior to NexLogic, Khan was general manager for Imagineering, Inc. in Schaumburg, Illinois. He has also worked on high-speed PCB designs with signal integrity analysis. He holds a B.S. in EE from NED University in Karachi, Pakistan, and an M.B.A. from the University of Iowa. He is a frequent author of contributed articles to EMS industry publications.