This month’s topic is smart factories, and this means it is not just because machines share data but because a company designed their smart factory and how the data usage was planned to enable them to do something unique like producing a single-panel lot size in a hands-off manufacturing environment. That sounds like a mouthful, and it is, but the gist is that when you plan a production facility with the mindset that connectivity and optimization will be key aspects of your operation, it will pay dividends in the form of lower production cost, better traceability, and higher reliability.
Okay, so that wasn’t any less of a mouthful, but it did help elaborate the point. When I look at a smart factory in the view of producing a quality product, I look at all the education and due diligence required. This is what I am looking for to determine if a facility is a smart factory or not.
Starting with the receiving dock all the way to final packaging, every process step is an opportunity to contaminate the final product. Conveying this information to everyone in production starts at the top of the quality department and should be part of an overall focus on reliability. Too many times I do process audits at contract manufacturers, and there are a lot of “box checkers” that are doing the bare minimum regarding cleanliness. Many times, that is exactly what is spelled out on the print, and in the end, you get what you ask for—nothing. That means they are building product that is shippable and not much more than that. That isn’t smart; it’s quite the opposite. I’ve said it a million times, and I’ll say it at least one more time that the cost of failure is always higher than doing it right the first time.
Having driven my point thoroughly into the ground, let’s look at the assembly process steps and what to be on the lookout for. This isn’t a fully comprehensive list, but it’s certainly enough to help clean up a lot of what is going on out there. Trust me, I’ve seen a few good processes, but I’ve also seen a lot of terrible processes—a lot.
Incoming receiving and storage may seem like a fairly sterile area since there aren’t any chemicals being used or boards being handled or parts that aren’t in some type of packaging. The issues with this process are how and where parts are stored. Many times, I see warehouse racks that are in the same room as the receiving dock that is fully open to the outside elements. This can create spikes in excessive humidity that can impact parts that aren’t marked for moisture-sensitive packaging.
For example, if it’s a dock in a normally dry climate such as Mexico and it rains for some reason, the air can become saturated with dust and contaminants from the typically extremely dry ground. If a breeze carrying some of the dust and contaminants settles on top of the stored raw goods, this can cause issues with solderability or other reliability-related problems. If partially used packs of bare boards or components are stored in this environment, the risk increases. That is why it’s important to have the smarts to separate the receiving dock and storage areas.
Moving on to the paste print process, the main thing to be smart about is controlling the humidity in the printer to help reduce solder balls and similar defect types. Beyond that, keeping track of the print life of the paste and the screen clean are the main opportunities for introducing some sort of failure.
The reflow process is the first opportunity to do some real damage by not being smart. The top things to think about are oven maintenance and proper thermal profiling of assemblies. Oven maintenance is something that I see scheduled anywhere from weekly to annually with the level being varied. It is essential to keep the chains oiled and the fans exhausting, but it’s also important to keep the interior free of excessive residues that collect and redeposit back on the PCBA surface. I normally see interior cleaning no more than twice a year, and depending on the number of pieces ran through, that may be woefully inadequate.
The paste flux residues are normally very concentrated forms of flux activators, and any of this material will be detrimental to reliability—no question about it. Properly profiling the assemblies is equally important because if the areas with the highest thermal mass aren’t monitored, the required temperatures for rendering no-clean flux near benign may not be reached and the activators will remain active. If no-clean flux residues aren’t properly processed, they can be as detrimental as leaving a water-soluble flux behind after wash. Not smart.
The PTH wave solder process is among the worst offenders of all as there are many ways it can go wrong. It truly takes a smart operator to know how to properly run a wave solder process. The main issues are excessive flux penetrating either the top side of the board or under selective pallet keep-out areas. If the pallets aren’t properly cleaned, flux residues can build up in the corners, and any part of the board that comes into contact with that is at a greatly increased risk of field failure (if it even makes it that far).
If the hold-down springs don’t have equal pressure at all points, there is also a risk that the board will raise up on the side that has less pressure. This allows spray flux to be deposited on areas of the assembly that were never intended to see flux. Any part of the board under the pallet will be shielded from the full thermal energy and remain active. What I am trying to convey is that it’s not smart to leave any active flux residues on an assembly; you don’t have to be smart to know that.
Next up are any hand solder operations that take place post-SMT or wave. This is where the smartest of operators are needed because there is nothing more dangerous to the reliability of a product than a human with unfettered access to flux. If a part is difficult to solder, the answer is clearly more flux, right? While that may be technically correct, it’s among the dumbest things in the history of electronics assembly.
There are a couple of good ways to use flux for hand operations, and they are to use the flux that is part of the cored wire or tacky flux used mostly for BGA rework. When using a cored wire with the proper feed rate, any flux residues will be near benign. The reason for recommending tacky flux is the application is very controlled, and the risk of flux spreading to neighboring components is much lower than using a bottle of liquid flux. Flux spread to surrounding components is a killer to reliability because it will be as active as flux gets next to right out of the bottle.
After some hand operations, there is a desire to clean any residues, which is another questionable operation. If it’s no-clean flux and coating or RF issues are of no concern, just leave it in place. That’s smart. If cleaning is required, it’s very important to be mindful of where any effluent is being deposited. It’s essential to effectively rinse the area that was just washed with IPA or some sort of solvent. Just be smart and see where all the effluent is draining. If that is under other components, they are now at an increased risk for electrical leakage. It’s vital to use the right amount of flux and know where 100% of that is spreading, and if cleaning is part of the process, be sure it’s effective and rinsed properly.
The final part of the assembly process that needs to be considered, at least for this month, is packaging. The points to consider are proper handling and using clean ESD bags/trays for shipping. I have seen many facilities reuse ESD bags that can collect processing residues and general debris over time. They can also lose their ESD properties due to damage. The same goes for trays used anywhere in the process including moving station to station or packaging to be sent to the next supplier.
Throughout all of the process steps I have mentioned, one more thing to think about is proper handling. Wearing gloves is essential, but knowing how to wear gloves is as important as anything else. Gloves should be changed any time an operator wipes their face, touches raw flux, or rubs a saltlick for some reason. Contamination is easily transferred to the PCBA surface and will greatly increase the risk of electrical leakage or electrochemical migration-related issues in the field. It’s smart to know how easily any assembly process can affect reliability.
Eric Camden is a lead investigator at Foresite Inc.