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SMT Solver: Choosing the Right Defect

I am intrigued by the theme of this month’s issue—“What You Need to Know”—because it is such a broad subject. There are lots of things we need to know. So, like most of you, I could hardly wait to read this issue of SMT007 Magazine myself. While others focus on addressing various aspects of this expansive topic, I will address some key issues that I think are important for us all to be aware of and learn about. And before I proceed, I am going to make some assumptions about my readers to narrow the focus of this column. The subjects are not technical but managerial, and the target audience is managers in the SMT assembly industry and engineers who aspire to be future managers.

Why is it important? Most companies attempt to achieve higher yield in SMT products through trial and error at considerable expense and frustration. Even though we have been manufacturing SMT products in high volume for almost three decades now, less than 10% of companies have first-pass yield (FPY) of more than 90%. In other words, 90% of companies are conducting too much rework. Rework adds to the cost of the product and reduces the reliability of solder joints due to an increase in intermetallic thickness each time the solder joint is reflowed. So, what are the reasons for this high defect rate? 

• The processes are at very high speeds, and machines must perform them

• The equipment must be characterized thoroughly; this can be defined as understanding all parameters that affect the equipment’s performance, and vendors may say it is easy, but it is not

• Most large companies have assigned engineers to optimize, and most small companies learn as they go, but learning as you go is not an option because revenue or product schedules (or both) may be impacted adversely

With the advent of fine- and ultra-fine-pitch, high-pin-count BGAs—0402, 0201, and 01005 resistors and capacitors—as well as the widespread use of no-clean flux, yield problems are getting worse, especially when you use large BGAs and BTCs in addition to a smattering of through-hole components on the same board in a lead-free process.

It is also worth noting that no matter what our job titles are in various departments—such as purchasing, design, manufacturing, quality, inspection, test, or repair—the key focus of what we do is to make sure no defective prod-ucts are shipped. That means the purchasing manager is responsible for not just focusing on cost but also on the quality of incoming mate-rial, such as components, PCBs, and other materials. And DFM managers are responsible for the overall defects.

1. Choose the Right Defect

Let me be clear; we all want zero defects in our final product, but good luck with that. We certainly can ship products with zero defects, but how can you do that if you don’t achieve zero defects in manufacturing? The obvious answer most people will have is to inspect and test followed by repair of defective units, but no; you cannot prevent the escape of defects through inspection and test completely, no matter how sophisticated or comprehensive those inspection and test regimes are unless you choose the right kind of defect.

Choose the right kind of defect? Yes. Even though there are hundreds of types of defects classified in industry standards, such as IPC 610 and J-STD-001, there are only two types of defects that test methods, including functional test and in-circuit test (ICT), are intended to flag. They are shorts (bridge) and opens. Fortunately, we have a great deal of control over which one we get. By the way, they are both bad, and the assembly will not function if we have either one, but one of them is better than the other: shorts.

Which one do you think is the predominant defect in every company? Nearly all companies, including yours, have way more opens than shorts. All you have to do is look into the defect data over the past six months to a year and put the defects in three categories: opens, shorts, and others. Whatever defect does not fit into either the “opens” or “shorts” categories should be put into an “others” category.

The defects in the “opens” category will be higher than the other two categories because they’re the type of defect that can pass even ICT since partial opens can appear as good joints when vacuum pressure is applied during ICT; opens often end up being discovered by the customer or in the field. And this is why it is not as desirable a defect as a short.

If you change your design and assembly process to focus more on solder paste deposit and better stencil design, most of your defect 1560s will revert back to shorts from opens and be caught before shipping. Your customers will never know your problems, and there’s an easy solution to a difficult problem, but it’s rarely practiced. You can, how-ever, learn to change that. You are not achieving zero defects, but you are choosing your defect wisely.

To read this entire column, which appeared in the December 2019 issue of SMT007 Magazine, click here.