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SMT Solver: Would You Prefer Shorts or Opens in Your Products?

In my last column, I discussed realistic goals to aim for regarding defect levels. I also mentioned, but it is worth repeating, that it may be unrealistic to have zero defect in products right after reflow, but we want zero defect in products we ship to the customer. That is why we spend so much time and money on inspection, test, and repair even though they are non-value added process steps, but they are necessary steps since you don’t want the customers to discover those defects.

As to the question raised in this column, would I prefer shorts or opens in my products? Neither, thank you. But if I do have to choose, I would choose a more desirable defect, if there is such a thing. But what is a desirable defect? A defect that would never escape inspection and test and would be caught before shipping the product to the customer. Before I answer this question, let me briefly review industry standards on defects and major types of defects that really matter in the functioning of a product.

Industry Standards and Major Types of Defects

There are three major types of standards in our industry: IPC, EIA, and J-STD. Each one has a different focus, reasoning, and target audience. Having been deeply involved with various standards, chairing many of them over the years, and still chairing many others, let us save this topic for future columns. For now, I will focus on two standards that deal with acceptance criteria for defects.

IPC-610 is a thick and widely purchased IPC standard that specifies acceptance criteria for all types of defects with color photos and very little text. It is easy to read and follow and is primarily targeted at inspectors and operators working on the SMT line.

The main standard for acceptance criteria of defects in electronics assemblies is J-STD-001. IPC-610 provides a visual representation of the acceptance requirements established in J-STD-001. This standard (J-STD-001) has very few photos and is full of text and tables targeted at quality and process engineers.

If you go through these standards, there are hundreds of different types of defects. There are week-long classes and certification programs for these standards. However, if you think about it, there are only two types of defects that matter for the functioning of electronic assemblies: shorts or bridges between adjacent leads or opens in solder joints.

All of the test methods, such as in-circuit test (ICT) and functional tests only look for shorts and opens because other types of defects cannot be detected by these tests and they don’t affect the functionality of the boards. Thus, it goes without saying that we can simplify IPC-610 and J-STD-001, but don’t hold your breath.

What Defect Should You Aim For?

As I just mentioned, ICT and functional tests can only find opens and shorts; they are not looking for any other types of defects because they cannot find them. If the defects that you see don’t fall into open or short category, you can safely classify them as “others.” Examples of “others” are insufficient, tombstoning, drawbridge, part movement, etc.

Bridge is the most objective type of defect. No two people will disagree if you show them a bridge. Meanwhile, opens, especially if hidden, can easily escape even ICT and functional tests because they are most likely to make intermittent connections due to pressures exerted during bed of nail testing or ICT.

There is another type of open called insufficient solder joint. Many people will disagree with me calling insufficient joint an open joint, but we all agree that insufficient solder joint is not a total open today but will most likely become an open tomorrow or months later. Therefore, we might as well call it an open. Unlike bridges, insufficient solder joint is the most subjective defect you can find. It is very likely that two well-trained inspectors will disagree whether an insufficient solder joint is acceptable or not. To be on the safe side, you might as well consider it an open.

Table 1: QFP defect levels vs. pitch.

Industry Findings: Which Defect Is More Prevalent?

In my last column, I quoted a paper by Stig Oresjo [1] and Table 1, which shows the level of defects depending on the pitch of a gull-wing device.

Based on Table 1, it is obvious that lower pitches will result in much higher defects because lower pitch devices increase the complexity of the manufacturing processes, such as handling, printing, placing, and soldering.

The same paper by Oresjo also analyzed the types of defects for all the components:

• Opens: 46%
• Shorts: 22%
• Insufficient: 17%

Figure 1: Shorts, opens, and others at a medical company.

As I noted earlier, insufficient solder joint is an open about to happen in the near future. So, if we combine open and insufficient, almost two-thirds of defects (63%) are opens, 22% are shorts, and the remainder are 15%. This is the average of all types of components in Orsejo’s paper.

The results in the paper on gull-wing devices were even more significant:

• Opens: 65%
• Insufficient: 11%
• Shorts: 16%
• Others: 8%

If you combine opens and insufficient, the number is alarming: 76%. Shorts are only 16% of the total.

My Findings: Which Defect Is More Prevalent?

Figure 2: Shorts, opens, and others at an automotive company.

This next section details my own findings at two major companies during my consulting assignments—one at a U.S. medical company and the other at a major Japanese electronics company that supplies to the global automotive company (Figures 1 and 2).

Recommendations

An assignment for you is to look at your own data for some of your higher volume products. Put them in three categories: shorts, opens, and others. Put insufficient, tombstoning, drawbridge, part movement, etc., into the open category. For the ones that you cannot put into short or open, call them “others.”

I bet that your findings will be no different than what I quoted from Orsejo’s paper or my findings. I am sure of it since I have looked at the data at many companies over the years. If I’m wrong, I will buy you a drink (nonalcoholic) if we end up meeting at any industry events, such as SMTAI or IPC APEX EXPO.

My recommendation is to have six to eight times more shorts than opens, but let me be clear that I am not saying have lots of shorts and opens; I am talking about the ratio of shorts to opens. You always want to work for the total to be on a declining trend, but the total should have more shorts than opens. Why? Shorts will never escape your inspection and test steps, but opens may escape no matter how rigorous your inspection and test regimes are. Opens will be discovered eventually either at a customer site, or even worse, in the field when it is too late and too expensive to fix.

The best part of my recommendation is that it’s very easy to achieve. All you need to do to achieve more shorts than opens is to design land patterns and stencils correctly. I may talk about those subjects in future columns.

References

1. Oresjo, S. “Year 1999 Defect Level and Fault Spectrum Study,” SMTAI Proceedings, 2000.

Ray Prasad is the president of Ray Prasad Consultancy Group and author of the textbook Surface Mount Technology: Principles and Practice. Prasad is also an inductee to the IPC Hall of Fame—the highest honor in the electronics industry—and has decades of experience in all areas of SMT, including his leadership roles implementing SMT at Boeing and Intel; helping OEM and EMS clients across the globe set up strong, internal, self-sustaining SMT infrastructure; and teaching on-site, in-depth SMT classes.

This article was originally published in the July 2019 issue of SMT007 Magazine.