SMT Solver: Flux and Cleaning—How Clean Is Clean? Part 2

The selection of a flux and cleaning process determines, to a large extent, the manufacturing yield and product reliability of electronic assemblies. In Part 1, I discussed various types of fluxes and why we use them, followed by various types of cleaning materials and processes. In this column, I will discuss cleanliness requirements to know whether the boards have been cleaned enough to meet their functional requirements for their intended applications.

I will discuss the cleanliness requirements in two parts. First, I will summarize the industry requirements as established in the latest industry standards, IPC 610 Rev H, and J-STD-001 Rev H, followed by my recommendations for cleanliness requirements without violating industry standard requirements.

Industry Cleanliness Requirements
Among many other requirements such as setting up accept/reject criteria for solder joints, IPC 610 and J-STD-001 also establish cleanliness requirements for electronic assemblies. Here is what the latest revision (Rev H) of IPC 610 and J-STD-001 have to say about cleanliness requirements. Please note the use of the term shall instead of should in the standards to avoid confusion.

General Industry Cleanliness Requirements
As is the case for many other quality acceptance requirements, IPC has essentially left it to the users and suppliers to determine the cleanliness requirements. I will highlight some of the major points in the standard (IPC 610 H and J-STD 001 H).

“Unless otherwise specified by design, or by the User, the acceptability of the residue condition shall be determined at the point of the manufacturing process just prior to the application of conformal coating, or on the final assembly if conformal coating is not applied.”

The requirement is essentially based on process control parameters, since changes in manufacturing materials or process parameters may detrimentally end item residue levels and product reliability, which may necessitate re-qualification. Manufacturing materials and/or process changes fall into two categories: major or minor, with major changes requiring validation, and minor changes with supporting objective evidence.

Qualification testing is generally considered to be more extensive in nature. Minor changes with supporting evidence are generally lesser efforts, involving shorter duration SIR tests or focused chemical characterization tests.

Again, the standard specifies that the degree of process change and associated required notifications of change are left to be established between manufacturer and user. However, IPC does provide some practical guidelines as to what to do. Here are some examples:

  1. You should not have discernible residue, but flux residues from no clean fluxes are acceptable.
  2. However, flux residues that inhibit electrical testing or visual inspection are not acceptable.
  3. Similarly, dull appearance is acceptable but colored residues or rusty appearance are not acceptable.
  4. Any foreign objects that potentially may be conductive are not allowed especially if they violate minimum electrical spacing requirements.
  5. White residues which may contain chlorides and cause corrosion are not acceptable.

Here are some more guidelines in the standard:

“Unless otherwise specified by the User, the Manufacturer shall qualify soldering and/or cleaning processes that result in acceptable levels of flux and other residues. Objective evidence shall be available for review. The use of extraction testing, i.e., ROSE, IC, etc., with no supporting objective evidence shall not be used to qualify a manufacturing process. 

“Supporting objective evidence shall be test data and/or other documentation demonstrating that the performance of the actual hardware is not adversely affected under conditions anticipated in the service environment. This may include:

  1. Surface insulation resistance (SIR), possibly in combination with ion chromatography testing, to demonstrate acceptable levels of residue. (Author’s note: However, no specific value is mentioned as to what the SIR value should be. I will provide some numbers as to what that should be based on historical data as to what you can expect if you follow some common process control.)
  2. Historical evidence, including field returns, warranty service records and failure analysis, demonstrating that ionic and other residues on delivered products have not caused failures in service.
  3. Electrical testing results, with power on, during extremes of temperature and humidity, which simulate the end use environment. Electrical failures should be subjected to failure analysis to determine whether ionic or other residues have caused the failure. This testing may occur during product qualification or outgoing acceptance testing. Rework processes shall be included in the process qualification.

“When it comes to visible residues, the requirement is that assemblies subjected to cleaning processes shall be free of visible residues. However, the visible residue requirements are established between Manufacturer and User.”

The bottom line is, IPC leaves it up to users and suppliers to establish mutually acceptable requirements based on the applications for their products. The standards also provide some references and white papers for additional guidance that users and suppliers may decide to use to establish their requirements. What it boils down to is that different users and suppliers can have different cleanliness requirements for the same applications and same products. I am not sure if that is a good thing.

My Views on Cleanliness Requirements
As discussed above, IPC does not tell you what the specific cleanliness requirements should be. But it does give a roadmap as to how to go about establishing the requirements. As noted earlier, IPC does have some very specific requirements when it comes to visually acceptable residues such as flux, white residues, and foreign objects, things you can visually see. However, when it comes to specific cleanliness requirements such as surface insulation resistance or micrograms of solvent extract requirements, you are on your own. 

Many would argue that the solvent extract test is relevant only for rosin. That must have been the reason for making the change in the J-STD-001, going as far back as 2005 when revision D was released. Now, more than 15 years later in 2021, we have revision H and the requirements for cleanliness are the same even though we have now more widespread use of components with practically no gap between the bottom of the component and the top of the board. But what have we been doing for the last few decades, starting in the 1980s? You guessed it: Solvent extract (aka ROSE), and I am talking about this test being used for all kinds of fluxes and all kinds of applications.

In addition to solvent extract, another test method that has been used extensively is surface insulation resistance (SIR). Previously, the industry used aggressive water-soluble fluxes; an SIR value of 500 MΩ/square (per square and not per square inch) on a production board under chip components, on a sampling basis, was the acceptance criteria. When I was at Intel, this test helped us discover many problems, such as poor adhesive cure profile that was trapping flux due to voids in the adhesive. We also had to make sure we didn’t ship any product with corrosion potential in the field.

The argument for not specifying a cleanliness requirement for RE and OR fluxes is that you cannot possibly develop acceptance criteria with repeatable test methods when you don’t really know what kinds of substrates, solder masks, and coatings these fluxes will interact with in some unknown environment. This argument may be valid, but we can ask the same question when it comes to RO fluxes. Besides, this problem can be solved by being more conservative and accommodating RE and OR fluxes in a very humid environment, as we have done for R0 fluxes. We can make an exception for applications where you are sending a manned mission to Mars, and you can afford to do all kinds of tests in the book if you are using RE and OR fluxes. But we are not all sending the assemblies to Mars.

If you are going to have a corrosion problem, does it really matter which kind of flux it came from? For example, you can have a corrosion problem from rosin flux if you use it generously for rework, leaving behind lots of flux that was never heat activated to become benign.

Recommendations for Cleanliness Requirements
Based on data over multiple decades now, my sense is that there are three simple requirements that should be considered (and may be given the force of an industry standard such as J-STD-001 and IPC 610 in future revisions).

  1. There should be no visible flux residue except for some no-clean flux residue. But there should be no white or corroded-look appearance no matter what kind of flux residue is on the board.
  2. Because solvent extract (ROSE) is commonly used, the 10.06 µg/in2 that has served the industry for so long should be used for all fluxes. However, if users and suppliers agree, they can use some other test, such as ion chromatography (IC) or other mutually acceptable tests, and the level of NaCl equivalent from 2.5 to 4.5 µg/in being used by some companies for IC.
  3. The most important criteria, at least to qualify the flux before use, the surface insulation resistance value taken in a humidity chamber at 100 VDC should be 500 MΩ/square to detect any trapped flux under components with practically no stand-off.

Of the three tests, given the fact that there is widespread use of low stand-off components such as BTC, LGAs and fine pitch QFPs with practically no stand-off (almost no gap between the bottom of the components and the top of the board), SIR test is the most reliable test. If there is any flux trapped under the components, SIR value will not meet the 500 megohm (10 to the power 8) requirements. Most boards, when properly cleaned, will show cleanliness as high as 10 to the power 12 or more, no matter what flux is used. There should be two SIR patterns on the board (on production boards for products meant for critical applications). One of those patterns will be covered with the components with lowest stand-off and the other pattern should have no component on it. This will serve as a control since you should always get very high SIR value as there should be no flux at that spot. Please refer to SIR pattern guidelines as discussed in the referenced chapter1.

Finally, itmust be noted that there is no such thing as the best flux, the best cleaning method, or the best method for determining cleanliness. These variables depend on the application. Thus, using the guidelines discussed in this column, the user must establish requirements for flux, cleaning, and cleanliness testing based on empirical data for a particular application. This means that the cleanliness tests (SIR, solvent extraction, and visual) should be performed on cleaned randomly-selected assemblies as a check on the process. There is no substitute for good process control because, if a bad board passes the cleanliness test, the failed assembly lot cannot be recalled, recleaned, or retested.

References

  1. Ray Prasad, Surface Mount Technology, Principles and Practice, second edition, Figures 13.13, 13.14 and 13.15.

 This column originally appeared in the October 2021 issue of SMT007 Magazine.

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2021

SMT Solver: Flux and Cleaning—How Clean Is Clean? Part 2

10-27-2021

The selection of flux and cleaning process determines, to a large extent, the manufacturing yield and product reliability of electronic assemblies. In this column, I will discuss cleanliness requirements to know whether the boards have been cleaned enough to meet their functional requirements for their intended applications.

View Story

SMT Solver: Flux and Cleaning—How Clean Is Clean? Part 1

07-07-2021

In this column, Ray Prasad discusses the various types of fluxes and why we use them, followed by various types of cleaning materials and processes. In the next column, he will discuss cleanliness requirements to know whether the boards have been cleaned enough to meet their functional requirements for their intended applications.

View Story

SMT Solver: Optimizing Your Manufacturing Operations

06-29-2021

We are not writing a book here but if you take a 50,000-foot view of the subject we should ask: What is the focus of manufacturing operations? The way I see it, there are only two things: quality and delivery. This is essentially the focus on the factory floor. Why? Because the customer cares about getting a quality product and getting it on time.

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SMT Solver: Major Drivers for Improving Yield and Reducing Cost

04-07-2021

In some of my previous columns I have gone into the details of types of defect that are predominant. There are many reasons we worry about defects, but the key reasons are increased cost and reduced reliability. These defects must be reworked so that you can meet the delivery requirement without too much delay.

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SMT Solver: How to Audit OEM-EMS Assembly Capability, Part 3

01-07-2021

Ray Prasad concludes his three-part series on how to audit OEM-EMS capability with questions on quality and RoHS compliance. The questions in this column are intended to help you generate your own questions relevant to your product and the manufacturing site you plan to audit.

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2020

SMT Solver: How to Audit OEM-EMS Assembly Capability, Part 2

11-08-2020

During the past two decades, there has been a tremendous increase in outsourcing by OEMs to EMS companies, which also results in a decrease in yield. In this column, Ray Prasad focuses on the technology and manufacturing capabilities of the supplier.

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SMT Solver: How to Audit OEM-EMS Assembly Capability, Part 1

09-13-2020

In Part 1 of this three-part series, Ray Prasad looks into how to assess the manufacturing capability of any company—OEM or EMS—and provides an overview of the audit process.

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SMT Solver: Industrial Revolution 4.0—Hype, Hope, or Reality?

08-02-2020

If you are in the electronics industry, you cannot help but notice the discussion about Factory 4.0. Ray Prasad discusses Factory 4.0 as he understands it and invites readers' comments on his interpretation.

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SMT Solver: Developing a Reflow Profile

05-15-2020

Developing a reflow profile involves ensuring all solder joints reach the minimum temperature to achieve good solder joints but don’t exceed maximum temperature to prevent damage to components or to solder joints. This is not an easy task. Ray Prasad provides specific guidelines and rules for developing a unique profile for each product without any damage and warpage to components and boards and with minimum possible profile related defects.

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SMT Solver: Developments in BTC Guidelines: IPC-7093A, Pt. 1

04-15-2020

As the chair of this IPC committee, let me share the latest developments in bottom-terminated component (BTC) design and assembly guidelines in this three-part series. In this first column, I will give you an overview of this technology and standard. In my upcoming columns, I will take an in-depth look at the design, assembly, quality, and reliability issues in BTC technology that have been incorporated in this latest IPC-7093A revision.

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SMT Solver: Dealing With Package Parasitics

02-03-2020

Packaging technology has constantly evolved over the decades from through-hole package to SMT with ever-decreasing pitches. There are many factors that play a role in the selection of a package, such as their cost and physical size, but the role package parasitics play in package selection has not changed over many decades.

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SMT Solver: Choosing the Right Defect

01-17-2020

Ray Prasad addresses some key issues that are important for all of us to be aware of and learn about, especially for managers in SMT assembly and engineers who aspire to be future managers. Topics covered include choosing the right defect and developing a DFM and process recipe.

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2019

SMT Solver: Today’s Soldering Options

11-22-2019

If you have to deal with mixed-assembly boards with both surface-mount and through-hole components—as is the case today for more than 95% of electronic products—the selection of a soldering process becomes more complex, especially if you use both tin-lead and lead-free components on the same board.

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SMT Solver: How Standards Impact You and Your Company

11-06-2019

Standardization is one of the key issues in promoting any new technology, but it is almost mandatory for SMT because of the need for automation to promote consistency in quality. Standards make the market grow faster than it would without them. A good standard benefits both users and suppliers. For example, if the package size tolerances are tightly controlled (within the requirement of the standard), the user can properly design the land pattern and use the same design for all suppliers of that package.

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

07-29-2019

Would you prefer shorts or opens in your products? Of course, neither. But what if you do have to choose? Ray Prasad says he 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. Read on why.

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SMT Solver: Benchmarking Defect Levels in Your Products

06-17-2019

In this column, Ray Prasad discusses why zero defects may be a desirable goal but not a realistic one. He also shares some industry data as proof, which you can also use to benchmark defect levels in your products. Finally, he also addresses the choices when selecting components that have a big impact on the level of defects you should expect.

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SMT Solver: Assemblers Can Help Customers Reduce Cost, Improve Reliability

05-08-2019

It is commonly assumed that the level of defects is primarily dependent on how the assemblers control their manufacturing processes. This sort of mistaken belief will cause you to never find the root cause of the problem. Hence, the problem will persist forever. And just because defects are discovered in manufacturing does not mean that they were created in manufacturing. Find out why.

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2014

Flux Classification

02-15-2014

In the previous column, I discussed flux functions and general considerations in their selection. In my next three columns, I will review various types of fluxes.

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Major Types of Fluxes

01-20-2014

Organic acid (OA) fluxes are stronger than rosin fluxes but weaker than inorganic fluxes.

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