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

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If you are one of the long-time readers of SMT007 Magazine, you probably know that there was a print version for decades known as SMT. I was one of the columnists for the print version, starting in the mid-1980s. I was looking at some of the columns that I wrote almost a decade ago to see how things have changed over time. It should be no surprise that there have been significant changes in many areas but practically no changes in others. 

For example, there has been practically no improvement in the percentage of yield despite decades of industry experience in high-volume manufacturing. Obviously, some companies are better than others in achieving higher and consistent quality than others, but overall the yield has not improved in percentage terms over the decades. Less than 10% of companies, OEM or EMS, have over 90% first-pass yield (see my June 2019 column). 

At a macro level, there are many reasons for this. For example, to keep up with Moore’s law, there has been significant shrinkage in size and pitches of packages while still dealing with old packages, including through-hole on the same board. This increases board complexity. In addition, 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. This is not because EMS companies are not good at what they do, but as I have explained in previous columns, EMS companies don’t have control over some of the key contributors of defects, such as DFM and in many cases incoming material quality. 

What I want to do in this column and the two follow-up columns is to look into how to assess the manufacturing capability of any company—OEM or EMS. When I say manufacturing capability, I am only talking about assessing or auditing, if you will—the capability of a company to produce consistent quality over time. In this column, I will provide an overview of the audit process. In future columns, I will delve into the details of questions that should be asked to assess the major areas, such as technology, manufacturing, and the quality of the company being audited.

Audit Process: An Overview 
We need to begin with defining the complexity of the product that needs to be built. Not everyone has the expertise to build all types of products, especially complex mixed-assembly boards. Also, they may not have skilled and experienced technical personnel. Even the best companies will find it challenging if the product requires the assembly of packages, such as fine-pitch, BGA, BTC, 0402, 0201, or 01005 discrete devices—especially when that board also has legacy components, such as J lead, gull wing, and even through-hole components. Add to this lead-free BGAs in a tin-lead process, which is very common in products for military applications. 

Keeping the complexity of the product in mind, the first thing to do before an audit is to prepare a detailed questionnaire for the audit. The questions should be targeted to assess not only the manufacturing capability for tin-lead and lead-free products but also the basic SMT infrastructure of the company, which is essential in building products with consistent quality over time. The answers to these questions will supplement the findings of the audit during the visit. The objective of the questionnaire is also to give the company being audited a heads up as to what you may be looking for during your visit. 

Some questions may be considered confidential or not applicable, and the company may choose not to answer them. You can draw your own conclusions on whether those questions are confidential or not applicable. 

If possible, the company should return the completed questionnaire to the auditor a few days before the site visit. Along with the completed questionnaire, it may be appropriate to ask for the following documents to review before the visit to make the on-site visit very productive, especially when you have limited time for the audit:

  1. Company-specific design for manufacturing (DFM) used to build your products. DFM means different things to different people (I will explore the details in the next two columns), but for now, we want to know the rules and guidelines in DFM for things such as component selection criteria, paneling considerations, fiducial requirements, land pattern design, solder mask consideration, via hole location, design for test, and anything unique to your design.
  2. Company-specific manufacturing process document(s), such as adhesive and paste application, pick-and-place, reflow, wave, cleaning, repair, etc., that are used to build your products. These documents are not dependent on the types of equipment being used in the line. They are essentially a recipe the company uses to build the product. An example would be targets for peak temperature and TAL, no matter which oven is being used. 
  3. Equipment operating procedures. This is equipment specific document. The objective of this document is to ensure all operators use the same procedure when operating the machines on the line. 
  4. Handling and storage procedure for moisture-sensitive components. This becomes even more critical for lead-free. Very few companies have good control on this issue.
  5. Typical wave and reflow profiles showing peak, TAL, and variations of different components seen in wave and reflow soldering. 
  6. The sum of all defect data collected from ICT, functional, AOI, and visual inspection for some of your key products. The defects should be summarized in four subcategories to help you analyze field failure potential and related consequences of these defects:
    • Shorts 
    • Opens 
    • Insufficient 
    • All other

In one recent audit, I asked for this defect data on assemblies made in China. The company responded that it had zero defects and, therefore, nothing to provide. With peak reflow temperature of 255°C and TAL of over two minutes, would you have zero defects? Since products were failing in some of the stress testing, you can draw your own conclusions. 

It is important that some products be built during the site visit so that you can get a realistic view of a typical production day. On the last day of the visit, findings of the audit should be presented to the company in an interactive and informal format. I find it most useful to give feedback face to face. All personnel who participated during the audit process and management should plan to attend the presentation. In some cases, a written report is necessary and can be provided a few days after the audit. 

It may be obvious, but it is important to confirm the details of your visit, including the dates of travel, dates of the audit, and the people you will meet. If possible, you may also want to have a brief teleconference with the people before the visit to agree on the format and expectations of the audit. The questionnaire can be very lengthy (I use a 30-page questionnaire, which I will summarize in a future column) and focus on the following areas:

Overview Questions: A Broad Overview of Your Manufacturing Capability in Both Tin-Lead and Lead-Free

  • General company and contact information
  • Types of components and products being built
  • Training program for engineers and operators

Technology Questions

  • DFM documentation and control
  • Laminate 
  • Surface finish
  • Components 

Manufacturing Questions

  • List of all assembly equipment (make, model, and year of purchase)
  • Materials management
    • Component procurement
    • Component storage and handling
  • Tin-lead and lead-free assembly 
    • Assembly materials (paste, adhesive, flux, cleaning)
    • Paste printing
    • Reflow soldering
    • Wave soldering
    • Rework
    • Inspection and test
    • Repair

Quality Assurance Questions

  • Materials assembly database
  • Quality monitoring and reporting database
  • Field returns
  • Materials declaration

RoHS Compliance 

  • RoHS compliance assurance system (CAS) 
  • Tests or procedures used to ensure RoHS compliance
  • Supplier reliability and selection of raw materials and components
  • Evidence and documentation
  • Staff training and experience

If you do not have a chance to review the various design and process documents (noted earlier) before the visit, you certainly want to review them during the visit. It is also important to review the training procedure as part of the audit and to talk to the engineers and technicians responsible for design and processes. And do not forget to spend time on the line alone, talking to the operators and inspecting some of the boards being built. If you only spend time in the conference room with the quality manager, you will never get the complete story. 

Asking the Right Questions
Building good quality products requires not only the right type of equipment but a solid infrastructure of skilled personnel, extensive documentation and process control, and equipment characterization. Characterization of process and equipment, which is necessary for higher yield, requires an investment in engineering resources that have not been made by all companies.

In my future columns, I will discuss specific questions that should be asked to correctly assess the capability of the company in building the products that meet your specific requirements.

This column originally appeared in the September 2020 issue of SMT007 Magazine.


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