Three Steps to Improve SMT Yield

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 more than two decades, 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.


What are the reasons for this high defect rate?
-The processes are at very high speeds.
-Machines must perform them.
-The equipment must be characterized thoroughly. This can be defined as understanding all parameters that affect the equipment’s performance.
-Vendors may say it is easy; it is not.
-Most large companies have assigned engineers to optimize; small companies learn as they go.
-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. With widespread use of lead-free, the yield problem will be compounded as we enter uncharted territory.

When yield problems persist, most people blame manufacturing. This is unfair and prevents companies from implementing the necessary corrective actions. Three things that control yield are design for manufacturability (DFM); quality of incoming materials; and manufacturing processes and equipment. Simply stated, we need a good recipe (DFM), good ingredients (incoming material) and a good chef (manufacturing).
If you look at the outsourcing trend, the “chef” (CM) does not have total control on quality because the customer (OEM) controls the “recipe” and “ingredients.” But the CM takes the blame. That is not only wrong, but this thinking will never get us to higher yield and lower cost. To solve yield problems, we must understand the interdependency of design and manufacturing.

Design for Manufacturability
DFM is a key driver, if not the most important of manufacturing yield. However, few circuit and board designers understand manufacturing processes. A DFM document must be company-specific. Using an industry standard such as IPC-SM-782 is a good place to start. Some major items that should be included in a DFM for
SMT products are:
-Establish design rules and guidelines while emphasizing the importance of differences between them;
-Component selection criteria, including consolidation of parts lists to reduce redundancy and eliminate obsolete parts;
-Paneling considerations;
-Fiducial requirements;
-Land-pattern design;
-Solder-mask consideration;
-Via-hole location;
-Design for test;
-Anything unique to your design.

With widespread use of high-pin-count BGAs that cannot be inspected visually, sufficient test coverage for in-circuit test (ICT) should be seriously considered. Keep in mind that no inspection method is perfect. The only way to prevent defects from escaping to the field is to rely on overlapping test and inspection methods. Once a DFM document developed by a well-trained team is finalized and released, the possibility of DFM violation generally does not arise.

Creating a DFM document is not easy; however, it will correct problems at the source and prevent their recurrence. This is critical in an environment where essentially all manufacturing is being outsourced or sent offshore.
Incoming Materials Quality
"Garbage in, garbage out" could not be more true than in the assembly of SMT components where pitches are shrinking and process windows tightening. As a result, there is no way to improve manufacturing yield if the boards and components have poor solderability or unacceptable co-planarity. Referencing industry standards such as J-STD-002/003 is a good idea.

Manufacturing Processes
How should one identify key manufacturing process issues? First characterize the process, then document the details of equipment- and non-equipment-dependent variables that control yield. There are some misconceptions that if you buy a particular convection oven, there’s no need to develop a unique profile for each product. This is not true, as each board has a different thermal mass.
In addition to having the right design, quality incoming materials and good manufacturing capabilities, trained personnel is critical to achieve high yields. Failing to realize we need a good recipe, the right ingredients and a good chef to improve yield makes the problem seem like the weather - out of our control.
Ray P. Prasad is an SMT Editorial Advisory Board member and author of the textbook Surface Mount Technology: Principles and Practice. He is president of BeamWorks Inc. and founder of the Ray Prasad Consultancy Group. e-mail: smtsolver@aol.com; Website: www.rayprasad.com for upcoming SMT and lead-free training schedules.