-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueIPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
The Cost of Rework
In this issue, we investigate rework's current state of the art. What are the root causes and how are they resolved? What is the financial impact of rework, and is it possible to eliminate it entirely without sacrificing your yields?
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
STEP 2: Process Control
December 31, 1969 |Estimated reading time: 7 minutes
This article discusses how lead-free material characteristics affect the process engineers responsible for implementing and optimizing the lead-free assembly process.
By Joe Belmonte
The transition to a lead-free electronics assembly has been discussed for many years. RoHS and WEEE regulations now have final implementation dates. Many electronics manufacturers have implemented lead-free process capacity; while others are making a significant effort to learn what is required. Other manufacturers are in the beginning stages of understanding the lead-free process.
It is time for process engineers to understand what is required in the processes that make an SMT manufacturing operation. Process engineers dealing with lead-free manufacturing need to put more attention into process details. The process window, process tolerance and margin of error are smaller in lead-free processes. Engineers are working with soldering materials that require increased temperature to reflow and do not wet or spread as well as tin/lead soldering materials. Increased solder temperatures result in solder processes that operate closer to the temperature tolerance of many components. Wetting characteristics of these lead-free materials are another area of the SMT manufacturing process.
The most widely used lead-free alloy of tin-silver-copper (SAC) reflows about 34°C higher then tin/lead alloys. However, as with a tin/lead solder paste, we must study and understand all aspects of the solder paste being used. The flux used in solder paste is a complex chemical product, making it unique. When working in the narrow range between the lead-free solder paste reflow temperature and a component’s temperature tolerance, process engineers must know what heating cycle or “profile” is required to produce the best possible solder joint at the lowest possible temperature. This also must be accomplished in the least amount of time. Process engineers must be aware of solder paste stencil life, storage requirements and print speed.
Prior to the introduction of lead-free soldering material, process engineers needed to have a firm understanding of the components used in products. If we were using miniature components, such as a 0201 or 0.4-mm Chip Scale Package (CSP), fine-pitch Quad Flat Packs (QFP) or complex components such as a column grid array (CGA) or large Ball Grid Array (BGA), we were concerned with process capabilities of solder paste printing, component placement and reflow soldering to continuously assemble these components. We seldom investigated component specifications to identify maximum component temperature tolerance or maximum temperature increase rate (degrees/second). We knew that these temperature specifications were important, but tin/lead reflow temperatures almost never exceeded the maximum allowable temperature, and seldom exceeded the maximum allowable temperature increase rate. We also were not concerned with the finish on the component leads. In most cases, we knew that it was compatible with our tin/lead soldering materials. Our major concern was that the component-lead finish was still “clean” (had no oxides) and solderable.
With lead-free materials, process engineers need to understand the specification of every component used on every product manufactured. Assuming that a component’s temperature tolerance and lead finish is compatible with a lead-free manufacturing process is a mistake. Many products may be produced that will not pass internal testing or will fail prematurely once they are delivered to customers. Early product life failures are the most costly defects and result in expensive repairs, customer dissatisfaction and customer loss.
The Printing Process
Experience has proven that there is no difference in the actual printing of lead-free solder paste vs. tin/lead solders. Formal testing has verified that printing either type of solder can be accomplished using the same stencil, printing equipment and boards. As with any solder paste, we have to read and understand the solder paste specification and adhere to the print speed, stencil life, storage and other performance specifications.
Lead-free printing, however, does require certain considerations. The performance characteristics of lead-free solder affect print accuracy - alignment of the printed solder paste to the PCB pad. Because lead-free alloys do not solder or wet as well as tin/lead, any solder paste that is not accurately printed onto the PCB will stay where it was printed after the reflow soldering process. Process engineers should be concerned with the accuracy of printing equipment to align the stencil apertures to the PCB pads, the accuracy of the stencil itself and the accuracy of the PCB. Products that use fine-feature or miniature components may require more accurate or recalibrated printing equipment to ensure optimum stencil-to-board alignment. PCB suppliers may be required to minimize board stretch and other circuit distortions, while stencil suppliers may need to verify aperture position accuracy for every stencil prior to shipment. Process engineers also need to increase the use of post-print inspection tools. In certain critical areas, a 2-D or 3-D post-print inspection is needed to verify solder paste to PCB pad alignment (Figure).
Figure. The requirement for accurate printing in a lead-free process is illustrated by the results of misaligned prints that do not wet back and fully cover the PCB pad.
Another issue with printing lead-free solder paste is stencil aperture design. Traditionally, stencil aperture size is reduced in relation to PCB pad size. This ensures the stencil aperture seals, or gaskets, to the PCB pad. Gasketing reduces solder paste that can get under the stencil and eventually cause wet solder bridges if not cleaned properly. Lead-free solder paste does not spread as well, so it is possible that with some lead-free solder pastes the entire PCB pad will not be covered after the reflow soldering process. The lack of pad coverage can result in exposed PCB pad material. Depending on the particular lead-free finish (OSP, Immersion Silver, Immersion Tin or nickel gold) this may be a cosmetic issue. Customers may not accept a product that has exposed copper. Depending on the final product application, exposed copper may create product reliability issues.
This brings up the dilemma of requiring coverage of the entire PCB pad with printed lead-free solder paste while minimizing the occurrence of wet bridges. There is no easy solution to this dilemma. Each product build should be evaluated according to the lead-free solder paste specification and printed board finish. Next, a stencil that will balance PCB coverage and stencil aperture gasekting should be assigned.
The Component Placement Process
In general, there is little impact on the component placement process when implementing a lead-free process. Components will have a lead-free finish, however the shape and size remains the same. One area of concern in the component placement process is placement accuracy. Several studies indicate that components that would center back to the PCB pads in a tin/lead process will not center back as well in a lead-free process. We will have to monitor the placement process to ensure components are placed onto the PCB pads.
The Reflow Soldering Process
The biggest impact of lead-free materials is on the soldering processes. The reflow soldering process is where the materials specifications and components specifications come together to create an optimum reflow soldering profile. The procedures and rules for creating an optimum reflow soldering profile are no different in a lead-free process than they are in a tin/lead process. The major and critical difference is adhering to these well-known reflow profile creation procedures and rules. We can no longer accept “group profiles” or “common profiles,” where one reflow soldering profile is used for a family or similar products. That slight difference in temperature exposure to a particular component on a particular product can damage or destroy that component. It is vital to understand the exact temperature exposure of each component on that particular product.
The use of nitrogen in the reflow soldering process also raises many questions. The first is whether or not nitrogen is required. This is not an easy question to answer. As with any process design, all process details must be considered:
- What lead-free solder paste is being used?
- What is the PCB finish?
- What is the lead-free component lead finish?
- What reflow soldering oven is being used?
Nitrogen can provide several known benefits, but do these justify the expense? Only a formal study of all factors in the manufacturing operation can answer this. The last issue to consider is the optimum-cooling rate needed to produce the most reliable lead-free solder joint. This, however, is another area that does not have a definitive answer. There are several formal studies that can provide guidance, but no generally accepted specification.
Conclusion
The implementation and optimization of a lead-free SMT manufacturing process requires attention to several process details. There are few new rules for process development and optimization procedures. For the most part, the existing procedures and rules remain. It is the execution of these procedures and rules that will dramatically change.
The bottom line is that the process window has narrowed and process engineers need to be more attentive to every process detail to achieve an organization’s quality and throughput goals.
Joe Belmonte, project manager, Advanced Process Group, Speedline Technologies may be contacted at (508) 541-4772; e-mail: jbelmonte@speedlinetech.com.