With all the focus on stencil printing to reduce defects and improve profits, electronics assemblers continually invest in custom tooling, new and improved stencil manufacturing methods or materials, high-capability printers, super accurate inspection machines, fancy statistical software and expert training. Ironically, many of the same high-tech assemblers that pour money into ensuring the robustness of their print processes actually skip the simple step of stencil verification, upon receipt.
Proper maintenance of equipment is often the most overlooked problem in hand-soldering operations. A few easy and repeatable practices regarding soldering iron tip care can eliminate a wide variety of issues. Proper tip care will extend the life of the tip and cut down on the need to frequently replace oxidized or pitted tips that are no longer usable.The process is flow-charted in FIGURE 2. The data analysis involves sampling volume readings from the more difficult to print devices, such as SOT883s, SOD882s and [mu]BGAs, as well as for two passive devices, usually 0201s and 0402s. For each component type, Cpks are calculated using control limits of [ or -]50% of theoretical volumes or slightly tighter. All Cpks must be greater than 1.33 for the stencil to be acceptable for production use. If any of the Cpks fall short of the 4-Sigma mark, the machine setups are checked and two more test prints are run. If they again fail to meet the mark, the stencil is held for further review. If the print quality is acceptable, the data are archived and the stencil placed into production.Each panel contains a collection of solar cells that are wired up with copper in series. Even a 2 hundred watt solar energy panel produces enough energy to either charge a battery for further use or to be converted right into AC voltage for domestic use.[FIGURE 1 OMITTED]* Use only sulfur-free sponges intended for electronics applications.* "Idle the tip!" This term refers to the practice of coating the tip with a thin film of solder before returning it to its holder. This thin film of solder will be oxidized instead of the surface of the tip, dramatically extending the life of the tip.Ray Whittier of Vicor (vicr.com) demonstrates how he uses his white light SPI system to verify new stencils before putting them into production: The official ISO 9001 documented procedure involves printing two dummy boards and inspecting them with the SPI machine. If the machine passes the two prints, the data are exported for further analysis. If the machine fails either print, the stencil is inspected for manufacturing defects. If no obvious defects are found, the machine setups are checked and two more dummy prints are run. If the prints pass on the second try, the data are exported for analysis. If they fail on the second try, the process is stopped and the stencil is held for further review.Depending on the size of your home and how much energy you need, you might want to make more than one home made solar panel.It takes about 10 min. on the assembly line to get the data, and another 10 min. to crunch the numbers. In addition to being faster and easier than measuring apertures, the SPI method captures process outputs instead of inputs. Where industry used to be limited to measuring the size and position of the aperture and using it as an indicator of a stencil's performance, we can now measure what really matters: the characteristics of the individual deposits on the PWB. That's where the rubber meets the road, or, specifically, where the solder paste meets the board.[FIGURE 2 OMITTED]The goal of any high-quality soldered joint is the formation of a good intermetallic bond (FIGURE 1) between the solder alloy and connection contact area (e.g., the wire or component lead, and the land, pad, or terminal). The following practices will help ensure acceptable soldered connections and prolong the life of the equipment used in common hand-soldering operations:
CHRYS SHEA is founder of Shea Engineering Services (sheaengineering.com); chrys@sheaengineering.com.
Author: Chrys Shea
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