Automated stencil printing is the heart of modern SMT production. The typical SMT line runs at 90-95% yield because the stencil printer is an incredibly challenging process to manage. Industry experts estimate that there are over three dozen different stencil printing parameters or variables that must be controlled. The whole system is so complex — and the cost of failures are so high — that once engineers find a process that works they rarely, if ever, revisit those parameters. But if companies upgrade their stencil printing process dramatic improvements in yields are possible, in some cases all the way to 99%+ yields.
Often the stencil printing isn’t flawed; it’s the stencil cleaning process that causes the problems. Stencil cleaning is an unloved process. The automated cleaning using under-stencil rolls is expensive, trouble-prone and time-consuming.
Unhappily, stencil rolls vary widely in quality, price and performance. Most purchasing managers opt for the least expensive rolls they can find. This is an understandable, but mistaken, conclusion. One customer in China recently discovered this phenomenon.
This is a large, well known subcontractor producing high-reliability PCBs for the auto industry. They have 16 SMT lines equipped with Ekra printers. The solder paste was a lead-free product from Hereaus.
They had been using inexpensive rolls and wetting the paper with a hydrocarbon solvent. They cleaned every three prints, using a “wet-dry-dry” cycle. They used 10-12 rolls/printer/day (about 5,000 rolls/month) at an estimated cost of $40,000/month. That was expensive, but that wasn’t the big problem.
The big problem came from a customer requirement that prohibited reworking PCBs. If the board wasn’t perfect it was scrapped. Even at a better-than-industry average yield of 98%, something over 100 boards per day were scrapped, costing more than $2,000 per day. Obviously, this was an avoidable expense the client wished to minimize.
The company switched to SMT stencil rolls from MicroCare and found many savings. They began wiping dry — without using any wetting fluid. They extended the cleaning cycles to every five prints. They only used two passes, not three, saving 5-6 seconds and extra paper per cleaning cycle. Stencil roll consumption plummeted to two rolls per machine per day.
But the big savings came when their scrap dropped to zero. That’s right — in one controlled test they made 2,000 boards and instead of the expected 40-50 defective boards there were none. So instead of scrapping $800-$1,000 of un-reworkable PCBs just in this one test, they shipped them all to the customer. This savings alone justified the move to MicroCare stencil rolls, and all the other savings productivity savings became profit.