Issue link: https://iconnect007.uberflip.com/i/989774
14 SMT007 MAGAZINE I JUNE 2018 reflow, and pick and place," explains Moody. "Rigid-flex design typically replicates rigid- board assembly, because most of the func- tionality on a rigid-flex board is all centered in the rigid area. The flex area of a rigid-flex is usually the interconnect between the flex areas. So, it becomes a little less problematic; but flex circuits themselves do lend themselves far more to the multiple process environment." Lenthor found flex circuit assembly challeng- ing initially. "Luckily for us, we hired a really good engineer who's been able to design a lot of the fixturing that we use, and that's what really makes us succeed in assembly," says Moody. "In the beginning it was difficult, there was far more rework than we have today. But the techniques, the equipment that we've got now, have really lent themselves to an assem- bly that's predictable. We get a very good first yield rate. I would guess it's 96-97% in the first pass yield. It wasn't that way 10 years ago." Indeed, when dealing with flex circuits, the overarching theme to ensure successful assem- bly is proper fixturing. According to Bob Wettermann, principal of BEST Inc., an electronics rework/repair and PCB assembly firm, "In the rigid world, you have a coplanar surface and components with known fixed dimensions, and so the pick and place machine knows exactly how and where to place the device. It's always a planar surface. Now, you add the three dimensionality as well as the lack of rigidity of the surface and it becomes very difficult to place micro compo- nents on essentially a noodle." "Fixturing is key to tackling this problem. Having a good coplanar surface to remove and replace devices is key to reworking complex devices on flex circuitry. In terms of physi- cal repair, such as IPC- 7721-related type of phys- ical repairs, there just aren't any standards for flex materials. You have to make the process up as you go along. The other day, a client sent us a case where we were refereeing the acceptability criteria for the solder joints of an SOIC placed onto a flex circuit. We were dealing with an epoxy attachment to flex. And guess what? There are no industry acceptability criteria. So, it's still a little bit of the Wild West, I would call it, in terms of standards, knowledge and practices. You make it up as you go along." Throw it Over the Wall In our conversations with industry experts, they always mention a critical factor to success when it comes to design, fabrication, or assembly—communication along the supply chain. Because most often than not, the indus- try continues to have a 'throw it over the wall' mentality. This case is no different. "It's just because of the kind of business that we are in in terms of assembly—we're the prototyping guys, so our job is to get the customer functional units so they can go to round two or round three," says Wettermann. "Generally, the boards are not ending up in end customers' hands. Our job is to get functional units." Moody echoes what Wettermann is saying. "In the prototype stage, a lot of customers are just simply looking to get some functionality out of the initial product, and they just want you to go figure it out. On the other hand, because we do get into some production stuff with our customers, and it's still interesting to me that when we begin the conversation, and we begin in sometimes even at the design stage, the assembly aspect of the build is still not being considered until it's almost neces- sary. No matter how much you try to drive the conversation in the beginning, I don't know why, but there is still some reluctance to concentrate on what the assembly is going to