|An installed SMT fastener allowing for components to be joined at a 90 degree orientation. |
Traditional hardware options for assembling printed circuit boards have always carried potential baggage. The tedious and exacting process of handling and installing loose hardware often drains productivity. Broaching fasteners, while typically a more efficient alternative, can damage fully populated boards if installed improperly. The resulting scrap costs can be excessive.
Recent advances in fastening technology have made it possible to stow this baggage by taking advantage of the infrastructure already operating in the marketplace. A variety of fastener types have been developed specifically for mounting to a PC board using the same soldering processes that mount other electronic components. Surface mount fasteners effectively become another board component that are just "along for the ride."
Such fasteners, when fed on a tape-and-reel, assembled with pick-and-place equipment, and reflowed alongside other electronic components, can eliminate costs associated with parts handling, contribute to dramatic reductions in quality failures, and accelerate production.
Many methods exist to fasten hardware to PC boards and each method has its advantages and disadvantages. For example, loose spacers (serving as plastic or metal columns used to offset two boards) usually require secondary operations: sometimes they are snapped onto the PC board; in other cases they may be joined with a screw to another component. The secondary operations, performed manually, rob time.
Difficult to Handle
Loose nuts and screws are handled in much the same way. During assembly, the handling process can become an even greater issue, due to the fine work necessary to align all parts, often on both sides of a PC board. Care must be taken to ensure that any dropped hardware is noticed and removed to prevent rattle or, even worse, shorts and product failures.
Striving to reduce the number of loose parts and simplify the assembly process, broaching spacers or nuts have enjoyed industry popularity by virtually removing handling issues altogether. These fasteners broach by pressing the fastener's knurled section into a slightly smaller hole in the board, forcing the knurls to cut into the board, installing themselves permanently.
But these types are usually installed after boards have been fully populated with electronic components, and the broaching process can subject populated boards to significant amounts of stress and ruin. Micro-cracks may form to sever traces in any of the board's multiple layers and boards may have to be scrapped, along with the pricey onboard components.
Board damage can additionally occur if broaching fasteners are misaligned while being pressed into the mounting hole during installation. This can cause large-scale cracking or breaking of the board and likewise lead to subsequent assembly issues. To reduce potential failures, sufficient "keep-away areas" should always be designated around the fastener to keep small traces clear of the broached area. Close-to-board edge distances must also always be maintained.
Other, less frequently used PC board attachment methods include adhesives, buttons, keyholes, and interlocks. All must be placed on the board or assembled and that means time, money, and challenges.
Surface mount fastening eliminates any need to manually place the hardware on a populated PC board. Instead, the hardware — supplied on tape-and-reel — is positioned while the board is being processed and then installed with the other electronic components using conventional surface mounting equipment.
Other relative advantages: quality issues related to board cracking and misalignment disappear, since the hardware installs automatically with the same pick-and-place robotic equipment used for a board's electronic components. Such a process is non-intrusive to the board and will not expose a board to cracking.
And with fasteners packaged on tape-and-reel (neither loose nor dumped into bowls), different parts will not get inadvertently mixed as can happen from time to time with disastrous results during production with loose hardware.
Several standard types of surface mount fasteners have been engineered to keep pace with application demands and interest.
Spacers enable board-to-board stacking; nuts provide an alternative to broaching counterparts or loose hardware to mount boards or attach components; panel fasteners will suit applications where easy removal and reinstallation of PC boards is required; and right-angle fasteners allow components to be joined at a 90° orientation.
Spacers and Nuts. These are favored for their capabilities to stack, space, and attach. Surface mount spacers and nuts with or without threads generally can be installed in boards as thin as 0.060-in. (1.53mm). Reels can carry up to 1,500 parts, depending on fastener size. A Kapton® patch is supplied to allow for reliable vacuum pickup. Standard steel fasteners get plated with tin to aid in soldering.
Panel Fasteners. These types in several variations will incorporate a threaded screw or pin that retracts and advances to engage a nut or internally threaded feature. Their design incorporates a steel retainer and metal Phillips drive screw, which is captivated in an ABS plastic cap.
These assemblies are mounted by snapping screw into soldered retainer. The screw thread and plastic cap initially are supplied separately so the retainer can be placed on the board and run through the oven. After reflow, the cap and threads snap onto the retainer, completing the assembly.
Right-Angle Fasteners. These offer an efficient and reliable method to create permanent right-angle attachment points on PC boards. They provide reusable threads parallel to the board for a component to be mounted 90° to the board.
These fasteners serve as viable alternatives to conventional angle brackets or threaded right-angle blocks for attaching board to chassis, chassis to board, or component to board. They can be installed on PC boards as thin as 0.040-in. (1mm).
The shape of the fastener readily allows the part to be picked up by a pick-and-place machine without a patch: The fastener is presented in the tape-and-reel with two small pins downward and the flat portion of the fastener's head exposed to the pneumatic finger. The two pins at the bottom act as two very small pilots, providing both stability and locational accuracy during placement.
|Installed and loose SMT spacers for board-to-board stacking. The supplied tape-and-reel also is shown. |
A step along the bottom allows the solder fillet to be formed along this edge, even while the face of the fastener is flush against an edge of the PC board. The fastener's rectangular hole reduces the mass and promotes quicker heating to minimize heat draw away from surrounding components on the board.
Other feasible surface mount fastener designs on the drawing board include snapping fasteners and externally threaded types, expected to surface soon.
Optimizing the Potential
While "cost" is one of the most important factors when evaluating fastening methods, a fastener's function is very much part of the equation. In the case of surface mount fasteners, function is represented by the part's ability to adhere to a PC board, which, in turn, is a function of the part geometry and surface finish, the solder paste, and the conditions of the surface mounting process. All should be taken into account to achieve desired results. Plating, feeding issues, and stencil design also should be understood to optimize the potential of surface mount fastening. Pure tin is virtually standard plating for surface mount fasteners. In applications where tin whiskers may be cause for concern, the plating could be matte tin and bake-annealed to relieve embrittlement as recommended by iNEMI (International Electronics Manufacturing Initiative).
On the feeding side, some surface mount fasteners lack a convenient, exposed flat surface to serve as the suction point for the pneumatic pick-and-place finger. This is most often a challenge as noted with threaded or thru-hole spacers and nuts. The solution is to introduce a patch to cover the part and achieve suction. The patch then must travel with fastener into the oven only to be removed when the thread must enter from the patched side of the hole.
While all types of surface mount fasteners can be soldered directly to the surface of a PC board, a pilot should be integrated to assist in stability and placement. A pilot sitting in a hole requires a stencil to mask the hole, while exposing the land to solder, and users will find that a spoke design can work well here.
Surface mount fastening technology has clearly made significant inroads, but every application will be governed by distinct parameters and requirements. Our best recommendation is to partner at the start of the design process with an experienced hardware manufacturer to maximize the opportunity.
Contact: PennEngineering®, 5190 Old Easton Road, Danboro, PA 18916-1000 800-237-4736 or 215-766-8853 fax: 215-766-0143 Web: http://www.pemnet.com