Circuits Assembly Online Magazine - Fastening Hardware Using SMT

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Why manually install hardware when existing placement lines can be used?

Traditional hardware options for assembling PCBs 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. Resulting scrap costs can be excessive.

Recent advances in fastening technology stow this baggage by taking advantage of existing infrastructure. A variety of fastener types have been developed specifically for PCB mounting using the same soldering processes that mount other components. Surface mount fasteners effectively become another board component. Such fasteners when fed on a tape and reel, assembled with pick-and-place equipment, and reflowed with other 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 PCBs and all have 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 PCB; in other cases, they may be joined with a screw to another component. The secondary operations, performed manually, rob time.

Loose nuts and screws are handled in much the same way. During assembly, handling can become an even greater issue due to the fine work necessary to align all parts, often on both sides of a PCB. 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 been popular 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, and installing permanently.

But these types are usually installed after boards have been fully populated with electronics 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 any placed components.

Further board damage can occur if broaching fasteners are misaligned while being pressed into the mounting hole during installation. This can cause large-scale board cracking or breaking 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 always must be maintained also.)

Other, less common PCB attachment methods include adhesives, buttons, keyholes and interlocks. All must be placed on the board or assembled; that means time, money and challenges.

Auto-Installation

Surface mount fastening eliminates any need to manually place the hardware on a populated PCB. Instead, the hardware (supplied on tape and reel) is positioned while the board is processed and then installed with other components using conventional surface mount equipment.

Other relative advantages: Quality issues related to board cracking and misalignment disappear, since the hardware installs automatically with the same pick-and-place equipment. 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 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 PCBs 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". Reels can carry up to 1,500 parts, depending on fastener size. A Kapton patch is supplied to permit reliable vacuum pickup. Standard steel fasteners are tin-plated to aid 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 a screw into the 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 PCBs. They provide reusable threads parallel to the board for a component to be mounted 90˚ to the board. These 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 in PCBs as thin as 0.040".

The shape of the fastener readily permits the part to be picked by a pick-and-place machine without a patch: The fastener is presented on tape-and-reel with two small pins downward and the flat portion of the fastener’s head exposed to the pneumatic finger. Two pins at the bottom act as two very small pilots, providing both stability and accuracy of location during placement.

A step along the bottom permits the solder fillet to be formed along this edge, even while the face of the fastener is flush against the PCB edge. The fastener’s rectangular hole reduces the mass and promotes quicker heating to minimize heat draw from surrounding components.

Other feasible surface mount fastener designs on the drawing board include snapping fasteners and externally threaded types. More will surface.

Case Studies

Two examples of real-world customer applications underscore the productivity, performance and cost benefits that can be realized from surface mount fastening technology.

• Telecom line card. Four surface mount spacers were specified for a PCB to mount a daughterboard in a line card for optical fiber telecom equipment. (Approximately 75% of all PCBs manufactured by this customer have been designed to host daughterboards.) The previous method involved loose hardware consisting of four standard hexagonal pillars and eight sets of screws and washers to secure both boards. The manual assembly process was slow and inefficient.

Results: Surface mount standoffs immediately provided a 50% reduction in required hardware and reduced cost more than 50%, compared with assembly by hand. Less handling has also improved unit quality and reliability.¹• External wireless modem. A surface mount right-angle fastener attaches a PCB to the inside housing of an external modem. The part is well suited for the manufacturer’s USB model because of space limitations. Attachment was accomplished previously using a custom-made bracket that carried an installed cost of almost $3.

Results: Installed cost of the surface mount fastener is one-sixth the cost of the custom bracket, and the production process has become more streamlined as secondary operations were eliminated.²Performance Considerations

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 PCB, which, in turn, is a function of the part geometry and surface finish, the solder paste and the surface mounting conditions. All should be taken into account to achieve desired results.

Plating, feeding issues and stencil design also should be understood to optimize 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 the International Electronics Manufacturing Initiative.³On 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 throughhole 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 PCB, 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 serve well.

References

Internal Penn Engineering study.
Internal Penn Engineering study.
International Electronics Manufacturing Initiative, “iNEMI Recommendations on Lead-Free Finishes for Components Used in High-Reliability Products, Version 4,” December 2006.