PCB Attachment Options Continue to Grow
By Bob Hult, Bishop & Associates Inc.

Packaging an electronic product to simplify manufacturability, repairability, and modularity has presented a challenge to system designers since the beginning of the electronic age. Early systems often interconnected individual components using hand-soldered, point-to-point discrete wires. This clearly was an expensive, error-prone process.

The solution came in 1936 when Paul Eisler, an Austrian engineer, built a radio around what was to become the printed circuit board. Mounting components on a rigid substrate, and creating electrical interconnects via a pattern of etched conductors, provides a uniform modular system design that offers both mechanical support and electrical interconnect.

Over the years, the method by which components are electrically attached to the PCB has evolved to address reduced centerlines, multilayer boards, automated assembly, field repairability, elimination of lead solder, high-speed performance, assembly cost, and system reliability. Today, design engineers choose between four basic attachment options: through-hole solder, compliant pin, surface mount, and compression.

Soldering a component lead in a plated through-hole (PTH) drilled in the PCB has been the gold standard attachment method for many years. The ability to visually inspect the electrical joint, as well as excellent mechanical attachment, has made this method particularly attractive for larger components, including connectors.

Unlike many solid-state devices, connectors are relatively large and subject to potentially damaging mechanical forces as they are mated and unmated. Through-hole solder joints provide a robust attachment that can solidly anchor the connector to the board.

Connectors can be either hand- or robotically-installed on the board, and then passed over a wave of molten solder, connecting all components on the board in one step. The preheat, fluxing, solder wave, and cleaning operations can be done in a single in-line process that is carefully controlled to insure consistent solder fillets at each connection to the board.

Problem joints, such as poor wetting, cracks, or voids, can be easily detected and repaired.

Another advantage of through-hole soldered components is that they can be repaired in the field by any technician with basic skills and a soldering iron.

Evolving system-packaging technology stimulated a search for alternative attachment processes that could address through-hole limitations in terms of footprint density, potential damage from heat, and high-speed performance.

As system complexity increased, and backplanes become a key interconnect between daughtercards, point-to-point wiring using wire-wrapped posts resulted in a rat’s nest of discrete wires, often hand-applied.




Although this process allowed a relatively simple method to make changes, labor costs became prohibitive, as production volume increased.

The answer came in the form of multilayer printed circuit boards, where interconnects are imbedded in multiple laminated board layers.








Today, backplanes in high-performance computers may be made up of more than 30 internal layers, making them 0.250-inch or more thick, with rows of connectors to support many high-density daughtercards. It becomes very difficult to heat a board of that size to solder reflow temperatures. Avoiding the need for heat also eliminates potential warping and damage to these expensive backplanes. The introduction of compliant pin termination addressed these issues.

Compliant pin or press-fit termination utilizes stored energy in a spring mechanism to establish and maintain a gas-tight connection to the walls of a plated through-hole.

The first attempts involved simply pushing an oversized square pin into a round hole. The corners of the post would wipe the walls at each corner, but the extreme pressure would distort the hole, or rip out the plating, causing serious reliability problems.

The problem was solved with the adoption of a contact structure that was designed to be compliant as the pin was inserted into the board. High-pressure contact areas could wipe surface oxides and contamination to establish gas-tight, redundant points of contact without damaging the hole. Connector manufacturers adapted their own complaint pin designs to typically 0.025²-inch posts, including split pin, bow-tie, and eye-of-needle.
 

Compliant pin termination has been applied to many standard and new connector systems for a variety of reasons. Connectors can be simply pressed to a board individually, or in mass, with automated equipment. Individual pins can be removed and replaced if a contact is damaged, although repair in the field can be more of a challenge. Since heat is not involved in the termination process, very large multilayer backplanes can be fabricated without inducing thermal stress or corrosion from flux residue. Thick midplanes can be built with connectors pressed into plated through-holes from both sides, effectively doubling the number of board slots and reducing signal path lengths. Experience gained over many years in applications that range from computing to automotive have proven long-term reliability in shock and vibration environments. Compliant pins have demonstrated compatibility with lead-free plated through-holes, although there is still some concern about the formation of tin whiskers.

Compliant pin termination is the method of choice for many connector types, but advancing technology has a way of adding new demands that require new solutions. Fine pitch connectors that feature centerlines of 0.5mm and less have no room for large square post tails. Processor sockets may have over 1,000 contacts on 1mm centerlines. It would be impossible to design a routing pattern in such a large grid of through-holes. Plated through-holes have also been recognized as a major source of distortion in multi-gigabit channels. Circuit designers must be careful to minimize stubs to reduce resonance and crosstalk, which restricts their ability to minimize board layer count.

Drilling out the plated barrel below the lowest point of layer contact has become a common practice in high-speed applications, but it adds some cost and the potential for damage.

Reducing the diameter of the PTH is one solution. The industry has seen a consistent reduction in hole diameter, from 1.5mm in the 1970s to the new Micro ACTION PIN (MAP) from Tyco Electronics, which requires a finished hole diameter of only 0.22mm. Drilling smaller holes in thicker boards and insuring that the finished hole is evenly plated adds complexity to the board fabrication process. Aspect ratio is the thickness of the PCB divided by the drill diameter. As ratio begins to exceed 10:1, fabrication costs go up, and the number of qualified suppliers goes down.




Another solution is to eliminate the press-fit hole and replace it with a surface mount pad with small internal vias for interconnection between board layers.

Small pitch semiconductors have relied on surface mount attachment for many years, as do many other leadless components.

 

Rather than inserting connector tails in plated holes, contacts are modified into a series of configurations designed to form a solid solder joint between the surface pad and the contact lead.

A variety of contact styles have been developed, including J-leads, flat tabs, and butt joints. In production, solder paste is screened on the exposed PCB surface mount pads. Components are installed with their leads dipping into the soft solder paste. The board surface is then exposed to carefully controlled heat that reflows the solder paste between the component lead and the PCB pad.


Very small components are often allowed to “float” onto the proper pad centerlines, while bulkier connectors may rely on separate board retention mechanisms to insure proper alignment, as well as isolate mechanical stress the connector may endure during the mating process.

Many standard through-hole connectors are now being manufactured using high-temperature plastics, which permits paste-in-hole reflow, a hybrid variation of surface mount.

Insuring that the proper amount of solder is screened on the correct locations can be a manufacturing process control challenge. A high-density connector that fails to form adequate solder fillets can result in an expensive field failure.

Several connector manufacturers have addressed the issue of insuring the proper amount and precise location of solder by supplying their connectors with solder mechanically attached to the surface mount tail.

Molex introduced its solder charge technology, where a precise slug of blanked solder is extruded through a hole in the surface mount (SM) foot. A symmetrical SM joint is formed at each junction. The Molex HD Mezz mezzanine connector uses this unique surface mount attachment method.

Samtec has developed a similar process with their solder crimp surface mount contacts, which feature solder pellets riveted to both sides of the surface mount tail.

Connectors that feature exceptionally small centerlines, such as flat flex connectors, make extensive use of surface mount attachment, but except for mezzanine styles, users tend to shy away from using SM with larger bulkier connectors for fear that thermal stress or differential expansion during the reflow process will put stress on the surface mount joints, increasing the potential for cracks and failure. Mechanical forces resulting from the mating/unmating process could also transmit stress to these joints. There are some valid electrical advantages of going to a surface mount connection, but the majority of two-piece backplane connectors remain attached via compliant pins or through-hole solder.

An alternative to the screened solder application is the use of ball grid arrays (BGA). Tiny solder balls are pre-attached to the component lead. When exposed to heat in a reflow conveyer oven, the balls reflow to form a column of solder completing the circuit. Processor chips typically utilize direct BGA attachment to the PCB. Chips that must be removable often plug into a Land Grid Array (LGA) socket that uses BGA termination to the board.







Mezzanine connectors are particularly well suited to BGA array connection, as they typically consist of many rows of high-density contacts, and unmating forces are in the vertical plane. FCI Electronics offers their Meg and Gig-Array connectors with BGA attachment.
 

The Amphenol TCS NexLev connector is another example of a mezzanine interface terminated using the BGA attachment method.

Contacts based on a compression beam to a PCB pad have made inroads where extreme signal density, low profile, and available space are top priorities. Connectors that use this technology typically act as an interposer sandwiched between two parallel boards.


The Neoconix PCBeam wafer can have raised beam contacts on one or both sides of a wafer to provide a very high density/low profile separable contact system. The signal path between the two surface pads can be extremely short, allowing excellent high-speed performance.

These contacts have been integrated into flat flex assemblies for ease of attachment.



 

Compressive connectors also include more exotic interconnect technologies, including plated and oriented balls in a elastomeric substrait, such as the system Pariposer offers.

Plated particle interconnects, C-shaped springs, gold bumps, and the Cinch CIN.APSE plated gold wire button contact are additional examples of compressive interposer component to PCB systems.

Design engineers have an increasing variety of options when selecting the connector PCB termination method. Careful analysis of the specific application requirements, including reliability, manufacturability, and applied cost, can point to the most effective solution.

Bishop & Associates Comments:

1. A variety of factors, including contact density, size of the connector, intended environment, pin count, availability of process equipment, and cost, influence the choice of PCB attachment method.

2. It took nearly 10 years for PCB assemblers to accept the reliability of compliant pin technology.

3. Surface mount termination is gaining acceptance, but concerns remain about applying large connectors using this method.

4. The replacement of lead with pure tin has been remarkably smooth, but concern persists that stress generated by a compliant pin on tin-plated walls may stimulate the growth of tin whiskers.

5. Compression termination of larger connectors has been attempted, but has generally not been successful to date. Insuring that adequate forces are achieved, contact registration, cost of external clamping hardware, and potential warping of the board, are yet to be addressed.

Robert Hult
Director of Product Technology, Bishop & Associates Inc.
Robert Hult has been in the connector industry for more than 36 years. Hult began his career as a sales engineer for Amphenol. He joined AMP in 1972 and served in several management positions through 1996. In 1997, Hult joined Foxconn as group marketing manager for Intel in Chandler, Arizona, U.S. Prior to joining Bishop & Associates, Hult was the regional application engineering manager for Tyco Electronics.

Hult graduated in 1968 from Bradley University with a bachelor of science degree in electronics technology and a minor in business.