Surface mount technology (SMT) has been practiced since the 1950's, but there has been little use of connectors until recently. This is probably due to the technical challenges that had to be overcome, plus the lack of direct real estate savings with surface mount connectors.
Today, however, interconnects are one of the last remaining through-hole components on many boards. Manufacturing efficiencies can be achieved by eliminating mixed technology boards that use both surface mount and through-hole components, and board populations can be increased by using the bottom side for components or interconnect traces.
Additionally, as reflow soldering of through-hole components gained acceptance, micro-centerline interconnect options became more prolific. These micro-centerline connectors were often designed to have a choice of either through-hole or surface mount terminations.
It should be noted that not all connectors promoted as surface mount are truly surface mountable. There are many differences between through-hole and surface mount connectors. This book addresses the major issues and challenges associated with the design, specification and use of surface mount interconnects today.
New four row shrouded surface mount interconnects (MOLC and FOLC Series) have tails terminating on .025" (0,64mm) pitch for very high density applications.
For the highest dual row density applications, connector sets on 1mm x 1mm grid (FTM and CLM Series) achieve mated profiles of only 3,50mm (.137") in height. To replace blade-and-leaf style connectors with a high quality pin-and-socket system, without changing the PCB layout, staggered 1mm (0.39") pitch interconnects (BKS and BKT Series) are available.
Many surface mount headers and socket strips are available on .050" (1,27mm) pitch for board stacking applications. These are available with both shrouded and unshrouded construction.
Dual row shrouded interconnects (TFM and SFM Series) are available with tails on .050" (1,27mm) pitch. Board densities can be further improved with unshrouded headers (FTS and FW Series) and ultra low profile socket strips (CLP Series). They can achieve a low 3,50mm (.137") profile, or up to 11,80mm (.465") in height.
Headers on .050" x .100" (1,27mm x 2,54mm) pitch are also available with surface mount tails (HTMS and HDWM Series) for mating with Samtec surface mount socket strips (RSM Series). Shrouded versions (TML Series) are also available.
A wide selection of both headers and socket strips on a 2mm (.079") grid are available for surface mount applications.
Surface mount headers include vertical designs (TMM and TW Series) for board stacking applications and horizontal designs (MMT Series) for perpendicular board mating. Shrouded post surface mount 2mm headers (LTMM Series) are available for mating with 2mm socket strips. Surface mount shrouded posts (STMM) for mating with 2mm IDC cable assemblies are also available.
Choices for surface mount 2mm socket strips include a low profile interconnect (CLT Series) for perpendicular mating or pass through applications, single and double row vertical socket strips (SMM Series) and horizontal socket strips (MMS Series). Each socket series has a different contact design for meeting different application needs.
Traditional .025" (0,64mm) square headers and socket strips on .100" (2,54mm) pitch are available in a variety of configurations. Headers include vertical and horizontal designs (TSM and HW Series) for board stacking and IDC cable mating applications. Standard and low profile (SSM Series) socket strips are available for perpendicular, horizontal and pass through applications.
Samtec has surface mount sockets for a number of traditional through-hole applications including DIP sockets (ICF Series) PLCC and SOJ sockets (PLCC and SOJ Series) which eliminate the problems of coplanarity across large areas and blind soldering of leads under the array.
Through-hole connectors are usually wave soldered. In this process the board is passed over a wave of liquid solder so that only the metal leads are heated above the solder melting point. Surface mount boards are heated in vapor phase (VP), infrared (IR) or infrared-convection (IC) ovens to reflow screen printed solder paste. Since the entire assembly must be heated above the solder melting point, and held there for sufficient time for the leads and pads to be wet by the solder, surface mount connectors must be able to withstand 230°C for 30 to 60 seconds. In some cases, IR or IC soldering processes can expose the connectors to temperatures as high as 260°C for 10 seconds.
While most surface mount connectors are designed to satisfy these process requirements, the amount of time and temperature should be minimized to only that which is required to wet the connector leads and printed circuit board pads. Extended time above the solder melting point can damage the board and sensitive components and form excessive intermetallic compounds in the solder joint, causing poor solderability and even solder joint failure.
New plastic materials have been developed over the past few years to specifically address the needs of surface mount reflow processing conditions. Liquid Crystal Polymers (LCP), Polyphenylene Sulfides (PPS) and Polycyclohexylene Terephthalates (PCT) have excellent high temperature properties due to their high Heat Deflection Temperature (HDT) and excellent moisture resistance. Additionally, they have excellent dimensional stability, low warpage and can be molded in thin sections to reduce the amount of mass to be heated.
Typical plastic materials used in connectors are shown in the accompanying chart.
[CHART]
Insulator materials such as glass filled polyesters, which are well established for wave soldered through-hole connectors, are generally not desirable for reflow soldering due to their low heat deflection temperature. Additionally, other high HDT plastics have proven unsuccessful in many surface mount applications. Heat deflection temperature can help predict the short term effect of high temperatures, but does not necessarily indicate when permanent distortion or thermal relaxation of molded in stresses might occur in an actual connector. Because of subtle differences in design or molding processes, two connectors molded with the same material could perform differently in the same application. Likewise, with two identical connectors except for the plastic, it is possible for the one with the lower HDT to outperform the one with the higher HDT in a particular application.
When specifying surface mount connectors, it is important that the actual processing times and temperatures be
Insulator bodies should be designed to expose the lead/solder pad interface, when possible, for easy cleaning and inspection of the solder joints and to reduce the effects of IR shadowing. (In IR systems not employing convection, components that are shadowed, or not directly exposed to the IR heat source, are heated more slowly than those receiving direct exposure, thus requiring longer soldering times or temperatures.) Socket and terminal strips are usually gull-winged with the leads out and exposed. Hidden leads, a concern with DIP and PLCC sockets where the leads are normally turned in to minimize board space, are exposed with open body designs.
The need to use different plastic materials for surface mount connectors has required manufacturers to make significant capital expenditures. The shrinkage rates and flow characteristics of most plastics used for surface mount are different than those used for through-hole connectors, requiring completely new tooling. In addition to retooling for surface mount interconnects, some manufacturers have tooled through-hole socket and terminal strips, DIP sockets and PGA sockets with high temperature plastics. These interconnects are useful on mixed technology boards which include both surface mount and through-hole components.
The design and construction of the connector lead has much to do with successful solder joints.
Surface mount components are commonly available with gull-wing, J-style and butt-lead configurations. Gull-wing style leads are the most popular design for interconnects. While it can be argued in some cases that a J-style lead offers a better solder fillet, in other cases they offer no technical advantages with connectors and almost always require more complex and expensive stampings. Butt-leads have less surface area than is desirable for good solder fillets, a particular concern with surface mount interconnects. Some tests have shown pull strengths of butt joints at less than half that of gull-wing and J joints.
Gull-wing style leads are relatively easy to stamp and form on standard and fine pitches, lend themselves to low profile constructions and have some ability to self-align on the pads during soldering. Depending on the insulator design, all styles allow easy cleaning of boards and visual inspection of solder joints.
One of the primary benefits of surface mount interconnects is the board real estate savings that can result from reducing the centerline spacing of the leads. While surface mount strips and DIPs on .100" (2,54mm) pitch might require slightly more space than their through-hole counterparts, 2mm by 2mm (.079" by .079") and .050" by .050" (1,27mm by 1,27mm) patterns have become quite common and offer significant space savings. As surface mount manufacturing processes become more precise, 1mm pitch is clearly on the horizon as the next generation of micro-surface mount interconnects.
Plating finishes, thicknesses and quality are critical in surface mount applications. Since surface mount components are exposed to higher soldering temperatures for longer times, improperly plated leads could become susceptible to excessive intermetallic formation, which could cause solder joint brittleness and solder dewetting.
Solder dewetting can be caused when the solder remains liquid for too long a time and reacts with organics to form gases. These gases can passivate the surface that was initially wetted by the solder. Organics can become trapped in porous plating if the process is not properly controlled.
A dense 50µ" (1,27µm) minimum nickel underplating is desirable to protect the copper-based lead from oxidation and leaching of the copper during the soldering process. The nickel underplating is then overplated with 150µ" to 200µ" (8,81 µm to 5,08 µm) of tin to protect the nickel and improve solderability.
An innovative feature on some interconnects are tails with micro slots in them. The solder penetrates the slot and wets a larger surface area on the lead for higher solder joint strength. It has also been reported that this lead construction can provide a path for organics to escape, reducing the occurrence of voids in the solder, a particular concern with reflow soldering. Also, connectors with slots in their tails tend to adhere to the wet solder paste prior to reflow better than flat leads.
It is more difficult to precisely position connector leads on pads, and to keep them in place until they are soldered, than to insert the leads through holes. The larger the connector lead count, and the smaller the lead pitch, the more difficult the task becomes. Automated pick and place machines can effectively resolve this problem with consistent, precise placement. For small volumes where hand assembly is required, optional alignment pins can help assure the leads are properly positioned on the pads for high quality solder fillets.
Metal clips with flexible barbs can be used to provide additional mechanical strength to the interconnect system.
Locking clips are designed to hold the connector in place prior to soldering and assure proper seating during reflow operations. This feature is especially useful when simultaneously soldering components to both sides of the board or for operations requiring multiple reflow cycles. Locking clips should be designed for minimum insertion force and maximum retention force. Generally accepted specifications are ten pounds maximum insertion force and five pounds minimum retention force per connector.
With smaller connectors, or connectors that are expected to experience a large number of mating cycles, the effect of the cycle stresses on the solder joints can become a concern. Since locking clips are metal, they offer the potential for additional strength by acting as a retention clip when they are soldered in plated through-holes. For particularly severe applications, mechanical fasteners or solderable tabs can be used to secure the connector to the board and reduce stress on the solder joints.
A minimum of .020" (0,50 mm) is usually required, from the bottom of the lead to the bottom of the insulator, to assure adequate room for board cleaning and solder joint inspection. Standoffs are not required on surface mount interconnects with large cross-section tails. The connector leads sit on the board rather than passing through it providing the necessary stand-off.
Leads that are at least 0.020" (0.50mm) in thickness have adequate structural strength to handle normal insertion and withdrawal without distortion. However, many micro sockets have leads that are only .005" (0,13mm) thick. For leads with these small cross sections, standoffs built into the insulator are recommended to absorb the stress of insertion. Additionally, they help protect the solder paste from being displaced off of the pad during placement.
Beyond the factors under the control of the connector manufacturer, one of the most critical variables affecting the quality of the solder joint is the design of the solder pad on the board. Pad layout for specific connector applications depends on a number of variables:
Because there is such a variety of factors that affect solder joint quality, it is important to specify and obtain connectors of the highest quality, designed to meet the stringent needs of surface mount applications, to avoid needless complications in the board manufacturing process.
Terminals are usually Phosphor Bronze, a copper alloy that is much stronger than brass, yet is easily formed for the surface mount feet. It can be drawn or stamped and coined to achieve the very tight tolerances often necessary with surface mount interconnects. The final finish can be specified as gold or tin, or more commonly, with gold selectively plated on the mating surface and tin on the solder tails. The underplating is usually nickel.
Contacts for surface mount sockets are usually available in Beryllium Copper and Phosphor Bronze. In addition to the forming and strength characteristics required for terminal manufacturing, contacts must be able to be stamped in very small dies and provide spring properties to provide a balance between insertion/withdrawal forces and normal forces required for minimum contact resistance.
Beryllium Copper (BeCu) is generally preferred, especially for micro-centerline connectors where the contact beam is very short and normal force must be maximized. BeCu can be heat treated after stamping and forming to optimize its spring properties for a given contact design. For larger connectors with longer contact beams, Phosphor Bronze is usually acceptable.
Again, contacts are nickel underplated, and then gold, tin or selectively plated. Contacts are usually selectively plated gold in the contact area and tin on the tail. Gold is used for high reliability applications, or those where frequent insertion/withdrawal cycles are expected. Tin plating in the contact area is acceptable in some applications where considerable normal forces are available.
All contacts are a compromise between minimizing insertion forces and optimizing withdrawal and normal forces. These properties can best be optimized through the use of multi-finger contacts. Double, triple or even four-finger contacts have now been fabricated using advanced precision stamping, forming and heat treating technology.
The contacts must be designed to achieve an optimal balance between a low withdrawal force and a normal force adequate to assure acceptable contact resistance. The solder tails must be designed to maximize the strength of the solder fillet when attached to solder pads of various size.
The most critical manufacturing tolerance is the coplanarity of the solder tails, which must be precisely controlled to avoid introducing weak solder joints into the system.
Samtec surface mount connectors have been thoroughly tested to assure that the pull force of the connector off the board is higher than the withdrawal forces of mated connectors. For Samtec interconnects, the force required to pull the connector off the board is three to six times greater than the force required to separate a mated connector pair.
Lead coplanarity is a measurement of the distance between the highest and lowest lead when the connector is sitting on a perfectly flat surface. Coplanarity of leads is critical for good solder fillets. Ideally, all leads should lie in the same plane because if even one lead is significantly higher or lower than the plane, it could lead to open solder joints. Most specifications call for coplanarity to be a maximum of 0.004 inch (0,10 mm) to .008 inch (0,20mm) deviation from the seating plane. For practical purposes, this means that the distance between the highest and lowest lead cannot exceed this amount.
Coplanarity is a difficult and time consuming measurement. However, due to the critical nature of this specification, and the difficulty in maintaining it, it is very important that it be controlled through statistical sampling and quality assurance procedures. Laser measurement equipment accurate to within .000001 inch (0,000025mm) is available and can be employed for this purpose.
Lead skew can cause process problems by causing a portion of the lead to land off of the solder pads. A small amount of skew can cause up to 50% of the lead to be off of the pad due to tolerance build up between the connector centerlines and board pad centerlines.
The gull-wing foot on surface mount leads can be out of square due to improper forming, improper assembly into the insulator or damage during handling or shipping. The maximum heel to toe angle is usually specified as 0 to 7 degrees.
With the large number of critical characteristics that can affect the performance of surface mount and micro-centerline interconnects, Statistical Process Control (SPC) is increasingly becoming mandatory in connector manufacturing. Coplanarity, plating thicknesses and a variety of other dimensions critical to proper performance are normally monitored using SPC methods.
Surface mount assembly of even relatively small volumes is almost always automated with pick and place machines. This is not only for speed and efficiency of manufacturing, but because the interconnects must be precisely placed on the pads, with the correct and consistent pressure into the solder paste and without damage to the leads.
Some interconnects, such as PLCC sockets, lend themselves to pick and place assembly because they have large open flat areas for the vacuum pick-up nozzle. Because of the open space between the rows of dual in-line sockets, pick-up pads can be easily added to the connector. However, strip interconnects, especially micro centerline strips, have no available area for the pick-up nozzle. Disposable pick-and-place pads for strips and micro sockets are available for these applications. Even strips on .050" by .050" (1,27mm by 1,27mm) centers have pads large enough to accommodate a 4mm nozzle. The chart shows typical interconnects and recommended nozzle sizes. Unfortunately, most pick-and-place machines today cannot accommodate alignment pins, locking clips or other through-hole components. This may require hand assembly or semi-automated assembly of larger connectors requiring these features.
Bulk packaging of most surface mount connectors is not acceptable due to the fragile nature of the leads and the stringent coplanarity, skew and angle specifications that must be maintained. Additionally, pick-and-place machines require components to be supplied in tubes, trays or on tape and reel.
Tape and reel is becoming the preferred packaging method for surface mount interconnects for a number of reasons. Connectors are placed in individual vacuum formed plastic pockets, covered with tape and rolled onto reels, hence the name "tape and reel." This packaging protects leads and eliminates the possibility of inserting a tube into the feeder backwards, causing polarization problems. Tape and reel also offers higher processing speeds by being able to load more interconnects onto a reel than can be fit into tubes.
Since the pocket for each different connector size must be tooled separately, tape and reel is relatively expensive when compared to the versatility and availability of tubes. Tube technology is older and more common than tape and reel and lends itself better to short runs. Many bench top pick and place machines can not accommodate tape and reel packaging.
Butt Joint. A solder joint where the end of the lead sits on the solder pad.
Body. The insulating part of an interconnect.
Capillary Action. The effect of surface tension that draws a liquid into a small opening.
Contact. The conducting part of an interconnect at the interface between the connector and the lead on the device being connected.
Convection. The transfer of heat by movement of hot air. Often used in conjunction with infrared radiation to reduce the effect of IR shadowing.
Coplanarity. The distance between the lowest and highest lead when the connector is laying in its seating plane.
Dewetting. A situation where a lead or pad was at one point in the soldering process wetted by the solder, but due to extended time or temperature, the presence of intermetallics, volatiles or other causes, has become withdrawn from the wetted surface.
DIP Socket. A connector for a Dual In-Line Package, or one that has its leads in two parallel rows.
Electroplating. A method of electrically depositing metals of very precise compositions and thicknesses onto a base metal.
Eutectic Solder. The most common solder alloy because of its low melting point (183°C), composed of 63% Tin and 37% Lead.
Fillet. A smooth, concave junction where two surfaces meet. The quality of a solder fillet determines the strength of the joint.
Fluorocarbon. The liquid vaporized in Vapor Phase reflow soldering.
Footprint. The pattern on the printed circuit board to which the leads on a surface mount component are mated. Also called a land or a pad.
Gull Wing. A surface mount lead configuration where the leads are normally bent out and are parallel to the bottom of the interconnect and to the board.
Heat Treating. A process that uses precise heating and cooling of metals after stamping and forming in order to optimize internal stresses and spring properties.
IR Shadowing. When connector bodies or other components prevent the infrared energy from directly striking some solder joints, causing non-uniform heating.
IR Reflow. A soldering process that uses infrared (IR) light with a wavelength between visible light and microwave radiation as its energy source.
Intermetallic. Chemical compounds formed between the metals present in the solder, base metal and protective platings. Intermetallic formation is necessary for good solder joints, but excessive intermetallics can cause brittleness.
J-Lead. A surface mount lead configuration where leads are bent into curves. Infrequently used on interconnects.
Land. The metal portion of a printed circuit board where the leads on a surface mount component are mated. Also called a footprint or a pad.
Leaching. The movement of metal atoms from the lead base metal into liquid solder. This is prevented by nickel plating. May also refer to alloying of a gold protective plating into the solder.
Pad. The metal portion of a printed circuit board where the leads on a surface mount component are mated. Also called a footprint or a land.
Pitch. The centerline spacing of the leads on an interconnect.
PLCC Connector. A connector that mates with a Plastic Leaded Chip Carrier.
Printed Circuit Board. An epoxy glass and metal composite on which circuits are etched and to which active, passive and hardware components are attached. Also called a PCB or PC Board.
Plated Through-Hole. A hole through a Printed Circuit Board that has been electropated and into which a lead is placed and soldered for electrical and mechanical connection.
Reflow Soldering. The process of screen printing solder paste and then heating it to cause it to melt, or Òreflow,Ó to wet the leads and pads around it.
Self Alignment. The tendency of leads to center themselves on solder pads due to the surface tension of the liquid solder.
Surface Mount Device (SMD). An active or passive device designed to be soldered to the surface of the printed circuit board.
Surface Mount Technology (SMT). The process of assembling printed circuit boards with components soldered to the surface rather than to plated through-holes.
Solder Paste. A mixture of solder powder, flux, solvents and binder that is screen printed onto the printed circuit board and then reflowed to form the solder joints.
Vapor Phase Soldering. A soldering process that uses the latent heat of vaporization of a liquid as its energy source.
Wave Soldering. The most widely used mass soldering process, primarily for through-hole boards, where the board is passed over a wave of solder which laps against the bottom of the board to wet the metal surfaces to be joined.
Wetting. The ability of liquid solder to attach itself to the surfaces being joined through the formation of intermetallic bonds.
© 1997 SAMTEC, INC.