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PURPOSE
Even when surface mount and micro pitch processes and materials are optimized according to the best theories and application experience, problems still arise. Following are some common conditions encountered in surface mount and IR compatible board processing and possible solutions. If you have questions or concerns not addressed in this Handbook, please see the last page and contact Samtec’s Interconnect Processing Group. We will be happy to work with you to improve your interconnect processing capabilities.
METAL PICK-AND-PLACE PADS
Many surface mount interconnects are available with optional plastic pick-and-place pads that attach to the metal terminals or plastic body. The smooth surface becomes a landing area for the vacuum nozzle. Once the connector is placed and the reflow process is complete, the pad is removed and discarded.
For a lower installed cost:
Metal pick-and-place pads, usually lower cost than plastic pick-and-place pads, are available on many interconnects.
NO PAD
Samtec has demonstrated the handling of some sockets by auto pickup equipment without any pick-and-place pads, custom nozzles or special machine programming. On some equipment, this can be achieved by programming out the vacuum loss sensor. Depending on connector configuration (longer, heavier connectors may not prove suitable for padless placement) and nozzle size, moderate vacuum loss still allows pick up and placement of the connector without a pick up pad. We recommend sampling the connectors and testing your equipment's capabilities.
If board space allows, two pins can be removed from the center of the terminal to provide a flat space for nozzle attachment. The cost of modifying the connector is less than the cost of the pick-and-place pad on .100" (2,54mm) systems. On smaller pitch systems this option may not be practical.
CUSTOM NOZZLES TO ELIMINATE PADS
For very fine pitches, multiple interconnects can be supplied on a single pick-and-place pad to enhance alignment. These interconnects reduce the number of board placements by allowing two components to be placed simultaneously. The pad minimizes shift and float of the connectors during the IR process allowing for tighter alignment tolerances when mating the PCBs after soldering.
INTRUSIVE REFLOW
When mixed technology boards have connectors with flat leads, paste can often be printed over the holes and the leads will not push out too much solder. Larger square pins can push the solder paste out of the round holes during component insertion and lead to solder contamination on the mating surfaces.
For a lower installed cost:
Intrusive reflow, metal donuts and solder preforms can allow soldering of high density and self-nesting interconnects without secondary soldering operations.With intrusive reflow, or Òpaste around hole,Ó paste is deposited on and outside small annular rings. The capillary action of the molten solder is depended upon to draw it to the lead and plated through-hole.
METAL WASHERS
Metal lock washers may be placed on the leads to provide an area directly above the annular ring that is in direct contact with the solder paste. The solder wets the metal donut and then easily flows to the lead and plated through-hole.
SOLDER PREFORMS
Another method of reducing the number of reflow operations is to apply solder preforms to the leads prior to their placement in the boards. These may be supplied on the connectors by the manufacturer, or purchased from solder suppliers.
STEPPED STENCIL
A modified stencil pattern can correct solder bridging problems. When leads of different widths or lengths are present, the thickness of the stencil can be stepped (layered) to deposit solder on the pads in different volumes.
MIXED TECHNOLOGY INTERCONNECTS
Samtec offers mixed technology interconnect systems with both through-hole and SMT leads. This design provides additional strength for high cycle, high vibration applications. It also facilitates PCB routings.
PAD ON MULTIPLE INTERCONNECTS
For very fine pitches, multiple interconnects can be supplied on a single pick-and-place pad to enhance alignment. These interconnects reduce the number of board placements by allowing two components to be placed simultaneously. The pad minimizes shift and float of the connectors during the IR process allowing for tighter alignment tolerances when mating the PCBs after soldering.
ALIGNMENT OF CONNECTORS TO PADS
Alignment pins located on the bottom of the connector body are available on most SMT interconnects. Alignment pins ensure proper registration, and therefore decrease the chances of misalignment during mating. This condition is of special concern when multiple interconnect sets are used to connect boards.
POSITIVE POLARIZATION
Samtec offers Òpositive polarizationÓ on many interconnect systems. Positive polarization assures proper mating, especially in blind mating applications.
STAMPED LEADS
Some interconnects are now being manufactured with a different stamping process that significantly improves connector lead coplanarity. Samtec has recently introduced both sockets and terminals that have metal leads stamped from flat stock, without any secondary forming operations, to achieve coplanarity tolerances nearly as good as the stamping die itself.
HEEL COPLANARITY
Ideally, all leads should lie on the same plane because even if one lead is significantly higher or lower than the plane, it could lead to an open solder joint. The heel is the critical site to measure coplanarity because a solder joint on the heel of the lead is stronger.
SQUEEGEES
While rubber squeegees perform well under most conditions, metal squeegees can be more reliable than rubber because rubber squeegees can deform and ÒscoopÓ or gouge paste out of the stencil. Metal squeegees may provide more uniform paste thickness than rubber, but can also wear out the stencil faster.
FLUX / PASTE RATIO
High flux contents in the solder paste can lead to opens. In general, solder content in the paste should be 85% to 95% by weight, or 40% to 70% by volume.
STENCIL APERTURE
Stencil aperture edges affect solder paste performance. Smooth aperture edges promote good paste release. Electro-polishing provides smooth, tapered edges to improve solder paste release. A stencil aperture aspect ratio of > 1.5 is desired (aperture width/aperture height > 1.5).
SOLDER THIEVING PADS
For high density through-hole connectors, solder thieving pads can reduce the likelihood of solder bridging. Immediately after the board passes through the wave solder, a debridging knife removes excess solder, leaving a uniform thickness solder fillet on all pins. The excess solder accumulates on the thief pad.
BOARD LAYOUT
In general, parts should be positioned on the PCB perpendicular to the direction of the PCB through the oven. This orientation increases the likelihood of uniform solder fillets and helps in uniform heating of the components.
APERTURE CONFIGURATION
SMT PCBs commonly have a variety of components with different pitches, lead sizes and coplanarity tolerances. A modified stencil pattern can correct solder bridging problems caused by this variety. When leads of different thicknesses are present, the shape of the opening in the stencil can be configured to deposit solder on the pads in different volumes to avoid floating the connector and leaving some pads higher than others.
BOARD WARPAGE
PCB layout can decrease the likelihood of board warpage. For example, the chances for board warpage are greater if the majority of the traces are on one side or section of the PCB. The traces attract heat and warp the board during the reflow process. One possible solution is to place dummy traces on the board to aid in evenly distributing the heat.
CONTACT OPTIMIZATION
Choosing the right contact for the application is crucial. All contacts are a compromise between minimizing insertion forces and optimizing withdrawal and normal forces. With Samtec you can choose the correct contact to optimize performance and cost for each application.
| Contact |
Type | Material |
Design | Availability
|
| Tiger Eye |
High Reliability |
BeCu | 3 or 4 finger |
2mm (.079") .050" (1,27 mm) |
|
Tiger Claw | Cost Effective | Phos Bronze |
Dual Wipe | .100" (2,54 mm) 2mm (.079") .050" (1,27 mm) 1mm (.039") |
| Tiger Buy |
Value | Phos Bronze |
Tuning Fork | .100" (2,54 mm) 2mm (.079") .050" (1,27 mm) |
END SHROUDS
For blind mating and ultra low profile mating applications, Samtec assures proper alignment through optional end shrouds.
ALIGNMENT PINS
Proper alignment of the connector lead to the pads is critical. The larger the connector lead count, and the smaller the lead pitch, the more difficult the task becomes. 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.
BOARD GUIDES
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 for double sided reflow (simultaneously soldering components to both sides of the PCB) or for operations requiring multiple reflow cycles.
LOCKING CLIPS
End shrouds with guide post are available to further enhance board alignment in blind mating and ultra low profile applications.
BOARD LOCKS
For high vibration applications, end shrouds with board locks assure proper mating as well as providing a mechanical locking of the boards together.
EJECTOR SHROUDS
For assuring proper mating of micro IDC assemblies, end shrouds with ejector latches may be specified. These allow removing the IDC from the assembly with maximum protection against stressing the cable assembly.
OPENS
Cause: Insufficient Paste
Solutions:
Cause: Board warpage
Solutions:
BRIDGING IR COMPATIBLE
Cause: High density design
Solutions:
Cause: Boards deep in wave
Solutions:
Cause: Dual wave / turbulence
Solutions:
Cause: Holes too small
Solutions:
BRIDGING SURFACE MOUNT
Cause: Pads too close