INTERCONNECT
PROCESSING
HANDBOOK

 


Introduction
Purpose
Lower Installed Cost
SMT Interconnect Placement
Metal Pick-And-Place Pads
No Pad
Remove Pins For Landing Area
Custom Nozzles To Eliminate Pads
Molded Pad
Single Pad On Multiple Connectors
IR Compatible Board Processing
Intrusive Reflow
Metal Washers
Solder Preforms/Stepped Stencil
Mixed Technology Interconnects
Preventative Measures
Registration
Pad On Multple Interconnects
Alignment Of Connectors To Pads
Positive Polarization
Coplanarity
Stamped Leads
Heel Coplanarity
Opens
Squeegees
Flux/Paste Ratio
Stencil Aperture
Bridging
Solder Thieving Pads
Board Layout
Aperture Configuration
Board Warpage
Contact Optimization
Flex Shroud System
End Shrouds
Alignment Pins
Board Guides
Locking Clips
Board Locks
Ejector Shrouds
Troubleshooting
Opens
Bridging
Interconnect Processing Group
Overview

 



INTRODUCTION


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.



LOWER INSTALLED COST


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.

 
REMOVE PINS FOR LANDING AREA

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

Many pick-and-place machine manufacturers offer custom nozzles for intricate or odd form-factor components. These nozzles fit around the top of the terminals and seal off the top of the body. They can be cost effective in medium to high quantities where the cost of the nozzle can be amortized over a large volume of boards.
 
MOLDED PAD

In medium and high volume quantities, the pick up pad (landing area) can be molded into the connector body. This reduces costs by eliminating the price of the pad, and the labor cost of placing it and discarding it after soldering.
 

SINGLE PAD ON MULTIPLE CONNECTORS

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 “paste around hole,” 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.

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PREVENTATIVE MEASURES

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 “positive polarization” 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 “scoop” 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)

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FLEX SHROUD SYSTEMS

 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.

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TROUBLESHOOTING

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