Composition of Z-Axis
Standard wires are gold, gold-plated copper and Z-Alloy but other materials can be incorporated (e.g. Chromel, Alumel, Inconel and polymer fibres) including wire supplied by customers as long as they are continuous.
Wire Size & Pitch
Standard wire is Ø 0.05mm but almost any wire shape of thickness can be made down to Ø 0.01078mm.
The pitch for most connectors is 0.125mm but this can also be customized. Wires can also be grouped in repeated patterns or positioned from a reference point or physical feature.
The standard core material is 50A Shore Durometer silicon rubber with a temperature range of -45ºC to 125ºC. Other materials can be selected for applications outside of the range of this material.
Basic Current Dimensional Limits of Connectors
Minimum and Maximum standard dimensions and tolerances are as follows:
In many cases it will be possible to produce connectors outside of these dimensional limits.
There are three basic dimensions to a connector: Length (Lc), Height (Hc) and Width (Wc).
In order to determine the correct dimensions for a specific application, a number of design guidelines and recommendations need to be examined.
Lc - Connector Length
The Lc is a function of the contact pad layout. In order to ensure that full contact is enabled, a certain amount of shift must be built-in and the rule is to add 0.64mm onto the overall length from the edge of the first to the edge of the last pad. This can be summarized as Lc = (N-1) Pp + 0.64mm where N is the number of contact pads.
Hc - Connector Height
This is the most critical dimension with respect to performance. Deflection in the z axis (or height) is essential to the functioning of an elastomeric connector with too much deflection resulting in the bowing of the PCB and too much deflection resulting in marginal contact which may be unreliable in certain conditions.
To prevent this, the Hc must be chosen carefully and must take into account the parameter of 'board separation' as described in diagram 3. The Z-Axis design team recommendation is to allow for a minimum deflection equal to 5% of the connector height. To calculate this the largest and smallest board separation must be compared including the tolerances. The tolerance of Hc is typically 0.125mm which leads to the following formula for determining optimal Hc:
Hc = ((Board Separation + tolerance)/.95) + Connector height tolerance where the default tolerance is 0.125mm.
The careful calculation of this parameter ensures consistent results even under extreme conditions such as mechanical shock and vibration.
Wc - Connector Width
The width of the connector is governed by other non-design related factors:
Manufacturability: At this time a minimum width of 0.5mm is achievable in large-scale production. This will improve in the future.
Force: Excessive width should be avoided as the silicon incorporated in the strip needs to be displaced when under compression and this needs to be kept to a minimum.
Stability: The ratio of Hc to Wc must not be too high to avoid buckling or folding of the connector when assembled. The housing design will improve stability but the following formula is used to achieve acceptable width values for given Hc values.
Wc = Hc/2.5 (can range from 1 to 5)
The connector housing may be a discrete piece as shown in the ensuing diagrams but ideally the housing geometry should be incorporated into the housing of the end product. The housing of an elastomeric connector performs a number of functions:
Deflection Stop: Flat landings at each end of the housing allow for very accurate control of board separation within the system.
Connector Support: The walls of the housing afford mechanical stability for the connector.
Connector Capture: Ribbing offers an interference fit that holds the connector in place during assembly.
Room for displacement: Space created by the ribbing allocates volume for the accommodation of the deflected silicon during compression/assembly.
Alignment of mating substrates: Pins protruding from both sides of the connector provide accurate registration leading to maximize reliability.
Facilitation of Clamping: Holes in the alignment pins permit the fastening of mating substrates within the same space allocated for the connector.
As for the connector material, one must again define Length (Ls), Height (Hs) and Width (Ws) for the housing slot.
Ls - Slot Length
Normally the slot is designed to be 0.25mm longer than the elastomer in order to allow for the tolerance in connector length.
Ls = Lc + 0.25mm
Hs - Slot Height
The connector housing may be used as a deflection stop for the mating substrates to accurately control deflection within the tolerance of the housing itself (normally 0.5mm or better). Wherever possible this will mean that Hs is equal to the board separation.
Hs = Board Separation = Deflected Height.
Where it is not possible to use the housing as a deflection stop, the Hs must allow for all possible variations in board separation to avoid obstruction during assembly. In such cases, the Hs should be as high as the system will allow providing maximum support for the connector while preventing an unnecessary fulcrum that may fold or pinch the connector between the housing and the mating substrate.
Ws - Slot Width
The Ws slot width is defined as the distance from the tip of the ribbing to the wall opposite. A slight interference of 0.1mm will hold the connector in place while justifying it against the flat wall.
Ws = Wc - 0.1mm
The ribbing inside of the housing slot is a critical factor in the design of the connector and performs a number of essential functions.
The number of Ribs (Nr) = Ls/Rp where Ls is slot length (see above) and Rp is Rib Pitch.
Round the Nr down to the nearest integer for the number of ribs.
Rib location is the location of the first rib from the edge of the slot and has the notation Rl. Rl = (Ls-Rp (Nr-1))/2
The radius of the ribs (Rr) is determined by the width of the connector (Wc). A larger radius creates a larger gap for the elastomer to fill when compressed. A higher Wc therefore requires a larger Rr. The following table contains some recommendations:
As a quick control one should verify that the volume of the slot is greater than the volume of the connector to ensure there is room to expand under compression.
The connector itself requires no alignment to the PCB or mating component. Thanks to the multiple parallel conductors several contacts are formed with the mated pad. Mating substrates should be aligned in order to maximize the number of potential conductors making contact and to eliminate any chance of shortening.
The alignment may be achieved by mechanical means such as alignment pins or by optical means.
Regardless of the alignment technique used the registration of the mating contact pads must be enhanced by adhering to tolerances for the location and diameter of alignment holes, the pad width, the diameter of the alignment pins and the surface of the pad registration as shown in the following diagram.
The contact surfaces should be flat and free of contaminants. If cleaning is required, isopropyl alcohol can be sued but the surfaces must be thoroughly dried prior to assembly.
When designing and assembling the elastomeric connector, one must avoid contact areas being coated in solder mask. The contact area includes in between contact pads and the area around the pads that could interfere with the proper sealing of the connector housing.
In the event that a PCB has been designed and includes solder mask above the level of the contacts, Z-Axis can overcome this by providing a stepped connector as shown.