The case for replacing AdvancedMC card edge gold pads
Increasing interconnection reliability
If only “gold” always meant “good as gold.” But it doesn’t. As Michael explains in this article, problems exist such as confusion over what constitutes “hard gold,” gold pads that can damage the surface of the connector contacts, chemical gold that falls shy of proper surface thickness, and even gold fragments that break off because they can’t stand up to frequent insertion cycles.
The MicroTCA market is growing as developers create more and more applications based on the PICMG standard. A high level of interest is extending beyond telecom to transportation, industry, and defense. With its compact and scalable form factor, combined with robust mechanics, MicroTCA has the potential to become an accepted standard across a broad spectrum of markets. However, despite the growth in MicroTCA, overall turnover is lagging behind the levels that were predicted five years ago. Examining the reasons for this shortfall reveals reluctance among several users to fully accept the card edge interface of the AdvancedMC modules.
The card edge interface has a long and successful history for example, in PCI and ISA. For a variety of reasons, PICMG defined a two-piece connector for CompactPCI and AdvancedTCA. For the AdvancedMC standard, PICMG again defined the card edge interface. But there are several differences between ISA and AdvancedMC. The most important of these differences is the contact pitch. While the contact pitch of ISA is 2.54 mm, it is only 0.75 mm for AdvancedMC. This small pitch, and the resulting small tolerances allowed by the PICMG specification, creates a number of challenges for the PCB manufacturer.
With the introduction of the MicroTCA backplane connector, with the con:card+ quality seal, HARTING increased the contact reliability of the interconnection significantly by implementing additional features like the GuideSpring. But with a card edge connection, the connector manufacturer can only control one half the side of the interconnection. The quality of the mating part, namely the AdvancedMC PCB edge, is only specified in general terms. Therefore to eliminate certain drawbacks of the card edge connection, it has to be replaced by a connector.
HARTING has addressed these drawbacks by developing the AdvancedMC Plug, which replaces the gold pads on the PCB (Figure 1). Rather than a situation in which the PCB inserts directly into the backplane connector, a module connector becomes the mating interface, creating a two-piece connector interface.
The major advantage is that a solid contact with a band plated surface mates with the backplane connector. PICMG specifies 30 µinch hard gold for the card edge pads, but no definitive definition of “hard gold” exists. A selective-plating process is necessary, because the so-called “pre-pads” do not provide an electrical connection. To avoid the complex and relatively expensive selective plating process, manufacturers often use chemical gold with an insufficient surface thickness. As a result, there are significant differences in the durability of the gold and the surface structure on the modules which are currently available. Market experience shows that PCB manufacturers are unable to guarantee 200 insertion cycles on AdvancedMC modules.
But even if the card edge is manufactured according to the PICMG specification, the user may face other problems. Gold pads, which are produced using a selective-plating process, often have exposed copper underneath the gold/nickel finish (Figures 2 and 3). Frequent insertion cycles and harsh industrial environments can easily lead to corrosion. Undercutting by the etching process may also occur, which in extreme cases can result in gold fragments breaking off during insertion.
The card edge itself can cause wear of the backplane connector. Rough and sharp gold pads can prevent the backplane connector from meeting the required 200 mating cycles. And it’s not just the gold pads that can damage the surface of the connector contacts. The lead-in chamfer of the card edge is a critical issue, too. It is milled from the PCB edge, and often the PCB plastic material contains glass fiber, which results in a very rough PCB surface. With every mating, this rough chamfer slides over the connector contacts and can cause abrasion of the contact surface, which will lead sooner or later to corrosion and the risk of contact interruptions.
One approach to the problems just noted is to use a connector that defines the mating interface. While every PCB card edge is different, the quality of the solid contact with band plated hard gold surface is constant. It is tested in the laboratory for 200 mating cycles, as well as undergoing an industry gas test, vibration, and other technical requirements. The backplane connector undergoes less stress because the contact glides over a smooth injection molded insulated housing, rather than rubbing across rough FR4 on a machined bevel edge. Consequently the connector manufacturer can ensure that a high quality interconnection is achieved on both sides, not only on the backplane.
In addition to the gold pad quality, a critical issue is also the production tolerance of PCBs, especially the width of the card edge and the position of the gold pad field on the PCB. Injection molding tolerances are much tighter than those that can be achieved in PCB production. PCB tolerances are a tenth of a millimeter, whereas injection-molding tolerances are only a few hundredths of a millimeter. These small tolerances enable a very good positional accuracy of the gold pads with the backplane connector contacts, even if the module is inserted into a backplane connector that does not have a GuideSpring.
Furthermore, the special design of the Plug insulator reduces the insertion force of the AdvancedMC module. Depending on the backplane connector, it can be up to 50 percent less. The insertion force is especially a problem for a module that is in the upper range of the thickness tolerance. The specification limits the thickness of AdvancedMC modules, because the backplane connector is only compatible with a thickness range of 1.6 mm ± 10%. However the Plug connector avoids the dependency on this specification, because the Plug defines the tongue thickness. Cards exceeding this thickness range can be used (as long as the mechanical environment of the rack does not cause problems). This is an advantage in situations where a substantial number of signal layers are used, as it enables the mating forces of the module to remain at a low level.
In order to achieve good mechanical stability, pin-in-hole reflow is used to solder the Plug to the PCB (Figure 4). The connector can be assembled to the board from either side. In general, this is the main component side (basic side). Pick and place systems can insert the Plug into the PCB, and it can be soldered in one pass along with the other components. As an alternative, a Plug version that is mounted from the bottom component side (extended side) is available.
By offering different many advantages during the manufacturing process, the deployment of HARTING Plug connectors also offers the potential to realize significant cost benefits. Selective plating of PCB card edges increases the cost of producing gold pads significantly. Tight tolerance specifications can also result in a large number of rejects, which increases either the PCB price or requires a detailed and expensive incoming inspection procedure. The beveled PCB edge is another critical area, because damage can occur to the contact pads. A simple board layout with through-holes is sufficient for the HARTING Plug, and these boards can be produced cheaply and with excellent quality control, thereby reducing the number of rejects. Furthermore the cost of a reject can be high if a defective PCB edge is not detected until the board is populated with expensive components. A HARTING Plug on a module can be replaced easily, reducing scrapping costs.
Improving the signal integrity to achieve high data rates is one reason to specify a card edge connector. PICMG defines the required transmission rate as 12.5 Gbps per differential pair, and the new Plug Connector from HARTING can transmit at this rate.
The Plug was initially developed for the MicroTCA Carrier Hub (abbreviated MCH), which is the MicroTCA management module. Here the PICMG specification recommends the use of a Plug connector for compensation of the mechanical tolerances. To achieve high contact density, it can have up to four tongues. The additional tongues can be stacked to the basic tongue using the stacking pegs and holes of the respective Plug connectors, with metal pins in the stack providing mechanical stability. (See Figure 5.) As a new option, a special connector is available that connects the third PCB directly with the fourth mating tongue.
The Plug connector is compatible with PICMG specifications MTCA.0 R1 and AMC.0 R2, and therefore it can be utilized in MicroTCA and AdvancedTCA applications. The HARTING Plug also ensures that the connection on AdvancedMC modules meets a defined quality level and eliminates the concerns of many end users about the card edge connection. The AdvancedMC module connector offers an alternative to the card edge gold pads in markets beyond telecom and will contribute to the acceptance and further success of xTCA and AdvancedMC modules.
Michael Seele graduated as an electrical engineer in combination with business administration at the University of Applied Sciences in Wilhelmshaven/Germany. He worked for several years in Product Management for connectors and gained experience in the field of telecom/datacom connectors. Since the conception of MicroTCA, he has been involved with the standard from the interconnection technology point of view. As a global product manager for HARTNG, he is responsible for the HARTING MicroTCA and ATCA connector range. He can be reached at michael.seele@HARTING.com