Table of contents:
- Structure of the connector
- Application: What is the connector used for?
- Checklist: How do you find the right connector?
Video: Checklist: How To Find The Right Connector
2023 Author: Hannah Pearcy | [email protected]. Last modified: 2023-05-24 11:12
Connectors are used whenever two electrical lines have to be connected or disconnected. The function of an electrical connector is defined as follows:
"An electrical connector is an electromechanical device that enables a separation point between two components of an electronic system without causing an unacceptable influence on the performance of the overall system." (According to Robert S. Mroczkowski: Electronic Connector Handbook)
Structure of the connector
A plug connection consists of different components, which are composed as follows:
- Connection technology pin contact
- Isolator pin contact
- Base material pin contact
- Surface pin contact
- Surface of socket contact
- Base material socket contact
- Isolator socket contact
- Connection technology socket contact
- Shielding the connector
- Housing and lock
Those who have the choice are spoiled for choice. This saying also applies when selecting a connector for a special application. However, the selection can be restricted by preselection. For this we want to develop basic criteria, the successive sequence of which makes it possible to make the right decision.
Application: What is the connector used for?
The application is the first criterion that the connector industry also offers as a selection filter on the Internet.
In order to find the right solution to the existing connector problem, the book "Practical Guide to Connectors", from which this article also comes, helps.
Input / output connectors: Input / output connectors (also referred to as I / O) are mostly used to connect cables and wires to a device or assembly. The signals can be forwarded to the permanently mounted pair of pairs inside the device using wires (flange connectors, also wire-to-wire or WtW). The fixed half can also be soldered or soldered directly into the circuit board. This in turn requires mounting the connector from the inside to the outside (rear mount).
Printed circuit board connectors : Printed circuit board connectors for connecting wires within the devices do not have the variety of peculiarities as just described and are also referred to as wire-to-board or WtB connectors. These connectors are usually not shielded and have integrated snap or snap locks. FFC (Flexible Flat Conductor) connectors also belong to this category, whereby here there is no mating connector, but a laminated ribbon cable with flat-pressed solid wires or a flexible foil reinforced on the back with gold-plated, tinned or printed with carbon-conductive lacquers (CCI - Carbon Conductive Ink) gives.
PCB connectors : PCB connectors for connecting two PCBs are also referred to as board-to-board or BtB connectors. These connectors often have integrated shielding for higher data rates. Unshielded multi-row pin and socket strips are also often used in pseudo-coax wiring. Parallel connection of (equally long) contacts sometimes avoids the use of high-current connectors.
Backplane connector: Backplane or backplane connectors are actually a subset of the BtB connectors. However, they have a special position in that these connectors mostly have integrated shielding, pre-centering with catch bolts, polarization, often also position coding, but no locking. There are also high-current modules to match the different systems. All of these connectors are mainly available in press-fit technology.
Mezzanine connectors: Mezzanine connectors have less of a problem with line losses than with the signal integrity of the connector pair due to unmatched impedances. With multi-row pin header connectors, a lot can often be achieved through clever wiring. With mezzanine connectors for higher data rates, particular attention must be paid to the impedance curve, reflection and crosstalk, which is why these connectors are used less often in press-in technology, but rather as SMT versions (two-row or multi-row with BGA-like soldering techniques).
You can look up which other connectors are available in the "Connector Practical Guide".
Checklist: How do you find the right connector?
A checklist can help us to find the optimal connector. We ask ourselves the following 15 questions:
1. Is the application I / O - WtB - BtB - Backplane - Mezzanine? The first criterion to find the right connector is the choice of application. With the preselection, you can reach the corresponding products from the respective manufacturers, taking into account that many connector families cover several of these applications.
2. What is the mechanical connection direction, what is the PCB arrangement? Mating direction and PCB arrangement are often essential for the decision to use a straight (vertical?) Or an angled connector. It is often confusing here that the direction of insertion and the design of the connector often lead to different results. An angled connector on the circuit board is to be viewed differently than an angled cable outlet of the I / O connector.
3. What about the spatial restrictions? Restrictions in the installation scenario often lead to more expensive solutions, to technically (electrically) questionable applications or to restrictions when plugging or unplugging the connector. Unfortunately, the connector is often the last component of the circuit board layout and has to make do with the remaining space.
4. What is the number of signals - mass or differential - what is the data rate? The number of contacts is a selection criterion for the connector, but you have to be clear in advance how many contacts are required for the transmission of the required signals. We differentiate the data rate for the differential signals and have to add the necessary ground contacts to avoid near crosstalk to the actual two contacts for the differential pair. Ground-related signals are slower and usually do not require any ground contacts for signal separation, but the entire reverse current should be conducted through more than one signal ground contact.
5. How many ground contacts do we need for the signals, do you need additional shielding? If you have analyzed the total number of ground contacts, you should again consider any differences in the grounding. The ground return of a differential pair may well be a different ground than the signal ground for ground-related signals.
6. Number of power supplies - amps → ambient temperature? When it comes to power, the first question is the maximum ambient temperature, which defines the nominal current of the individual contact via the derating curve and the stripped wire cross-section. In hybrid arrangements, one often comes to the conclusion that a hybrid connector should be used, which, however, may record bottlenecks in terms of costs and procurement.
The parallel connection of contacts with BtB connectors is absolutely permissible, but you should make sure that the current paths are of identical length. In addition, it is recommended to calculate a reserve of approx. 20% on the temperature-related total current. In the case of WtB connectors with crimp connections, the bridging of the contacts on the cable side must be examined technically. If this is possible, a power reserve of approx. 30% should be calculated.
7. Reverse current of the power supplies? The reverse current for the power supply is often forgotten when determining the minimum number of contacts, but must be taken into account - especially if several different voltage potentials are combined on one reverse current line and this carries a higher current than the individual supply lines.
8. Are there voltages higher than 50 VDC due to touch safety? At voltages above 48 VDC, more stringent regulations apply regarding safety during operation or the maximum permitted nominal voltage of the connector. In addition, it must be taken into account that inductive loads when pulling in operation can cause the contact to be destroyed by an arc from as little as approx. 20 V.
9. What soldering techniques are there on / through the circuit board (SMT – wave soldering – PiP – THR – THT)? When selecting the connector, it must be known which soldering processes are carried out during the manufacture of the circuit board. Smaller SMT connectors on the B side of the circuit board can definitely go through the second soldering process on the A side in a hanging state. If these connectors become too heavy or they have too little soldering area, then it is worth considering whether this connector is not available as a press-in variant and can be re-fitted after the double-sided soldering process.
10. Are we talking about lead-free soldering processes (re ﬂ ow +260 ° C or wave soldering +300 ° C)? Nowadays, lead-free soldering processes are the standard. Thus, all connectors soldered using the SMT process should survive this lead-free soldering process. As already mentioned, some of the data sheets have not been revised. Therefore, it makes sense to adjust the temperature curves of the SMT soldering process of the SMT connectors again.
11. What is the maximum component height allowed for automatic assembly? Connectors are often the largest components in automatic assembly. For this reason, both the contact surface for the vacuum suction cup and the overall height of the component must be checked so that the pick & place machine can correctly place the connector.
12. Are there cross-sectional restrictions or regulations for the wires (solder - crimp - IDC)? Since the struck wire cross-sections have a direct in ﬂ uence on the current carrying capacity of the respective connector contacts, it must be known when selecting which wire cross-sections are used. Increasing the cross-section is often the cheaper solution than choosing the next larger connector. The crimp contacts should be identical on both sides of the lead wires to simplify the manufacturing process. With insulation displacement connections, both sides are usually identical.
13. Is gold or tin surface preferred? Do you need an office or an industrial atmosphere? The decision whether to choose gold or tin or silver as the contact surface is often located in the decision tree above, because many connector families are only available in one or the other contact surface. Of course, gold is always more expensive than silver or tin, but it cannot be replaced if the connector in the field (service connection) has been exposed to the environment for years without being plugged in and then has to function when servicing. However, if the connection is plugged in and you have the choice between gold and tin, the 50:50:50 law applies:
- Does the connector have fewer than 50 contacts? (Mating forces become too great due to the higher normal contact forces with tin.)
- Can the electronics handle end-of-life contact resistance of 50 milliohms?
- Are fewer than 50 mating cycles to be expected over the life of the connector?
If these three conditions are met, there is usually nothing to prevent the use of cheaper tin-plated contact surfaces.
14. Are there any legal restrictions or standards (UL - GWT - VDE - LV)? Of course, devices must comply with the relevant safety regulations. An overview of the most important:
- UL:In the simplest case, UL regulates fire behavior and means the flammability of plastics in devices and components if the fire comes from outside. We all know UL 94 and its classification levels HB (horizontal burning test) as well as V-0, V-1, V-2. Plastic with UL94 V-0 level should be used for connectors. It is even better if the connector is classified as a UL product. Manufacturers typically provide this information on the website (e-number or "yellow card"). For applications in frequency converters or motor drives, UL508 must be followed, i.e. an E number alone is no longer sufficient, but wall thicknesses and creepage distances within the connector must be analyzed relative to the plastics used to avoid leakage currents and failure.
- GWT: The next higher security level for fire behavior is the so-called glow wire test (GWT) according to IEC 60 335-1. This test simulates the burning of a contact within a connector using a glowing wire. The fire behavior of the plastic used in the connector is examined for ignition and subsequent self-extinguishing within a specified time window. GWT is mandatory for all connectors in so-called unsupervised household appliances. This includes large appliances such as washing machines and dishwashers, but also coffee machines and cleaning devices.
- VDE: VDE guidelines apply wherever mains voltage is distributed within the device and the entire device does not have a type test. Then all line voltage-carrying components must have VDE certification marks.
- LV: In automotive engineering there are LV regulations (for LV214 connectors with test speci ﬁ cation LV214-2 and test procedures according to LV214-3) for the German market and SAE /uSCAR-2 speci ﬁ cations for the US market.
Of course, there are additional standards depending on the application, from medical requirements to food suitability to customer speci ﬁ cations, which then have to be confirmed by appropriate product releases.
15. Is there a preferred contact grid (for what reason?) Preferred contact pitches limit or simplify the connector selection. This can be due to the circuit board layout, soldering processes or simply from historical experience.
What is the most critical parameter from the 15 questions above? The above order does not necessarily have to be correct, hence the question of the most critical parameter that must be fulfilled.
If all questions are answered honestly, you are prepared to find the optimal and available product for your application from the estimated four million basic products that the connector industry offers.
This article is an extract from the "Practical Guide to Connectors". Further basic knowledge about connectors and in-depth expert knowledge can be found in the specialist book.
* Ing. Herbert Endres, freelance consultant for questions about connectors under the ConConsult logo.