Tolerances of Reed Switch Lead Formations
Reed Switches may need to be mounted suitably on various different types of PCBs, both SMT and on through hole types, and at times, even inside custom plastic housings that need to be encapsulated. This requirement means that reed switch leads should be modified suitably. The leads may need to be cropped for PCBs with slots, or SMD formed, or goal-post formed for horizontal mounting, or welded with longer leads for vertical mounting. Cropped versions are also soldered with longer cables.
Cropping or forming the leads of a reed switch removes low reluctance Iron and introduces high reluctance air into the magnetic circuit, much the same as adding resistance to an electrical circuit. The result is that a greater number of equivalent Operate AT is required to actuate a reed switch with cropped or formed leads. Since the Operate AT is altered, it is evident that this increases the release AT as well. This would mean that the actuating distance will decrease. More information and videos on this phenomenon can be found on our Reed Switch FAQ page.
When forming the leads of reed switches and reed sensors, this usually requires cropping, bending and probably flattening if required. These collective operations require a lot more force to be thrust on the leads than simple cropping of leads. Extreme caution should be taken to ensure that shock is not transferred to the delicate glass to metal seals, as this could damage the hermetic integrity and cause the reed blade gap to change.
As PCBs and mounting spaces are getting smaller and smaller, it is important that reed switch also shrink in size and have the lead formation tolerances tight to ensure requirements in mounting are met. This is especially true when assembling large volumes of reed reed switches or reed sensors. We have listed tolerances that can be met for the various types of lead formations that we offer.
Cropped
The best method is for the reed switch manufacturer to use specially designed cropping tools where the leads are gripped just outside the glass seals to prevent shock from transferring to the contacts, and the unwanted leads cropped in one swift motion. It is also recommended that clamping of the leads be done by giving a gap from the seals, approximately equal to the glass diameter as this ensures there is less scattering of the AT band after cropping. It is also recommended that such tools only crop only lead at a time to reduce the impact forces of the cropping operation. When requesting for cropped reed switches, the tolerances that can be achieved are shown above.
SMD Formed
SMD and other complex lead formations are always best done by the reed switch manufacturer. The special tools used ensure that the leads are gripped just outside the glass seals, cropped to the required length, and then SMD formed, in one swift motion. It is always better to select reed switches with manufactured flat leads as no extra flattening is required during SMD formation. Any extra flattening done will most certainly add more shock the products, resulting in AT band scattering. When reed switches with pre-flattened leads are used, the SMD formed reed switches have oriented internal blades for maximum in-group sensitivity. When requesting for SMD formed reed switches, the tolerances that can be achieved are shown above.
Goalpost Formed
Goal-post formation for horizontal mounting is required when PCBs use plated through holes for soldering. Special goal-post forming tools ensure that the leads are gripped just outside the glass seals, cropped to the required length, and then formed into a kind of football goal-post, in one swift motion. As is generally the norm, it is always better to select reed switches with manufactured flat leads. This ensures that the blades are oriented to the PCB and collective sensitivity of a batch of goal-post formed reed switches, but this is true only when using magnets where both poles are parallel to the length of the reed switch glass. When a goal-post formed reed switch is being actuated with a magnet, oriented with only one pole towards the reed switch, it does not matter if the blades are not oriented parallel to the goal-post formation. When requesting for goalpost formed reed switches for horizontal mounting, the tolerances that can be achieved are shown above.
L-Formed and Welded
When reed switches need to be used on PCBs with plated through holes, but vertically, some extra operations are required. The reed switches need to be L-formed first, and then welded with a wire to help with vertical mounting. The same care that is required for goal-post formation of reed switches is taken for L-formation. After this process, cropped wire which is not ferro-magnetic is welded to the L-formed reed switch. Special care is to be taken during welding and more information is available here. When requesting for welded reed switches for vertical mounting, the tolerances that can be achieved are shown above.