ZI-formed lead resistor doesn’t compromise performance

Author : Eugene Davis

26 February 2016

Wirewound resistors with ZI-formed leads
Wirewound resistors with ZI-formed leads

Axial-leaded resistors frequently offer performance characteristics that are either not achievable in surface-mount form or are considerably superior to their surface-mount equivalents.

The downside of axial-lead components is the added complication and cost of circuit board assembly, which requires plated-through holes, more time-consuming component insertion and a wave solder operation. 

This article looks at the benefits of improving the PCB assembly process by employing the ZI-formed lead resistor to replace its through-hole equivalent, thereby easing implementation, reducing cost and increasing throughput.

What is ZI-forming? 

Engineers have been designing with axial-lead components for many years and understand the advantages that axial products such as wirewound resistors provide. For example, compared to carbon or even metal-film, wirewound resistors deliver better accuracy, higher stability (over temperature and age), and can handle higher powers and transient pulses. Wirewound resistors meeting UL approval for safe fusing in mains input protection applications are also well established as axial-lead components. The problem with these axial components is that they have always been either manually fitted by operators or have required the use of sophisticated and expensive lead forming machines, significantly increasing assembly time and cost.

What was needed was a way to pre-form the leads of axial resistors, not for through-hole mounting but to create a surface-mount technology (SMT) style component able to stand in solder paste deposits and be reflowed in a single pass rather than having to have additional solder added or require subsequent wave soldering. The ZI-formed lead (Figure 1), developed by TT Electronics, addresses this need. It removes the lead-forming step from the assembly operation, eliminates plated through holes from the circuit board and streamlines the whole manufacturing process.

ZI-form development

With the new ZI lead form, the development engineers also increased the stand-off between the resistor and the PCB to improve the heat dissipation performance of the resistor and in turn protect the PCB from heat induced damage.

This feature enables other resistor types to benefit from this unique lead forming capability, such as the UL recognised parts where an increased stand-off allows these fusible resistors to fuse without scorching the PCB below. Such UL recognised components are commonly found in white and brown goods to protect the PCBs inside from current surges.

Benefits to assembly process

What is probably most important to the design and process engineers is the benefit of picking the right component for the job and being able to bring the time to produce a single board down to the speed of their pick and place machine and reflow profile. 

With a standard through-hole assembly there has to be a final wave-solder operation, so the process flow would be: 

Print --> Pick & Place --> Reflow --> Component Insertion --> Wave Solder

This process flow doesn’t take into account any of the additional automated optical inspection (AOI) requirements between process steps nor the extra time it takes to insert through-hole components. 

Currently, where through-hole components are still required in a circuit board design, efforts have been made to eliminate the wave solder step from the SMT assembly flow. A commonly used technique is called “Pin-in-Paste”, which employs a technique of printing solder paste onto the pad and into the hole, sometimes even printing beyond the solder pad. All this is done to ensure that there is satisfactory solder fillet formed on the top and bottom side of the PCB. Much has been done in the design of stencil technology to maximise the deposition of solder paste onto the PCB, including multi-level stencils and nano-coatings. With the Pin-in-Paste process the process flow now looks like this:

Print --> Pick & Place --> Component Insertion --> Reflow

With stencil technology getting thinner with smaller apertures, the process of printing sufficient paste deposits becomes more challenging. Some solder suppliers have come up with solder preforms in tape and reel that can be added to the existing solder deposit as part of the pick and place operation. These aim to increase the solder volume in forming a good solder joint. This technology however does not remove the requirement for manual or automated insertion of axial leads into the PCB, which both reduces assembly throughput and the real estate available on the bottom of the PCB for other SMT components.

The ZI-forming of axial-lead components on the other hand was designed to eliminate the need for wave soldering, manual handling, on-line automated lead forming and the Pin-in-Paste process. Aside from the much-simplified assembly flow shown below, the most immediate benefit is that the board no longer requires plated through holes, which saves design time and lowers PCB manufacturing costs:

Print --> Pick & Place --> Reflow

To some the idea of picking and placing a cylindrical part may seem complex, however there are already nozzles designed for picking up MELF (metal electrode leadless face) components that are ideal for this. Unlike MELF components, the ZI-form leads provide stability so that this component can stand up in the solder paste deposits allowing the solder to form a reliable joint on the leads of the component during reflow.

The nature of ZI-forming also delivers a marked improvement in solder joint quality. As a surface mount component where the body of the component doesn’t come close to the PCB, the ZI-form eliminates the need for over-printing, placing additional solder preforms or wave soldering, as well as not having to implement special pad designs to eliminate mid-chip solder beads. Hence what ZI form leads now allow engineers to do is to be able to solder the entire assembly in a single reflow stage, which is a cost saving and streamlines the process. Alongside easier placement and being compatible with the reflow process, ZI-form components can be supplied in tape and reel packaging, which is ideal for integration into existing pick and place manufacturing operations. 

There are many benefits of utilising ZI-form leaded resistors, both from a PCB design and manufacturing standpoint. From a designer’s perspective the reduction or elimination of through-holes in the PCB simplifies the design task and frees space on the reverse of the board for other components. The ability to include UL recognised resistors in ZI-form is a further advantage.

From a manufacturer’s point of view, no through-holes translate to lower PCB costs. More importantly, the benefit of ZI-form components being fully SMT-compatible eliminates the additional component insertion and wave-soldering steps that were previously required for the assembly of circuit boards that include leaded components. ZI-form components also avoid the alternative Pin-in-Paste assembly flow as they can be placed directly on to standard solder pads and still achieve reliable solder joints. ZI-form resistors can be handled with regular vacuum pick and place equipment and are available in tape and reel packaging for ease of use.

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