ZI-formed leads simplify process flows

The focus of the article however is taking a look at the benefits of improving the assembly process of PCB boards by employing the ZI-form resistor to replace its through-hole equivalent, in ease of implementation, reduction of cost and increasing throughput.

What is ZI-forming?

What TT Electronics has done in developing the ZI-form lead is looked at the needs of today’s design and process engineers. Axial products that engineers have been designing in for years, such as the WP-S and RC series resistors, have always been either manually fitted by operators or using more sophisticated and expensive lead forming machines. These all add to process time and therefore cost.

By taking on the task of forming the leads of the resistors, the component goes from requiring plated through holes and time consuming placement to an SMT style component able to stand in solder paste deposits and be reflowed in a single pass rather than have to have additional solder volume added or to be wave soldered.

ZI-form development

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

What this has also allowed is the inclusion of more varieties of resistors to have this unique lead forming capability, such as the UL recognised ULW and EMC series resistors, into the portfolio. An additional advantage with the increased stand-off is to allow the fusible resistors to fuse without scorching the PCB below. These 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 standard through-hole assembly where there would be a wave solder operation, the process flow would be;

This process flow doesn’t take into account additional AOI processes between steps nor the time it takes to insert through-hole components.

Today, through hole components are still used and there have been a number of different steps to eliminate the need of wave solder technology in SMT. Commonly used is a technique called 'pin-in-paste' which employs a technique of printing solder paste onto the pad and into the hole, and also it has been seen to be printing beyond the solder pad, all this is done to ensure that there is satisfactory solder fillet formation on the top and the 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 would now be;

With stencils 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 (such as Solder Fortification or Exactalloy) in tape and reel that they can add to the existing solder deposit to increase the solder volume in forming a good solder joint that fit into the pick and place operation. This technology however has not removed the requirement of manual or automated insertion of axial leads into the PCB, that both reduce throughput and PCB real-estate on the bottom side.

ZI-form for axial leaded components was designed by TT Electronics to eliminate the need for wave soldering, manual handling, on-line automated lead forming machines and the 'pin-in-paste' process. The first change that can be implemented by the design engineers is that there needs to be no through holes for this component, reducing the cost of manufacture of the boards and increasing the available space on the bottom side of the PCB.

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

The next marked improvement is the solder joint quality, with the ZI-form being a surface mount component where the body of the component doesn’t come close to the PCB, this 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. So what the ZI-form leads have now allowed engineers to do is to now be able to solder the entire assembly in a single reflow stage, which is both a cost saving and also streamlines the process. Alongside easier placement and reflow process, the product comes pre-packed in tape and reel, ideal for integration into the existing pick & place process.

Summary

To summarise, there are many benefits of utilising ZI-form leaded resistors in your design, not just from a PCB design point of view, but from a process standpoint. From a design standpoint the advantages are:

• Increasing the available PCB area on the bottom side to allow for more components to be placed;
• Allowing design engineers to include UL recognised resistors as surface mount components; and
• Reduction in PCB manufacturing costs due to the reduction, or even elimination, for the need of through-holes in PCB.

From a process engineer’s standpoint, the ZI-form components have improved the assembly process:

• ZI-form eliminates the need for wave soldering of axial leaded resistors;
• Improves the throughput of the assembly process;
• The ZI-form eliminates the need for pin-in-paste technologies as it can be placed on to standard solder pads;
• Eliminates the defect of insufficient barrel fill from pin-in-paste and wave soldering;
• No need for specially designed pick and place nozzles as it can be placed using standard MELF nozzles; and
• Comes packaged in tape and reel for ease of implementation.