Zulki’s PCB Nuggets: Consider the Integrity of Wire Bonding

As we said in an earlier column, wire bonding plays a central role in PCB hybrid manufacturing, meaning the merger of traditional SMT manufacturing and microelectronics. Reliability and integrity are inextricably intertwined when it comes to wire bonding—especially for small PCBs, including rigid, flex, and the combination of rigid-flex.

While reliability and integrity can be regarded as synonymous as far as PCB manufacturing with microelectronics assemblies is concerned, the integrity of wire bonding takes on other reliability-associated process qualities. Wire bonding is the methodology of interconnecting the wire to the bond pad. In short, wire-bond integrity is about the quality of wire-bond interconnect that determines how good or how bad it is by verifying the dependability and reliability of it.

Three Main Factors

To create the integrity of wire bonding, three main factors need to be implemented on that PCB hybrid manufacturing floor.

First, when the pads are designed for wire bonding in the layout phase, assurances need to be made that wire length is properly calculated and verified. This is a very important step because when the wire loops are performed, technicians have to ensure wire loops are not overly extended. If they are, it creates the probability of wire breaking when bonding is performed or afterward in the field due to a latent flaw in wire bonding.

Also, stacked wire-bond loops need close attention. This becomes extraordinarily tricky and integrity becomes very challenging because now you are dealing with two to three sets—and in some cases, four sets—of wire-bond loops, each of different lengths. These sets of loops cross-cross over each other, going on top of one set of wire bonds on top the second, and then going on top of a third (Figure 1).

ZulkiKhan_Figure1_12Jun2019.jpgFigure 1: Sets of wire loops crisscross over each other.

This loop set crisscrossing creates a series of wire bonds on top of each other. What results is an angle on which the bond is stitched on the pad. That angle has to be carefully and accurately calculated and verified by process engineering. There shouldn’t be too much of a crisscross. Pad length from the first set of wire bonds to the second and third needs to be very accurately and correctly calculated and verified to create a highly acceptable staggered wire bonding product. Unless those factors are kept at the forefront, wire-bond integrity is severely jeopardized.

Second, the proper PCB or substrate surface finish must be selected at the manufacturing level. When it comes to surface finish, you have to make sure the amount of nickel, copper, and gold is properly specified in the fabrication drawing, depending on the application of the end product.

Based on our experience and success, the surface finish choices that worked best for our projects are immersion gold over nickel or ENIG, ENEPIG, and in some cases, immersion silver. These different surface finishes have different strengths, behaviors, and properties according to their chemical composition under which they operate and behave. This means it’s critical to ensure either ENIG or ENEPIG is used for wire bonding.

As far as the surface finish of the PCB is concerned, ENEPIG is suitable for gold wire bonding, aluminum wire bonding, and contact resistance. Sequential deposition of electroless nickel (120–240 microinches) is followed by 2–10 microinches of electroless palladium with an immersion gold flash of 2–6 microinches on the top. For ENIG, the deposition consists of 2–8 microinches of electroless gold plated over 120–240 microinches of nickel.

Third, how do you make sure the wire bonding to be created maintains high levels of integrity? In some cases, the amount of gold is not enough. Other times, the surface is oxidized, especially if the PCB has an immersion silver surface finish. Also, the board has been sitting in a non-humidified environmental control chamber or ambient room temperature for a while, and the surface finish has been contaminated or oxidized. In cases like these, the PCB has to be cleaned with either argon gas, IP solution, or another solvent. The surface finish has to be clean to create a wire bond strong enough to pass strength and pull tests.

A physical pull test measurement is the final judge as to the integrity of the wire bond. Here are the specific numbers a pull test provides to measure the integrity of wire bonding and pass it with flying colors. For example, if you use 1.0-mil gold wire, you need to have wire bond withhold a 3.0 pre-seal and 2.5 post-seal grams force of strength. In the case of 1.0-mil aluminum wire, you must have 2.5 pre-seal and 1.5 post-seal grams force strength. These are for test class C or D per MIL-STD-883 Rev. G.

Conclusion

So far, I’ve covered the aspects and measurements going into ensuring wire-bond integrity. However, there is yet another factor involved—the human factor. Engineers and technicians doing the programming for those wire bonding stitches and loops must be sufficiently experienced to perform effective and optimal programming of the wire bonder.

Zulki Khan is the president and founder of NexLogic Technologies Inc.

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2019

Zulki’s PCB Nuggets: Consider the Integrity of Wire Bonding

06-12-2019

While reliability and integrity can be regarded as synonymous as far as PCB manufacturing with microelectronics assemblies is concerned, the integrity of wire bonding—the methodology of interconnecting the wire to the bond pad—takes on other reliability-associated process qualities. Here are three factors that need to be implemented to create the integrity of wire bonding.

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Zulki’s PCB Nuggets: Avoid PCB Wire-bond Loop Failures

05-30-2019

Today, hybrid PCB manufacturing is making greater inroads into our industry, which is the marriage of traditional SMT manufacturing together with microelectronics and wire bonding. In many cases, the OEM working with EMS providers doesn’t fully understand the nuances of effective wire bonding and related failures.

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PCB Hybrid Manufacturing: Coming Your Way

05-15-2019

Today, PCB manufacturing is rapidly evolving to include SMT as well as microelectronics, such as chip-on-board (CoB) installation, flip-chip assembly, wire bonding, and die attach. SMT merged with microelectronics is also known as hybrid manufacturing. This is occurring because PCBs have begun shrinking at a faster pace in recent years due to the introduction of wearables, IoT devices, and portables demanding smaller circuit boards.

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2014

Tighter Scrutiny Needed for PCB Cleaning Agents

05-13-2014

PCB cleanliness on the assembly floor is now getting more attention, due to tiny residues and contaminants being left on assemblies after new, advanced assembly processes. Cleaning methodologies, testing, analysis, and special chemistries are being taken to a new level to assure customers of ultraclean boards to avoid costly latent issues.

View Story

Zulki's PCB Nuggets: Tighter Scrutiny Needed for PCB Cleaning Agents

05-13-2014

PCB cleanliness on the assembly floor is now getting more attention, due to tiny residues and contaminants being left on assemblies after new, advanced assembly processes. Cleaning methodologies, testing, analysis, and special chemistries are being taken to a new level to assure customers of ultraclean boards to avoid costly latent issues.

View Story

Uncovering Assembly Problems of High-Speed PCBs

03-12-2014

The high-speed board may be perfect when it comes to BGA assembly. All the balls properly collapse; all the thermal profiles are accurately determined and performed. All soak temperatures, pre-heat, soak, and cool-off periods fall within manufacturer limits and ranges. Yet, this high-speed board fails at high speed at the time of system functional level testing in the system.

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Zulki's PCB Nuggets: Uncovering Assembly Problems of High-Speed PCBs

03-12-2014

The high-speed board may be perfect when it comes to BGA assembly. All the balls properly collapse; all the thermal profiles are accurately determined and performed. All soak temperatures, pre-heat, soak, and cool-off periods fall within manufacturer limits and ranges. Yet, this high-speed board fails at high speed at the time of system functional level testing in the system.

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EMS Discovers Mature IC Technologies

01-14-2014

Columnist Zulki Khan asks, "Did you know that really new, up-to-the-moment PCB technologies are nesting on the doorstep of PCB assemblers?" In fact, he says some of these technologies are very mature, but they're completely new to the assembly side of things.

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Zulki's PCB Nuggets: EMS Discovers Mature IC Technologies

01-14-2014

Columnist Zulki Khan asks, "Did you know that really new, up-to-the-moment PCB technologies are nesting on the doorstep of PCB assemblers?" In fact, he says some of these technologies are very mature, but they're completely new to the assembly side of things.

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2013

Another Look at AOI

11-13-2013

PCB inspection is taking on greater significance as boards and packaging become increasingly smaller, with greater functionality. Automated optical inspection (AOI) and its backup associate, X-ray, team up to catch a variety of board assembly problems. But it's AOI that's at the forefront of this process.

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Zulki's PCB Nuggets: Another Look at AOI

11-13-2013

PCB inspection is taking on greater significance as boards and packaging become increasingly smaller, with greater functionality. Automated optical inspection (AOI) and its backup associate, X-ray, team up to catch a variety of board assembly problems. But it's AOI that's at the forefront of this process.

View Story

Zulki's PCB Nuggets: ECOs Reviewed - The Importance of Accuracy

09-11-2013

Designers can perfectly layout a design and, in theory, follow written specifications to the letter, but when one factors in the practicality of that design, virtually everything associated with it has its limitations--ranging from the material used to make the board to assembly, machine tolerances, and process limitations.

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