Tag Archives: surface finishes

Keepin’ it Smooth: How Surface Roughness Impacts High Performance PCBs

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First a disclaimer: I am not an electrical engineer. I am only fluent in my knowledge of PCB fabrication. In other words, much of what I am about to share is borrowed from well- educated and experienced EEs who don’t share my aversion to math. I am, in effect, jumping into the deep end of the pool, but only because I am wearing my (virtual) brain floaties! For this reason, I ask for your forbearance as I attempt to translate this left-brained subject matter into my right-brained mother tongue. Here we go…

Substrate copper application methods. I have been discussing surface finishes for the last couple of posts, and I would be remiss if I didn’t cover the crucial topic of copper surface roughness and how it specifically impacts high performance PCBs. Most substrates are copper clad with either rolled annealed (RA) copper, electrodeposited copper (ED) or reverse treated copper (RT). I have put some links below should you want to learn more about each type of copper and the resulting surface roughness of each.

When the copper surface is rough, even at microscopic levels, the effective conductor length grows and the resistance increased as the signal must move up and down with the topography of the copper surface. To the naked eye, copper-clad substrate appears very smooth but when you view the surface under magnification, the copper can look like something akin to the Himalayas! For this reason, some choose RA copper that is both smooth and consistent in thickness. RA can be more costly, however, and not an option for all. ED copper has the roughest surface but, depending on the application and speed requirements, may be perfectly adequate. RT copper is smoother, and doesn’t cost more than ED. Yet with RT copper you need to be careful about potential delamination and poor peel strength. Once again, we are back to trade offs and specific design needs!

Rogers has a “LoPro” laminate series that is extremely smooth while Taconic offers an extremely smooth “reverse treated/ED” clad laminate. These laminates aid in good, sharp etch definition as well. In some cases, these are smoother than RA copper and can be less costly.

Skin Effect Considerations

  • Michael Ingham of Spectrum Integrity shared the following illustrations with me in regards to the skin effect and how a signal flows through a conductor. In the first illustration, he shows a rough cross-section of how a very high frequency signal tends to run on the outermost areas of a conductor—creating the skin effect. However, he notes that this is only true when there is not a ground plane underneath.
  • In the second and third simulation illustrations he used a full 3D field solver for a top layer trace that has a ground plane beneath.
  • The second illustration shows how a current will flow through the entire trace cross section area at low frequency.
  • Finally, in the last illustration, he shows the current distribution at 20GHz, which is mostly at the bottom and sides of the trace.

Fig 1 Current Flow

Michael’s point is that when it comes to surface finishes and texture, the most critical issue appears to be the smoothness of the copper. He has raised the question that if the high-frequency current mainly flows on the bottom and sides of a trace—is using nickel really causing the unwanted losses, or is copper roughness the culprit? Many fear using ENIG-plated traces and go to great measures to avoid and resort to using costly mixed plating, etc. Below is measured data of a RF trace with standard ENIG plating. The overall loss may be surprising.

Fig 2 Insertion Loss

This is just scratching the surface (no pun intended!) in regards to the topic of conductor losses and copper profile. When dealing with coupled trace structures, the effects of nickel could have a big impact due to having adjacent trace walls interacting. But I will leave this topic for Michael to cover at another time!

Spectrum Integrity has been very successful in high-speed/high performance designs, and has done so focusing more on design technique and paying great attention to all the properties of materials such as the smoothness of the copper rather than focusing on surface finishes or restricting to very low dielectric loss materials. They have enjoyed very good success with avoiding costly mixed plating and with the use of smooth copper laminates like the Taconic RT/ED material.

When it comes to the smoothness of outer layer copper traces, a board fabricator can go a long way to hurt or help your desired results! The higher the speed/frequency and more critical the application, the more you need to be working with a company like Transline Technology who understands high performance board fabrication. For instance, when we clean the outer surface of the boards, we do it with chemical cleaners that are non-abrasive and maintain the smooth copper outer surface. Additionally, there are many points throughout the manufacturing process where standard practices of PCB handling can also compromise the otherwise smooth copper outer surface that can create havoc for your design performance. High performance boards can appear deceptively simple in their design, but there are many intricate details that must be considered when making a sound high performance board. Many a board fabricator does not possess the knowledge or the trained eye for the subtleties of high performance PCB fabrication.

Conclusion. Skilled and well-informed partners are the key to success when it comes to choosing surface finishes and materials for high performance designs. These issues are complex and have many nuances that must be considered to create successful products. As such, it is critical to forge strong working relationships with both your advanced material suppliers and your board suppliers. By doing so, you will save much time and money and avoid a host of needless headaches. I have listed some additional resources below. Many thanks to Michael Ingham of Spectrum Integrity, who is always teaching me something! I highly recommend Spectrum Integrity for RF/MW and high performance design. Their website link can be found below. As always, I welcome your input and comments! [email protected]

I also invite you for lively discussion regarding High Performance board design and fab on Linked In: http://tinyurl.com/85ymddk

–Judy

 

Additional Resources

http://tinyurl.com/7he2lmv

http://tinyurl.com/7wnewcq

http://www.polarinstruments.com/support/si/AP8155.html

http://tinyurl.com/7yblg4m

http://www.spectrumintegrity.com/

http://www.rogerscorp.com/documents/1183/acm/RO4000-LoPro-Laminates.aspx

http://www.taconic-add.com/en–products–material-view.php

Off to the Races: Creating Winning Finishes

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As promised, we are off to the races, untangling the convoluted subject of PCB surface finishes. This is a complicated subject, so bear with me as I try to break it down into bite-sized, manageable portions! This week I will discuss each available surface finish and the pros and cons of each one. Below is my version of SparkNotes for surface finishes:

PCB Surface Finish Comparison Chart

HASL

Tin/Lead

HASL

(Lead-free)

ENIG

Immersion

Silver

Immersion

Tin

ENEPIG

Electrolytic

Nickel-(Hard or Soft)Gold

RoHS Compliant

No

Yes

Yes

Yes

Yes

Yes

Yes

Fabrication Costs

Low

Low

Med

Med

Med

High

High

Shelf Life

1 Year

1 Year

1 Year

9-12 Months

9-12 Months

1 Year

1 Year

Themal Excursions

In Assembly

Multi

Multi

Multi

Multi

Multi

Multi

Multi

Wire Bonding

No

No

Yes/No

Yes/No

No

Yes

Yes

Low Resistance/High Speed

No

No

No

Yes

No

Yes

Yes

Soundness of Solder Joints

Excellent

Good

Good

Excellent

Good

Good

Good

Coplanarity

Poor

Good

Good

Excellent

Excellent

Excellent

Excellent

Wettability

Excellent

Good

Good

Excellent

Good

Good

Good

As I mentioned in my previous blog post, many designers working with high speed digital, RF or Microwave applications are using ENIG, ENEPIG and Electrolytic Hard or Soft gold these days. From the chart above, you can see why; these finishes offer many desired properties. However, if wire bonding is required, the field is narrowed to Immersion Silver, ENEPIG, and Electrolytic gold. Here at Transline we are often asked what the difference is between hard and soft gold: quite simply it is the purity of the gold; the purer the gold, the softer the finish. Among all finishes widely available, silver has the best conductivity.

The Tradeoffs

Many engineers and designers report significant signal loss from the nickel that is plated beneath all gold applications. As the desire for greater speeds increase, many are looking for ways to eliminate, or reduce loss due to undesirable resistivity from plating finishes. In the plating process, gold cannot be plated directly over copper for a couple of reasons. First of all, some copper gets chemically dissolved into the plating tanks, tainting the gold purity and the very costly gold bath. Secondly, on the board surface the copper and gold become diffused and mingled not allowing the gold to plate over the top of the copper. Nickel, therefore, is applied over the copper to act as a barrier to prevent these two undesirable effects. It is for this reason that many are looking to ENEPIG (Electroless Nickel, Electroless Palladium, and Immersion Gold). The Palladium is far less resistive, and due to the “skin effect,” the signal travels to the outermost areas of the circuit—through the low resistive Palladium and Gold. The only downfall for this finish is that it is more costly, and may or may not take more time since most PCB suppliers do not have palladium tanks in their facilities, and rely on outside plating services.

With greater frequency, designers are turning to Immersion silver. The drawbacks to Immersion Silver include the inability for fine wire bonding and oxidation. Oxidation can be avoided or delayed by packaging with sulfur-free packaging materials and storing in temperatures less than 95 degrees Fahrenheit. Some mild oxidation can be easily cleaned away. Any unused boards should be resealed for proper storage.

Conclusion

As you can see, plating finishes offer a series of tradeoffs in cost, performance and function. Each designer needs to carefully consider the requirements that are unique to a given project or application. I hope this information helps to clarify what the general pros and cons are of the various surface finishes available.

Next Installment

Next time, I will dig a bit deeper into this subject and discuss the impact of copper smoothness and “skin effect” and how each may help you make decisions about the surface finish. Thanks to all of you who sent me emails and questions on this subject! Keep the comments and questions coming here, or to my email: [email protected]

Further reading:

www.taconic-add.com/pdf/technicalarticles–effectsofsurfacefinish.pdf

www.ddmconsulting.com/Design_Guides/hasl_alt.pdf

— Judy

Getting the Right Finish, Before You Start

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If you are a fan of John Wooden, the celebrated UCLA coach, you will know that he had more than his share of clever sayings. My husband, who played basketball all his life, and who went to college on a full-ride scholarship—nearly worships Coach Wooden. For this reason, I have heard many of these wise and witty Wooden-isms over the years. One that comes to mind is: “Be quick, but don’t hurry.” With the speed of advancing technologies, we all have to be quick, but we cannot afford to hurry—or be hasty—putting revenue and market share opportunities at stake.

In regards to RF and microwave printed circuit boards, there seems to be some confusion about PCB finishes and their affect on the high performance requirements of these applications. Too often, when considering the available finishes and the potential impact they have on performance, many engineers become both quick and (unwittingly) hasty when they make finish choices based on information, which is conflicted, at best.

I think it’s time to clear up some of these issues, so I am going to spend the next few blog posts talking about these issues. Hopefully, by the time I’m through you will have a much clearer understanding of finishes and which to choose for your product, before you start! I will be drawing from our real-world experiences, as well as looking to experts in the substrate and RF design industries.

Today, I am just going to cover the major available finishes, and which ones seem to be preferred by those with high speed applications:

  • Tin (Lead free)
  • HASL (Hot Air Solder Leveling) Tin/Lead 63/37
  • ENIG (Electroless Nickel Immersion Gold)
  • ENEPIG (Electroless Nickel/Electroless Palladium/Immersion Gold)
  • Hard Gold
  • Soft Gold
  • Immersion Tin
  • Immersion Silver

In high-speed applications, the prevailing wisdom suggests ENIG, ENEPIG, hard gold, soft gold and immersion silver are the best choices. Gold is a natural choice due to the fact that it does not oxidize and that it is wire-bondable. Immersion silver is gaining some traction due to the excellent conductivity, but it oxidizes and it is not wire-bondable, which keeps many from choosing this option.

Unfortunately, I must leave us barely posed in the starting blocks, in regards to finishes! In two weeks, however, I will sound the starting shot, and we will be off to the races. I will discuss each finish in more detail with the pros and cons of each.

If you have specific questions you would like to submit about this subject, please post them here in the comment section or email me at: [email protected].

I’m looking forward to tackling this complex and critical subject together!

 

–Judy

Which Surface is Best?

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RoHS has been in effect since, when, 2006? Pretty close to five years now. It’s been around long enough that there’s even talk of follow-on legislation. All of the PCB fabricators have pretty much figured out how to deal with RoHS. There are Pb-free versions of every PCB finish at this point. But, we still get questions about the best choice of PCB finish.

I don’t think industry has selected one PCB finish as the “standard” preferred choice. A lot depends on the application and the componentry being used. For large parts, HASL, leaded or lead-free, is a good choice. It’s inexpensive and works well. For leaded work, HASL still seems to be the most common finish. We don’t see quite so much Pb -free HASL, though.

If you’re working with small geometry parts, then you really need to go to immersion silver or ENIG. The consistently flat surface of those finishes will help keep the small parts on the pads where they belong. The disadvantage of silver is that it requires a little more care in handling and storage. It can oxidize which will make soldering more difficult. ENIG is more expensive, but it tends to work real well and is easier to store. Fingerprints can be a problem though. We’ve seen the oils from a fingerprint essentially etch the gold surface off. Weird.

OSP becomes a viable choice with high-volume, cost critical applications. It used to be very sensitive to storage and handling, but has gotten a lot better over the last few years. We don’t see immersion tin much at all. It apparently is harsher on the environment to produce than other finishes.

All that makes it more understandable that we don’t have one preferred finish. It seems confusing, but really it’s not that different than any other product. There certainly isn’t just one preferred style of tire for all motor vehicles.

Duane Benson
Gotta have those monster truck tires if you live in Kelso

http://blog.screamingcircuits.com/