DFM 101 DFM 101: Final Finishes—HASL

One of the biggest challenges facing PCB designers is not understanding the cost drivers in the PCB manufacturing process. This article is the latest in a series that will discuss these cost drivers (from the PCB manufacturer's perspective) and the design decisions that will impact product reliability.

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Final Finishes

Final finishes provide a surface for the component assembler to solder, wire bond, or conductively attach a component pad or lead to a pad, hole, or area of a PCB. The other use for a final finish is to provide a known contact resistance and life cycle for connectors, keys, or switches. The primary purpose of a final finish is to create electrical and thermal continuity with a surface of the PCB.

HASL (Hot Air Solder Leveling)

The next finish to discuss in this series is HASL. Choosing a surface finish means weighing the pros and cons associated with each, typically a combination of application, cost, and the properties of the finish. For example, Lead-free HASL is considered &#;RoHS&#; compliant (< 0.1% BW of finish, for Pb, Hg or Cd), while SnPb (tin/lead) HASL is not.

HASL is available in two versions: a tin/lead alloy and a lead-free option. HASL is a variable thickness metal alloy coating, typically 25 to µ in [0.65 to 50 µm] thick, designed to create a solderable surface and to maintain solderability over an extended period. Using specialized equipment, panels with exposed copper (pads and/or traces) are inserted into a vat of molten solder. A series of compressed hot-air knives then remove excess solder and &#;level&#; the surface before the solder solidifies and cools. HASL is the most prevalent and most solderable finish. Boards that have been HASL leveled will have a bright, silvery pad coloration. Solder thickness and uniformity on SMD pads is a function of pad size, pad orientation during processing, and HASL equipment. As pad size increases, thickness and uniformity decrease. This is caused by the high surface tension of solder and the airflow over pads that remove excess solder. Small pads (0.025" pitch SMD) will have a more uniform solder coating than large pads (0.050" x 0.050"). Large pads may be partially covered with a mound of solder while the rest of the pad is thin eutectic solder. Small pads may have the same condition depending on pad orientation during processing.

 

 

Pad orientation also contributes to solder thickness and uniformity. Fine-pitch SMD pads perpendicular to the direction of processing will have a uniform solder coating covering the entire pad. Fine-pitch SMD pads that are parallel to the direction of processing will have the leading half reduced to the thin eutectic solder with a mound of solder on the trailing half (eutectic solder is the optimum metal alloy to maximize solderability). Angled processing exposes both pad directions to similar air blasts resulting in more consistent pad thickness for similar pad sizes. 45-degree angle processing minimizes the pad orientation difference. HASL is not used for fine pitch components. This is because it increases fabrication and assembly rework for boards that have fine-pitch SMT devices with lead centers below 0.020" and presents coplanarity issues.

Pros and Cons of HASL

Pros

 

 

 

 

 

 

 

 

 

 

 

• Economical and widely available
• Excellent solderability
• Can be reworked at the fabricator
• Presents a very flat solder deposit

 

Cons

 

 

 

 

 

• Limited assembly reflow cycles, as the HASL process introduces a thermal shock before it gets to assembly

 

If you want to learn more, please visit our website lead-free hasl.

 

 

 

 

 

 

 

 

 

• Not suited for fine pitch (due to uneven surface height) or high aspect through-holes
• Standard SnPb HASL is not RoHS compliant
• Lead-free HASL requires different assembly soldering parameters

 

Understanding the cost drivers in PCB fabrication and early engagement between the designer and the fabricator are crucial elements that lead to cost-effective design success. Following your fabricator&#;s DFM guidelines is the first place to start.

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Is HASL a Good Choice for Surface Finish?

We are using a lead-free hot air solder leveling board finish. Our boards have bottom-terminated components. Is this the best choice? The Assembly Brothers, Jim Hall and Phil Zarrow, discuss this scenario and share their own suggestions and recommendations.
Board Talk Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes Phil Zarrow
With over 50 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes. Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript

Phil

Welcome to board talk with Jim Hall and Phil Zarrow of ITM Consulting, the Assembly Brothers. Jim, what's today's question?  

Jim

It comes to us from D.S. We are using lead free HASL (Hot Air Solder Leveling.) Is this the best choice for a surface finish?

We have several bottom-terminated components on our board and our production is low volume and the PCB's are often stored for a year or more.   

Phil

Once you mention surface mount specifically, bottom terminated components, the answer is no it's not. HASL was originally invented back in the days of through-hole in the mid '70s and it was a welcome change from methodologies we were using before like IR fusing and hydro squeegee and all those other fun methodologies of putting a tin-lead coating on.

But once surface mount came around what we found is unlike through-hole we are concerned about the topography and the smoothness of the board, especially after we started to get finer and finer pitches. Look how much we critique solder paste stud, deposition and solder paste height, how important that is.   

So if you're starting off with a lumpy, bumpy finish you're going to have problems. And that was even before the advent of bottom terminated components, well before they became so popular in area arrays. So it becomes even more critical. 

Even with the lead-free versions, some of the mythology out there is that lead-free HASL is smoother than tin-lead HASL and Jim and I have attended enough seminars and propaganda sessions and stuff like that to say no it's not.

If they're still using the vertical methodology for a hot air solder leveling, whether using an SN100 alloy or tin copper, it's basically from what we've seen not going to be any smoother than tin-lead HASL. So HASLis definitely, we don't believe that's the way to go.

What are some of the alternatives? Well there's OSP (Organic Solder Preservative.) Jim, what are some limitations with OSP?

Jim

What jumps right out, shelf life. Nobody recommends that you store OSP coated boards over a year so based upon the data given here in the question, eliminate OSP.

OSP works great for single soldering applications but most people don't like to store to for more than six months or so. It's very inexpensive. It's the least expensive.

We don't know if their cost is an issue here. The other question that people will raise, are there multiple soldering operations?

Does it maintain solderability through multiple heat cycles? Most specifically, if you have a board where you have to reflow both sides and then do a wave soldering or selective soldering operation, can you get reliable hole fill after the barrels of those holes have gone through two reflow cycles.  

Phil

You want the advantages of OSP and it's come a long way in that it is the cheapest finish, but as I said there's a lot of caveats. 

Then we get into some of the immersion finishes, immersion tin, immersion silver. The jury's still out on those and with regard to some of the pros and cons.

There is shelf life concern with immersion silver. Certainly it's a lot more forgiving and it's longer duration than OSP, but it also I would say from assembly standpoint, you better have your act together as far as handling.

You know there's still oxidation issues and there's tarnishing issues.   

Jim

I disagree. I feel that first off immersion tin is not an acceptable finish. It just doesn't solder well. It suffers many of the problems that OSP does with multiple heat cycles.

It's used a lot in Europe but most people in the U.S. don't like it and most people I've heard that have tested it say it doesn't solder well which is surprising for tin. 

I feel that although the limitations that my brother outlined with immersion silver are true, many people feel comfortable for specific applications.

Certainly not if you're going into a high sulfur atmosphere because it is the most prone to creep corrosion of all the surfaces.

But some of the server people and other high rel people use immersion silver and they are comfortable with it so I feel that it is a valid choice depending upon your application.

Phil

I would tend to lean to tried and trusty ENIG (Electroless Nickel Immersion Gold) surfaces. Been around awhile, it's time tested.

Certainly storage concerns are most nominal because oxidation's not really that much of an issue. But again the only caveat with ENIG is make sure you got a good fab shop. We've talked about black pad and some of the other concerns. Make sure they've got the process under control and particularly as it pertains to things like gold thicknesses and nickel thicknesses, all should be in conformance with the IPC specifications. 

So I tend to favor ENIG but again it's your choice. But I think my brother and I definitely concur with regard to HASL. Run away, run away, HASL is not the way to go particularly for surface mount with bottom-terminated components, area arrays and certainly any pitch below 20 mil and below.

Well we thank you for listening to Board Talk and regardless of the surface finish you use, whatever you do don't solder it like my brother.  

Jim

And don't solder like my brother.

Comments

Another point I want to clarify about the use of Immersion Silver that the brothers brought up that I hear often; "it should not be used for circuit board assemblies that may see a high-sulfur environment." To clarify, bare PWBs with IAg finish should not be STORED in a high-sulfur environment because the immersion silver finish will tarnish.

Once soldered, however, there is no issue with using the finished assembly in a high-sulfur environment, because the IAg is not going to tarnish with solder wetted to it.

Richard Stadem, Analog Technologies Corp.

We have been happily using lead-free 50u HASL for 8-layer double- sided, fine-pitch pcbs for over 20 years. Lots of 0.65mm and 0.5mm 480, 672 pin bgas, 0.5mm QFN parts, etc on both sides. We use vapor-phase soldering, one side at a time. We had trouble with ENIG in the beginning since the PRI-side gold seemed to vaporize while we soldered the SEC side. Several pcb manufacturers could not produce a smooth enough HASL surface, but we found a few (the hard way) who can do it reliably.

When we tried electro-tin, it did not seem to provide enough wetting. We produce small series, and appreciate HASLs long shelf-life. Once we re-tinned some boards after they sat (wrapped) for 5 years. They worked perfectly. Tin-on-tin seems the most logical to us.

John Fullemann, Corelatus AB

Not sure why boards are sitting idle for a year or more. That would be where I start. Further, the finer the pitch on components/pads the greater need for a nice flat surface. I have not seen a HASL board that ever looked good enough to me to process fine pitch type boards. Think about what a sloppy job yields, opens/shorts/voiding, endless rework, should have just spent the money on some nice gold. N2 cabinets are nice.

We stored our bare copper in there until ready for processing and we did wire bonding as well as SMT, no issues. Boards sat idle no more than a few hours to overnight. Each process and board types for businesses are unique. I agree with the no one size fits all approach.

Dave Kearns, TinyCircuits

The comments from Richard Kincaid and Richard Stadem on Lead-free HASL align with the book chapter on lead-free board surface finishes from Rick Nichols in the Wiley book "Lead-free Soldering Process Development and Reliability" from last year. In this chapter, it mentions that Lead-free solder levelling is seeing a resurgence due to easier removal of the excess solder compared with tin-lead solder combined with the use of state of the art equipment which improves coplanarity. Typical applications for lead-free HASL include Consumables (Cellphones, Laptops), Industrial and Military/ Aerospace.

Jasbir Bath, Bath Consultancy LLC

I believe tin lead 63/37 is the best, it is only problem is environmental. Tin lead and Lead free HASL process have a thermo-shock cycle. it will help to eliminate the future problem. ENIG, immersion tin or OSP, the boards on those process will see more innerlayer delamination, internal open or short after assembly.

Quang Uong, Coatek Inc.

"Nothing solders like Solder"

Bob Lazzara, Circuit Connect, Inc.

Robin Taylor and Richard Kincaid and many other like them are absolutely correct; the newer HASL machines and processes make both SN100 and SN63 solder finishes much easier and better to use than the older HASL machines. If your PWB house has the right HASL machine and the right HASL product, you can be much better off than ENIG or IAg for certain designs, and especially much better off than Immersion Tin or OSP, which are completely different finishes. Work with your PWB supplier candidates. Review their processes. You may be very surprised. The notion that HASL or SN100 cannot be applied flat enough is definitely no longer true.

Richard Stadem, Analog Technologies Corp.

Jim, your comments on immersion tin are just plain wrong I'm afraid. Its widely used in Asia as well as Europe and solders extremely well. Press fit applications need immersion tin and provided you have a stable process and can eliminate whiskers its a very useful product. Multi solderable for sure at 1.0µm of tin also. The US market is not a great indicator of global trends as, with Europe, its a very small portion of PCB manufacture. There is no "one fits all" approach for final finishes as one other comment alludes to.

Robin Taylor, Atotech Deutschland GmbH

In regards to you talk on best solder finish, I would like to comment on why I disagree with you on Lead Free Hasl. We have a Newer Machine that the air knives are adjustable front and back. We are getting great results, and have converted many customers from Immersion Gold to Lead free Hasl more and more. The Alloy consists of 93% TIN 0.07% Copper O.05% Nickle The changes to the machine has improve from the past. So far we have gone down to 16 mil ,I will be following up with more customers and their results, and get back with you on what I here. We are now tracking and calling customers that give us under 16 mil But I would have to believe they would say something if they had a issue.

Richard Kincaid, K & F Electronics, USA

The saying goes nothing solders like solder so if done correctly HASL lead free or tin lead has the best shelf life otherwise no one would use it (dirty process). There are negatives. Ionic contamination and co-planarity. However 20 mil pitch should be no problem. With ENIG besides cost the possible issue I do not hear a lot is the inter metallic is more brittle than any of the other finishes. All finishes except ENIG the inter metallic is tin copper. So if there is a concern about cracking the solder joint this should be considered.

Glenn Sikorcin, Florida Cirtech, USA

The best finish doesn't exist. It depends on the type of application and from many variables that must be considered during the design of the product. I agree that the HASL will not the future considering the miniaturization of the electronic components and the advent of lead-free on all sectors.

Alessandro Barbieri. AEB SPA, Italia

ENIG is the best finish for a shelf life of up to a year and has a surface flatness that is great for BGA and fine pitch devices. We have been using ENIG finishes in particular due to shelf life and BGA use. Bottom side components are almost guarenteed on our designs, so we have had great experience with ENIG.

Theresa Chartienitz, Doble Engineering, USA

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