Let’s Talk about HAL – For Big Parts Only

The board surface names: HAL and HASL (hot air leveling and hot air surface leveling) refer to the same thing. They are interchangeable terms. With that out of the way, I’ll get to my point, which is that HASL is not the right surface for all applications.

Take a look at the photo on the right. This is a 0.5 mm pitch BGA land, using lead-free HASL. Don’t expect good results with this board. It’s a good quality HASL board. Even the bumps on the pads are not out of line for a HASL PC board. It’s not a defect. It’s the HASL works.

The catch is that, while the PC board is perfectly good, it’s not the correct board surface to use for all parts. HASL is fine for larger parts, but for small components, it’s archaic and not reliable.

BGAs require a flat surface (also called a planar surface). With the bumps common on HASL boards, the BGA won’t have a flat surface. The solder paste won’t adhere evenly to the pads. The BGA will probably slide off the pads before reflow. It may end up far enough off that it can’t self-center, as BGAs usually do.

The HASL pads won’t all have an even amount of solder left on the board. Some pads will have more, some less. When added to the solder paste, the pads with more solder may end up bridging.

All of the issues become even more severe as the parts get smaller. Wafer scale parts, 0.4 mm pitch parts, 0201 passives, and other similarly or smaller sized components are essentially incompatible with the HASL surface.

So, what do you do? Order your boards with immersion silver or ENIG. Both give a nice flat surface that BGAs like.

Duane Benson
Open thse Posd Basy Doors Hasl

http://blog.screamingcircuits.com

Proper PCB Storage — The Top 3 Hazards

It’s late. Do you know where your printed circuit boards are? Let me rephrase that: Can unused PCBs be stored for future use?

Yes, they can – if stored properly. Keep them wrapped up, or sealed in a bag. Anti-static isn’t necessary in this case, but it won’t hurt. Keep them in a cool, dark place. Keep them clean. Do your best to avoid dropping them on the floor and stepping on them.

The board in this photo was left out on a desk for a while, and then shoved into a desk drawer. The environment took its toll on the immersion sliver finish, making it very much unusable.

PCBstorage

What can go wrong:

1. Fingerprints. The oils on your finger can etch fingerprints into ENIG or immersion silver board surfaces. If you plan on committing a crime go ahead and do this so we can catch you. If you aren’t going to start a life of crime be careful to not get your fingerprints on the board surface. Handle on the edges, or at least, don’t touch any exposed metal.

2. Moisture. Moisture is good for your skin but not for your PCBs. Over time, PCBs can absorb moisture, especially in a humid location, or the ocean. If thrown into a reflow oven they then might laminate. Store boards in a dry environment. If stored for a long time, you may want to pre-bake them prior to use.

3. Atmosphere. Sometimes dirty air can contribute to tarnish or corrosion on the exposed land pads. Dust can settle onto the boards as well. Tarnish and dust can usually be cleaned off, but corrosion can’t. Wrap up your boards for long-term storage.

Treat your boards well and you can likely use them at a later date. Don’t treat them well and you may need to replace them, wasting a bunch of money. Often, the damage isn’t as clear as in the above photo, but could still lead to poor solderability.

Duane Benson
Don’t surf on your silver

http://blog.screamingcircuits.com/

Open The Pod Bay Doors, HASL

Does anyone use HASL (hot air surface leveling) anymore? It’s also known as HAL.

Prior to the RoHS days, HASL was probably the most common surface finish. You can get it lead-free, but most boards seem to use immersion silver or ENIG (electroless nickel immersion gold). HASL has traditionally come at a lower cost than those other two finishes, but immersion silver can generally be found at the same price now.

Our friends at Sunstone.com, for example, charge the same for silver and tin/lead HASL. ENIG is still more expensive no matter where you go, though.

One of the chief disadvantages of HASL these days, is the lack of planarity on the surface. (Note the bumps on the BGA land pattern in the image on the right.) With through-hole or large components, an uneven surface doesn’t matter so much. With the increasingly smaller BGAs and QFNs, however, surface irregularities can cause big problems.

Both immersion silver and ENIG have nice flat surfaces. OSP (organic surface preservative) has a pretty flat surface too, but it’s not used much except in high volume consumer goods or specialized applications.

Duane Benson
Oh, the pain! Save me, William.

http://blog.screamingcircuits.com/

Off to the Races: Creating Winning Finishes

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: judy@translinetech.com

Further reading:

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

www.ddmconsulting.com/Design_Guides/hasl_alt.pdf

— Judy

Revelations at ACI

Folks,

I’m taking a few moments from Wassail Weekend, held annually in my village, Woodstock, VT (“The prettiest small town in America”), to write a post about the recent workshops at ACI.

Indium colleague Ed Briggs and I gave a three-hour presentation on “Lead-Free Assembly for High Yields and Reliability.” I think Ed’s analyses of “graping” and the “head-in-pillow” defect are the best around.

There was quite a bit of discussion on the challenges faced by solder paste flux in the new world of lead-free solder paste and miniaturized components (i.e., very small solder paste deposits.) One of the hottest topics was nitrogen and lead-free SMT assembly. There seemed to be uniform agreement that solder paste users should be able to demand that their lead-free solder paste perform well with any PWB pad finish (e.g., OSP, immersion silver, electroless nickel-gold, etc.) without the use of nitrogen. Not only does using nitrogen cost money, but it will usually make tombstoning worse. However, in the opinion of most people, nitrogen is a must for wave soldering and, since it minimizes dross development, it likely pays for itself.

After Ed and I finished, Fred Dimock, of BTU, gave one of the best talks I have ever experienced on reflow soldering. He discussed thermal profiling in detail, including the importance of assuring that thermocouples are not oxidized (when oxidized they lose accuracy). He also discussed a reflow oven design that minimizes temperature overshoot during heating, and undershoot when the heater is off. Understanding these topics is critical with the tight temperature control that many lead-free assemblers face.

Fred Verdi of ACI finished the meeting with an excellent presentation on “Pb-free Electronics for Aerospace and Defense.” Fred’s talk discussed the work that went into the “Manhattan Project.” A free download of the entire project report is available.

There appears to be agreement that acceptable lead-free reliability has been established for consumer products with lifetimes of five years or so, but not for military/aerospace electronics where lifetimes can be up to 40 years and under harsh service conditions. These vast product lifetime and consequences of failure differences are depicted in Fred’s chart (see the pdf link). Commercial products are in quadrant A and military/aerospace products in quadrant D.

One of the greatest risks faced by quadrant D products is tin whiskers. Fred spent quite a bit of time discussing this interesting phenomenon. One of the challenges of this risk is that there is no way to accelerate it, so you can’t do an equivalent test to accelerated thermal cycling or drop shock. Fred mentioned that there have now been verified tin whisker fails, the Toyota accelerator mechanism being one.

In addition to tin whiskers, lead-free reliability for quadrant D products (with a service life of up to 40 years) in thermal cycle and other areas remains a concern.  I mention that tin pest was not on the list of issues for this quadrant.

Fred and the Manhattan Project Team have identified many “gaps” that need to be addressed to determine and mitigate the risk of lead-free assembly for quadrant D products.  They plan to start this approximately $100 million program in 2013.

For those that missed this free workshop, another is planned in about six months.

Cheers,

Dr. Ron

Which Surface is Best?

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/