HASL Surface Finish and Coplanarity

Mr. Zarrow has been involved with PCB assembly for more than thirty years. He is recognized for his expertise in troubleshooting SMT manufacturing and lead-free implementation. He has extensive hands-on experience with set-up and troubleshooting through-hole and SMT processes throughout the world.

Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.

Bob has been in PCB design and fabrication since 1976. He has held elected positions with the SMTA, is a member of the MSD Council, has served as a committee member for various IPC standards and is a Certified IPC Trainer.

Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

Subrat has 10 year of extensive experience in PCB assembly process optimizing for quality, process includes screen printing, wave, reflow. He has a copyright in stencil design published in Apex Expo2010 at Las Vegas US.

Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.

As mentioned above, if you have a 0.6 mm ball, then you are dealing with a rather large pitch BGA (above 0.8mm pitch). With such a 1.0 or 1.27 mm pitch BGA you could actually be OK with even Vertical SnPb HASL, which I have been using down to 0.8mm pitch BGA with good success. Of course, you need to make sure the "other parameters" are tweaked for good results, also as mentioned above (solder paste type, stencil thickness, aperture size, reflow profile, etc).

But the more obvious improvement to make is to ensure the PCB is done with Horizontal SnPb HASL. This process method results in much flatter pads. We have been doing down to 0.8 mm BGAs and 0.4 mm QFPs for many years with excellent yield results, in fact even better than with the previously used ENIG on some PCBAs (and at a much lower PCB cost).

There are many reasons to avoid the ENIG surface, but for Industrial applications it is extremely important to utilize surface finishes that have the much more robust SnCu Intermetallics (vs SnNi IMCs) and are the most long term corrosion immune (such as HASL - SnPb or Pb-free, or ImSn). ENIG fails badly in both respects (plus the cost, black pad, rework and many other issues/challenges).

And if you are needing a RoHS compliant finish, then the Pb-free HASL actually provides you with a further benefit for your fine pitch concerns, as it results in even flatter pads. The Vertical Pb-Free HASL process creates pads that are in the same range of Flatness as the Horizontal SnPb HASL Process, and the Horizontal Pb-free HASL process is even flatter.By the way, the Pb-free HASL processes are also resulting in slightly thicker minimum pad coating dimensions, which can accept/accommodate more IMC growth.

The Vertical Pb-free HASL process is more common presently, but there are some of the large high quality PCB suppliers that have already switched over some of their Horizontal machines to Pb-free HASL (Nihon Superior SN100C being the preferred choice). And they are seeing continued increase in demand for Pb-free HASL (as the old SnPB HASL, of course, decreases in demand and is actually being banned in certain areas of China, such as Dongguan, in the near future).

Either Pb-free HASL type is easily usable down to 0.4mm pitch QFP and 0.8mm BGA (and possibly less for the Horizontal type).Ironically, over the past couple years I have had 2 of the largest and highest quality PCB suppliers (globally) perform in-depth analysis of these various HASL surface thickness and flatness dimensional issues, so my comments above are based on actual extensive factual data collection.

The only thing I would avoid is the use of Pb-free HASL on high complexity/layer count PCBs with Blind-vias, especially stacked blind vias. The increased thermal shock can be an issue with such fragile blind via PCBs. The recommended choice here would be Immersion Tin, as it will provide great flatness (basically equivalent to ENIG), very good long term Industrial market Corrosion resistance, a strong SnCu IMC interface, low cost and it will avoid a high thermal shock in the process.

I would however recommend against using Immersion Tin on very Heavy Copper PCBs, as you will have full via fill challenges for any thru-hole joints (this is where the Pb-free HASL really saves the day). Immersion Tin is also a bit of a tricky process for PCB suppliers to keep under control (although so is ENIG), so make sure you use a high quality PCB shop.

You will also have much better and more consistent results if you have your boards leveled on a horizontal leveler. Acceptable results can be achieved in both lead free and tin lead horizontal leveling.

Here is what I don't understand, I know that HASL is highly uneven but when the PCB undergoes reflow does not the SnPb flow over the pad surface evenly due to surface tension, etc. and coplanarity is maintained?

Is the issue actually that sections of the pad may not have any SnPb on them and therefore the Cu and SnPb will not intermingle?

The big issue with using HASL for this application is can your PCB house provide consistent coverage on your BGA pads. I have seen in past experience where an acceptable level of co-planarity can be achieved only to have the pads on the board oxidize and not be able to produce reliable solder joints. A quality board house, will be able to answer your question.

I have seen the whole co-planarity argument made by industry experts. My main issue, ask them for a paper where boards have been produced to show the effects on end of the line quality. In other words, we built a given number of boards using a number of finishes, and we experienced the following defect rates. At the end of the day, we are all concerned with building quality boards without defects.

The price difference between HASL and ENIG is well documented. My argument has always been, will the cost of going to an alternative finish may erode margin verses defects caused by HASL.

In answering Robert's question, the HASL boards will fail to gasket the stencil to the board surface. This will allow solder paste to escape around the edges of the pads and can lead to solder balls, and in extreme examples solder bridges.

It is very very difficult to get a highly coplanar area for populating a 256-ball BGA component with 0.6mm pitch in a PCB fabricated through HASL process. This is by virtue of the HASL process itself. A slight offset in the planarity can eventually result in the formation of cracks in the device, because of the difference in the stiffness of the solder joint, the associated lead frame and the over molded epoxy. I would suggest to use an ENIG PCB for the application.