Comments

HASL flux will easily remove fingerprints from either brushed or micro etched copper prior to HASL, these fluxes are based on Hydrobrominc Acid or Hydrochloric acid and have some pretty strong wetting agents incorporated. Most likely what has happened is the Air Knife has been set too powerfull and removed the HASL Solder coating down the the IMC layer and that then becomes unsolderable with anything even a decent soldering iron has a job to build the solder back onto the pad. The Paste will wet to the component body not the pad.

Gregory York, BLT Circuit Services Ltd

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It could be any number of things, but if it's made overseas, look into the simple things first. What are they using to micro-etch the copper just prior to flux and HASL? Are they controlling the chemistry and the wait time prior to processing? It could be as simple as the operator went on a break right after micro-etch and let the boards oxidize for 30 minutes before they reflowed them. In my experience with off shore suppliers, they don't do a good job of controlling bath chemistry either. Maybe the PH was off in the nitric tank?

Bradley Fern, Entrust

Might be a plating. If the copper was plated, with copper, as is the general case to create the copper via's, then the plating bath may have been depleted of acid. This depletion will allow copper to be deposited but it does not properly bond with the substrate copper and forms a layer which is mechanically weak. Looks good, but quite defective.

The HASL process then bonds properly to the surface layer of copper but, as you note, the components can easily break free. Examine the component which broke free and see if copper is present on the HASL separation point. If so, then it is clear that the copper plating failed. In any event the boards are inherently defective and must be scrapped.

Jaye Waas, Renkus-Heinz

This is not necessarily a HASL issue. This could also be a soldering issue sometimes called secondary reflow. This frequently occurs when reflow soldered surface mount parts are passed over a wave for soldering through hole components. George Wegner performed investigated this problem back in the 90's. The crack initiates when the top side solder joints come close to the solder melting point. The strength of the solder is zero at the melting point and is close to zero as it approaches the melting point. A CTE mismatch between board and component puts enough strain on the intermetallic layer to initiate a crack. Once formed, the crack will propagate. A search of the Technet archives should find more discussion on this point.

Donald Vischulis, Woodward

I agree; this was some type of contamination and could very well have been a fingerprint. In fact, it most likely was.

As far as the comments about a supply chain issue, well, nearly all board fabricators have a defect slip by them from time to time, and this one was certainly in the "undetectable" category. I would not blame a supply-line manager because a single PWB had a single defect. Anyone with any experience performing non-conforming product disposition knows strange single-incidents happen sometimes. While HASL is usually quite reliable, ENIG is also a very good finish with its own design advantages (especially for very high-sped or lossy circuits), but only if plated by an experienced and diligent fabricator. ENIT is also very resistant to corrosion; I suspect someone meant immersion SILVER is susceptible to sulfur products, but not so ENIG. ENIG does require a longer time in reflow because nickel's rate of dissolution into molten solder is 10X longer than that of copper. Nobody said anything about BGAs, so I am not sure what that comment is about. Bare copper was seen per P.T., so not sure why someone would assume a "thick, brittle IMC layer" was the cause.

The IMC layer is a very self-limiting phenomena, primarily completely formed in the first reflow process. After that, there is very little further IMF growth, not even during subsequent soldering, rework, or baking operations.

So the only question still in my mind after all that is: Whatever led P.T. to attempt to pull the affected SOIC off with a tweezers in the first place?

Richard Stadem, General Dynamics Mission Systems

Every board manufacturer tries their best to ensure that after HASL the pad is as flat as possible for the SMT process.
The problem with this is the hot air knife used on the board as it is lifted out of the solder is set far to strong. The result is a flat pad, however they have actually blown all the solder off of the pad and you are basically left with the Cu6Sn5 intermetalic layer. This layer you can not solder onto and create any reliable bond. If you look at the board under a high magnification microscope you will notice that it has a copper sheen in the HASL finish. They might have provided you with a flat pad but they have also provided a finish that is not possible to create a good solder joint on.

From my experience the solder joint is basically adhered to the pad by flux and a tiny amount of fine solder points. This of course offers no mechanical strength or even electrical connection. I have also found that these joints and up as high impedance joints especially with time.

The lesser of the 2 evils is to have a bump on the one side of the pad and reflow that solder with the solder paste, rather than a flat pad that does not take solder.

If you have found 1 SOIC that comes off that easy, the entire batch of boards will be suspect.

Les Watts, Testerion

I concur with the original analysis, but I would first eliminate the following. Did you check to see if other components could be removed with the same method? Did you check the component to see if there was bare copper on the component pads. I would suspect a copper laminate to plated copper adhesion issue IE peeling copper as a possible cause.

Ed Uslar, TriTech, USA

Based on the fact that the user clearly states that the pads were "bare Copper" after lifting the SOIC off, I would have to agree that Jim and Phil are definitely correct in their answer. This is clearly a case of the PCB supplier not getting the HASL to Solder to the Copper in this particular area for whatever reason (most likely local contamination). But what might be the true "root cause" is a Supply Chain Management decision to use a some low cost, low experience and low quality PCB shop? Because if this was a good experienced high quality PCB shop, it's rather embarrassing for them, unless this was an extremely rare event!

The other topic related to this that has come up in this blog thread is the subject of "brittleness of IMC". In reality, the IMC formed with the HASL is the rather strong, non-brittle Tin/Copper type versus the truly brittle Tin/Nickel type IMC formed when using ENIG surface finish! I have first-hand experience with this difference on a PCBA with double-sided reflow and BGAs. This PCBA was subject to bending during installation, and when we were building the prototypes made with ENIG surface finish, 60% of the boards were having BGA joints fail due to the bending forces.

When I then recommended a switch to SnPb HASL (and actually vertical in this case) for production, the bending caused failures went to zero. In fact, I did a bending experiment comparing the PCBA finish types on this PCBA. With the ENIG finish the failures would start occurring at around 1mm of PCBA deflection (on a credit card sized PCBA). Testing of the HASL coated PCBA went to 5mm (in both directions on the same PCBA) without any failures (not that I recommend allowing this amount of bend in an actual product)! Of course this is an extreme level of bending, something like a HALT test approach, but it clearly showed the large improvement with HASL vs ENIG. The more brittle nature of Tin/Ni IMC compared to Tin/Cu IMC is a well know and established in the PCB/PCBA industries.

The incident my former co-worker Fritz describes is one of a rather high level of IMC growth in a finish of any type. A 4um / 158uinch IMC layer on a HASL finished PCBA is very high, but as Fritz indicated, this PCBA had been "over-exposed to multiple heat processes."

Typically a HASL PCB will arrive from the supplier with 10 to 20uinch (0.5um) of IMC, and as long as it is covered with at least a thin layer of non-IMC solder then it will be solderable. And IMC itself is in fact solderable, as long as it is not oxidized (which a solder layer above it prevents for quite some time, as the oxygen absorption rate is pretty slow). And of course the IMC is the most important part of the joint, as it is the actual critical "glue layer" that holds the entire joint together! So, IMC is not inherently "bad", it is rather actually "very good."

After a properly temperature controlled Double-sided SMT Reflow processing, and a Selective-wave Solder process (RoHS or non-RoHS), the normal and expected intermetallic growth should result in IMC thickness levels in the 30 to 60uinch (1.50um) range. Each process step will grow a bit more IMC. But this is also why each Reflow process should be optimized to only expose the PCBA to the minimum level of heat input needed to create all the joints and no more! Temp Moles should be utilized and one should definitely not just simply use one common SMT Reflow Profile, especially if it was developed for ENIG surface finish PCBs (which require more heat energy)!

This is one of the key items to realize, and actually a benefit for all the SMT parts being soldered to your PCB, the fact that the actual Solder-joint / IMC has already been created at the PCB shop during the HASL process! As noted, all the SMT process is actually doing is "reflowing the solder". This noticeably reduces the heat energy input needed when "PCBA level processing" a HASL coated PCB vs a ENIG surface finish PCB, since with ENIG you have to create the Tin/Ni Solder-joint / IMC during the PCBA level processing (and thus all the components are exposed to this extra heat).

So, you will get some IMC growth in the 1st Reflow process, and then you will get some more during the 2nd Reflow process. But if you have controlled your temps correctly, and you are using a HASL or Pb-HASL dedicated Reflow profile, there is no reason you can't achieve fully IPC Class 3 acceptable joints (even on the 2nd side) and with a highly reliable Tin/Cu IMC!

Depending on your thermal masses, pad sizes and paste volume, once in awhile you may see some small areas of "non-wetting/de-wetting" on the outer perimeters of the 2nd Reflow side SMT joint pads due to small areas of IMC finally coming up to the surface in the thinner HASL coating areas. But these are "fully acceptable", even to IPC Class 3, as long as the actual SMT joint meets the IPC requirements for wetting angle and % perimeter, which they seem to always do (my IPC Certified coworkers, our supplier IPC Master trainer, and myself have inspected thousands of such joints and they all were still better then Class 3 requirements).

The topic of these "small areas of 2nd reflow side non-wetted/de-wetted areas" has also been thoroughly and deeply investigated in the past (by some other former co-workers of Fritz and I), as the question was asked: "what if there are also areas of non-wetting/de-wetting under the joint that we can't visually inspect?"

Well, as I said, the investigation was very elaborate and included; hundreds of multiple cross-section slices, 5D-X-ray, and CSAM, thru many joints that exhibited such areas on their pad perimeters, by a large and highly experienced team of experts on PCB/PCBA subjects. The clear conclusion was that it was not a concern. There were no such areas found under the actual solder joints. And when you think about it there are some likely good logical reasons for why this the case, and why the actual solder joint under, and around the perimeter of the Component body/lead is formed before any IMC growth to the surface becomes an issue (body/lead thermal mass, shadowing, latent heat of vaporization of flux, solder paste wicking to the body/lead, etc.)

And once you have created an "IPC acceptable", and actual good solder joint with the HASL coating, even if you have some of these small visible areas of non-wetting/de-wetting on the perimeter of the pads on the 2nd Reflow side, these joints will still also perform acceptably in any subsequent needed rework process. This is because the Solder above the IMC in the actual joint area is now much thicker because of the addition of all the solder from the Solder paste. Rework will be reflowing this solder and any further additional solder paste applied. And in general, and many others have expressed this, reworking of a HASL joint is a much more reliable enterprise then doing so with an ENIG surface finish PCB (and fact some experts even consider ENIG a "non-reworkable" surface finish for their products). There is also the subject of the vastly superior nature of HASL and Pb-free HASL vs ENIG in medium to long term exposure to corrosive industrial G2/G3/GX environments (such as paper mills, tire plants, China, etc) but that is a detailed topic for another day.

The summary is that if you use a high quality experienced PCB shop, control such items as shipping and storage temp, use proper dedicated/optimized HASL/Pb-HASL reflow profiles and understand correct IPC inspection requirements, you shouldn't have problems with excessive IMC growth, and you also shouldn't have problems with areas of no HASL or poor adhesion of HASL due to pre-HASL contamination! As the old saying goes, nothing solders like solder! Although the use of non-RoHS HASL will continue to decline (and drop sharply in mid ), according to my most experienced large high quality PCB suppliers, the use of Pb-free HASL is already significant and continues to grow substantially (in particular the Nihon Superior SN100C which is a very good choice).

Steven R. McLaughlin, ABB, Switzerland

As a board manufacturer I was wondering if this board was made in the USA? For a BGA I find it rare to have a HASL Finish, because of the flatness needed.

Richard Kincaid, K & F ELECTRONICS, USA

Gents, I think you may have gotten this one wrong. I suspect that the culprit is a thick, brittle IMC layer. This can happen when the PWBs are baked for long periods after HASL. Heat cycles at the end user's site such as pre-assembly bakes, reflow, wave soldering, post-cleaning bakes, etc. can make it worse.

I once had a bad case of this on a particular PWB part number, so bad that just flexing the board would cause SOICs to fly off! The user can confirm (or reject) my hypothesis by doing the following:
1. If the land surface is dull gray and will not take solder from an iron immediately after removing one of the SOICs, the land surface is IMC.
2. If IMC on the land surface is confirmed, do a vertical cross section through a land, joint and lead. Measure the IMC thickness. It is probably at or above 4 micrometer.

Fritz Byle, Astronautics

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January 26,

Flex Talk: Final Surface Finish&#;How Do You Choose?

There are so many final surface finish options to choose from today. How do you decide which is best? HASL&#;both tin-lead and lead-free&#;immersion tin, immersion silver, ENIG, OSP, and ENIPIG are the primary finishes used in PCB fabrication. Fabricators and assemblers generally work with the majority of these surface finishes to support their customers&#; requirements. So the question is, with all of these available, how do OEMs select their preferred surface finish?

In the past, the primary function of the surface finish was to protect the copper from oxidation prior to the soldering of components. Today&#;s expectations also include: superior solderability, contact performance, wire bondability, corrosion and thermal resistance, and extended end use life. Designs have changed. Lines and spaces are reduced, solder types and flux chemistries are different due to no-lead requirements, the number of assembly cycles has increased, and the product may need to carry highfrequency signals.

For more information, please visit multilayer pcb design tips.

Additional reading:
What is 2.54 mm pitch connector?
IDC Connectors - Wiring Supplies

Things to think about when selecting a final surface finish: 

• Does the application require tin-lead or lead-free assembly?
• Will the end environment have extreme temperatures or humidity concerns?
• What shelf life is needed? Will it be months or years?
• Volume and throughput
• Does the design have fine-pitch components?
• How many assembly cycles will be required?
• Is this an RF or high-frequency application?
• Will probe-ability be required for testing?
• Is thermal resistance required?

Once the project requirements have been identified, the surface finish options can be reviewed to find the best fit.

HASL&#;Hot Air Solder Leveling

Let&#;s start with HASL. Fifteen or 20 years ago, HASL was the universal go-to surface finish. Today, that is not at all the case. A couple of things greatly influenced this change. The first was RoHS and lead-free requirements. The second is miniaturization and the need for tightpitch components. HASL is blown from the PCB surface to remove excess; this can create uneven coverage, which makes placement of these tight-pitch components difficult at assembly.

This finish is used in aerospace, defense and high-performance electronics as well as lowerend consumer markets.

Things to keep in mind: 

• The oldest surface finish
• Tin-lead and lead-free versions are available
• Tin-lead HASL currently in limited use due to RoHS and WEEE initiatives &#; Currently exempt: industrial vehicles, military, aerospace and defense, high-performance electronics
• Leaded versions are harder to source
• Long shelf life
• Not suited for fine pitch 

OSP&#;Organic Solderability Preservative

OSP is the highest volume surface finish worldwide, with applications spanning data/telecom, automotive and both low-end and high-end consumer products. Older versions of this chemistry were not thermally resistant and were not able to resist more than one reflow cycle. Improvements have been made to allow higher temperatures and multiple reflows without degrading. This finish does well as a selective finish. For example, when ENIG is applied as a surface finish and OSP is used selectively, it will not adhere to or stain any of the gold surfaces, so there is no need to plasma clean.

Things to keep in mind: 

• Highest volume surface finish worldwide
• Applications range from low end to high-frequency server boards; also used in selective finishing
• The latest versions are copper selective and more thermally resistant for high-temp, no-lead applications
• OSP is applied through chemical absorption on the copper surface; there is no metal-to-metal displacement
• Inexpensive surface finish
• Limited shelf life

Immersion Tin

Applications for immersion tin are predominantly in automotive, U.S. military and aerospace. One caution at the assembly level is the fact that pure tin thickness is lost to the copper intermetallic with time and temperature. Loss of pure tin will degrade solder performance. The first reflow exposure will dramatically reduce the pure tin thickness and deposit stress could result in tin whiskers. This is a naturally occurring characteristic of tin in direct contact with copper.

Things to keep in mind: 

• Applications are predominately automotive, U.S. military and aerospace
• Excellent for press-fit applications (i.e., large back panels)
• All contain anti-whiskering additives, but tin whisker elimination is not guaranteed
• Low-cost, flat and suited for fine-pitch use
• Aggressive on soldermask 

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