We search for industry news, so you don't need to.
Ask the Experts  
Questions Index  ■  Submit a Question  ■  Experts Panel  ■  Join the Panel

February 2, 2017

Gold Edge Contact Flux Contamination Failures

We received some returned circuit board assemblies that failed after some time in the field. These assemblies were originally processed using no-clean flux. The assemblies had visible contamination on the gold edge contacts and analysis confirmed that there was flux residue on the gold edge contacts.

We sent the assemblies back to the EMS supplier to properly clean them. The EMS supplier claims that the residue on the gold edge contacts alone would not cause functionality problems. What do you say?

L.C.

Experts Comments

I'd say you need to run some tests. If the flux residue is causing interruptions or intermittent contact, then the residue should be cleaned.  

If you and your supplier are working IAW IPC-A-610E, you'll see in section 10.6.1 that the defect for all classes calls out "Note 3: Processes designated 'no-clean' need to comply with end-product cleanliness requirements." While that doesn't see like it's much help, it does give you, the user, the responsibility of defining how clean your "no clean" assembly needs to be. If that hasn't been done already, the supplier has a bit of wiggle room on "How clean is clean?"  

What seems more applicable to your situation is section 10.1 for Gold Surface Contact area. The defect condition states, "Solder, any metal other than gold, or any other contamination in the critical contact area of the gold surface fingers, pins or other contact surfaces such as keyboard contacts."  

Notice, "... or any other contamination in the critical contact area ..." Here's where your tests will come into play. As I stated above, If the flux residue is causing breaks in the contact, then the residue needs to be cleaned.  

Unfortunately the marketing people coined the phrase "No Clean" as a sales gimmick for the more properly named, "Low Solids" or "Low Residue" fluxes. Just because it's called no clean doesn't mean the assembly doesn't have to be cleaned.  

Check out the new IPC-CH-65B for a more in-depth discussion of cleaning "no clean" assemblies.
image
Kris Roberson
Manager of Assembly Technology
IPC
Kris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.
Boards that have been manufactured with "No-Clean" flux may still require cleaning prior to delivery to service. This depends on the environment to which they will be placed. For example, if the "No-Clean" boards are placed into a humid environment without a conformal coating, with temperature variations, and (optimally) a continual low-power current, the conditions are perfect for the dendritic growth to occur.

If the operating environment has these characteristics, then you should be cleaning your "No Clean" boards. Others may tell you different, but you have the proof in your hands. The gold fingers can easily be wiped with a good solvent blend and retested. If the problem still exists, you probably have some shorted areas on the board. If you need more info on remedies, feel free to contact me.
image
Rick Perkins
Chemical Engineer / Owner
Chemical Logic Inc.
Rick Perkins is a chemical engineer with more than 25 years of Materials & Processes experience. He has worked with Honeywell Aerospace in high-reliability manufacturing, as well as with several oil-field manufacturing companies. He also has a good understanding of environmental, health, and safety regulations.
I cannot agree with the EMS supplier. The gold edge contact is a critical interface surface for mating with the connector. No clean flux while not necessarily having a corrosive nature can have an insulating effect which could cause intermittent failure between the contact and the connector. From IPC-A-610E section 10.1 contamination on contacts is a defect.
image
David Bergman
VP International Relations
IPC
David W. Bergman is Vice President of International Relations for IPC. He has worked at IPC for more than 30 years responsible for IPC standardization efforts, education and certification programs. Currently, David is responsible for globalization activities, including IPC's China and India offices and reps in Europe and Russia.
No-clean fluxes come in different classes, the ROL0 or ORL0 is preferred for reliability. This would need to be verified. Functionality may or may not be affected since this is due to many factors, however all no-clean fluxes need to see soldering heat to de-activate completely.

If flux gets into regions which do not see soldering heat, these areas would normally need protection from flux ingress, if there is a potential for corrosion or electrical conductance.

If flux is suspected to be present in excess and has not seen soldering heat, it would be good to first confirm the presence of the flux's weak organic acids. Determine the WOA using ionic testing such as ionic chromatography is a method often used. It should be very low and the region suspected of failure should not exhibit electro-migration issues when examined under a microscope.

If it is determined that flux is the issue for the failure, it would require either the gold fingers to be covered to avoid flux contact or reduce the amount of flux applied to reduce the likelihood of excess flux permeating areas where it is not desired. Also increasing heat contact by either increasing the preheat temps and times or solder wave contact times will help de-activate the flux.
image
Peter Biocca
Senior Market Development Engineer
Kester
Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.
Since the edge connector contacts wipe across the surface they open up new material so they might not cause initial functionality problems but the residue on the surface might very well cause corrosion/long term reliability problems down the road. A lot depends on the reliability requirements and expectations for the part. You need to understand the long term implications, not just the short term effects.
image
Lee Levine
President, Consultant
Process Solutions Consulting Inc.
Lee Levine has been a Process Engineer and Metallurgist in the semiconductor industry for 30 years. He now operates his own company Process Solutions Consulting Inc where he consults on process issues and provides SEM/EDS and metallography services.
It really depends on what the gold pads will be used for when the product is in use.  Most typically, these gold edge pads (or "gold fingers") are used as a press fit connector into another assembly, and it is critical for the gold to make good electrical contact into whatever type of substrate or mother board it is being plugged into. For this reason, most assemblers try to prevent anything at all from contaminating the gold fingers. The prevention of debris onto these gold surfaces will insure reliable contact when they are "plugged in" to another board.

If cleaning is an option at this point, I would suggest pursuing that. Contaminated gold fingers can impact functionality in the field.
image
Brian Smith
General Manager - Electronic Assembly Americas
DEK International
Mr. Smith has been supporting customers in the electronics assembly industry since 1994. His expertise is focused on solder paste printing and reducing soldering defects. He holds a BS in Chemical Engineering and an MBA in Marketing. He has authored several papers in trade magazines and at industry conferences. He is an SMTA Certified Process Engineer.
Find a new EMS supplier. Flux residues on the laminate will cause premature failures and also promote ionic migration, pending signal frequency and premature failure. A response like this, tells you a great deal about the competency and integrity of your assembly house.
image
Rodney Miller
Capital Equipment Operations Manager
Specialty Coating Systems
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.
A number of questions came up as I read about your problem. I will attempt to touch on all of them as well as answer your question. Many, if not all "gold plated edge connectors" consist of fingers or pads on the PWB which slide into a mating connector, which is itself mounted onto a backplane that holds all of the circuit card assemblies (CCAs) in the device or product.

The mating connector is usually either press-fit or soldered to the backplane. I assume you are speaking of this type of design? Or could it be an edge-mounted pin connector, rather than gold-plated PWB pads?

This may be irrelevant, because nearly all connectors have a very small contact area, or pinch point, if you will. Very few, if any, connector designs make contact over the full length of the pads or pins, with the exception of mating pins/socket types.
  1. It is always possible that the target mating connector (the one mounted to the backplane) may be at fault due to flux from rework, or flux from rework/assembly of adjacent components. This could be the possible source of the no-clean flux found on the edge-connector on "your" CCA.
  2. No matter the source or type, any flux or any other FOD that is visible or verified present on the edge-connector or in the mating connector is sufficient to cause a malfunction. This is because the pinch point area is very small. A single fiber or very light residue of no-clean flux can very easily cause an open connection, or a high-resistance connection, leading to electrical malfunction. At room temperature, the no-clean flux residue may be of a "greasy" appearance, but at hot or cold operating temperatures it may be a very hard residue. Even if only slightly sticky, any particle of FOD can stick to it and roll under the pinch-point, causing an electrical malfunction.
  3. There should never be any evidence of flux residues, conformal coating, FOD, or anything else on the connector contact lands or pins. Small solder splatters are allowed as long as they are not in the contact area (connector mating area where the pinch point is located), per IPC standards. Careful masking of edge connectors to prevent contamination is required, and if tape is used it should be a low-residue, ESD-safe tape made just for this purpose, or some type of slip-on gasketing material, perhaps as part of the assembly fixturing. The fixturing in turn must also be cleaned. For pin connectors, some type of shroud should be used to prevent solder splatters or other contamination. These are available in ESD-safe plastics for most connector types.
  4. Sometimes, but very rarely, press-fit mating connectors (in the backplane) can develop an open condition.
  5. Sometimes connector contacts in the pinch-point area can develop opens due to vibration in the device while in operation (especially true of any automotive, machinery, or avionics application) which causes the pinch point to rub back and forth across the gold pad, exposing the basis metal underneath the gold plating (almost always nickel). This is called fretting. The exposed nickel oxidizes very quickly, creating an open or high-resistance contact. In this case, any other FOD or flux can wrongly be blamed. Fretting can sometimes be very difficult to detect, even at 30x magnification. Remember, it only has to be present right at the really small pinch-point location to cause problems. Keep in mind on some types of connectors, a gel is purposely applied to the connectors to lubricate them, to prevent fretting. While it may serve that purpose, it can also attract and hold dirt particles (FOD). 
Or it can be a combination of any or all of the above conditions occurring. Hopefully this and others contributions will provide a list of root causes to investigate and eliminate. But if contamination can be seen and detected, I would start by eliminating that first. Go for "pristine appearance" at final inspection.
Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.
LS this failure scenario is likely a result of several factors. A no clean flux when processed correctly will meet many acceptable standards, Bellcore, IPC, etc. "No Clean" is actually "Low Residue". However now you still have a contaminant residing on a circuit. Is it benign and non-conductive? More than likely yes it is considering the hours of development by the flux and solder paste suppliers, so according to your EMS he is also correct, yes it is perfectly acceptable to leave this low residue on the pcb.

In a perfect world with a clean environment and atmosphere you will likely get many years of successful operation. However, place this circuit out in the field the tables are now turned against you. Salt air along coastal regions, smog and exposure to a variety of other industrial pollutant and fine air particles in our environment can result in a visible contaminant now seen on your pcb's that will result in a failure. This flux residues can act like a sponge and absorb atmospheric contaminants. For many of my customers No Clean flux does not mean no cleaning at all. What are you building? Is the extra cost of cleaning up front justified or do you risk a product recall? The added cost to clean insures in your favor that no field returns will occur. In this specific case there is hope. I have seen field returns in the past cleaned of the offending flux residues and recovered at >90% success rate. If you wish to discuss this further please do not hesitate to contact me.
image
Charlie Pitarys
Technical Expert Sales Support
Kyzen Corporation
Charlie Pitarys has over thirty years of industry experience and has been with KYZEN for twenty-one years. Charlie is a former Marine and a retired Sargent First Class in the Army Reserves. His previous employers include Hollis and Electrovert. Charlie continues to use his expertise on cleaning processes and machine mechanics to help KYZEN customers and partners improve their cleaning operations.
The only way to find out whether flux would cause functionality problems or not later on is to assess the assembly cleanliness level or the electrical performance before it is sent out to the field.

Climatic chamber  test (i.e. Surface insulation resistance test) could be conducted  first to investigate the detrimental effect of any potential contamination ( normally done at 85C and 85 % relative humidity). Contamination reduces the insulation resistance. Here, you could test the resistance of the unclean (no clean) board versus a cleaned board and confirm how cleaning would improve your surface insulation resistance. At the same time climatic chamber test would also predict whether the no-clean board could fail in the field later on if it is not cleaned.  

If the assemblies fail the climatic chamber test, as a next step, the root causes need to be investigated. If suspected contaminant is ionic, then the ROSE test or an ion chromatography test should be conducted. Ion chromatography test is a more detailed oriented test method which would provide the information on the amount of cations, anions and weak organic acids potentially present on the board surface one by one. 

For testing non-ionic (organic, rosin etc.) species, the most common method is Fourier-Transform Infrared (FT-IR) spectroscopy.
image
Umut Tosun
Application Technology Manager
Zestron America
Mr. Tosun has published numerous technical articles. As an active member of the SMTA and IPC organizations, Mr. Tosun has presented a variety of papers and studies on topics such as "Lead-Free Cleaning" and "Climatic Reliability".
Submit a Comment

Your Name


Your Email


Company Name


Country


Comments




Authentication

Please type the number displayed into the box. If you receive an error, you may need to refresh the page and resubmit the information.