Ask the Experts
INDEX
ASK
PANEL
JOIN
COMMENT
June 8, 2021

Oxidation of Solder Balls During Secondary Reflow

Can solder balls oxidize and create opens during a second reflow if the circuit board assemblies were properly cleaned after the first reflow?

M.A,

Expert Panel Responses

So, let's set out a few assumptions on which my answer is predicated:
  • "Properly cleaned" assumes all flux residues were removed, leaving no ionic materials
  • The second reflow takes place without a nitrogen-inerted atmosphere
  • The reflow process is under control for peak temperature and time over liquidus
Under the above assumptions, there will be some oxidation of BGA balls, however this should never cause opens unless something is really out of control. If you are experiencing opens after second reflow that you can show were not there after first reflow, the list of possible causes includes:
  1. Poor initial wetting, with minimal contact that was broken after second reflow
  2. De-wetting of pads on second reflow (this should also be affecting visible SMT pads). This is a less-likely scenario
  3. Vibration in the reflow oven, causing loss of contact (this is really unlikely)
My personal course of action would be to perform careful x-ray analysis of the solder joins after first reflow to identify poor wetting conditions. If this is a fine-pitch component, look at the paste printing process carefully to ensure that you don't have low volume on some pads.

image
Fritz Byle
Process Engineer
Astronautics
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.

Hope I am reading this correctly - if the solder ball flowed properly during the 1st reflow, then no it could not oxidize and cause an open. If the solder ball did not flow properly, then you already have a reject condition.

If you are having a test failure and pressing on the BGA changes the test results, then you most likely have solder balls that did not fully collapse/flow.

image
Tod Cummins
Director of Corporate Quality Assurance
Delta Group Electronics Inc.
Tod has been working in the Aerospace Electronics Industry for 25 years, beginning with 4+ years working for PCB fabricator ending as the Quality Manager and 20 years with Delta Group Electronics Inc. an AS9100 registered electronics contract manufacture. Currently position is Director of Corporate Quality Assurance.

You didn't provide much information along with your question, so I'll have to assume you are referring to the solder balls in the solder paste used during first-side processing.

Those solder balls should have already been formed into solder joints during first-side reflow, so external oxidation should then be irrelevant for second-side processing.

image
Rick Kompelien
Principal Product Engineer
Benchmark Electronics, Inc.
27 years experience working with electronic and electro-mechanical manufacturing and design (medical, automotive, military, computer, and industrial controls). Military veteran - served as a Combat Engineer with the United States Marine Corps.

In general, proper cleaning would remove solder balls from the solder mask. Balls, like the solder joints created during the first reflow probably won't oxidize. You may want to look at the root cause of the solder ball creation.

Printing solder paste on to solder mask is a common cause, stencil design can fix this. Another thing to look at is the temperature ramp up in the first reflow. If it is too fast, solder balls may be formed.

image
Mitch Holtzer
Director of Reclaim Business
Alpha Assembly Solutions
I've been in the soldering materials/applications industry for 25 years. Since joining Alpha, Ive been the global product manager for preforms, wave soldering flux, solder paste and more recently the Director of the soldering materials reclaim business.

If you are talking about solder spheres on BGA components, then oxidation can and will occur to the metal surfaces during 2nd reflow. This will not normally result in an open circuit assuming the solder joints were properly formed during 1st reflow.

Cleaning no clean flux residues off of the circuit board removes the potential protection from oxidation that the no clean flux itself may provide. Water soluble flux residues are potentially corrosive and must be removed, and typically do not help with oxidation protection as well as no clean fluxes.

image
Tony Lentz
Field Applications
FCT Assembly
Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

I can't understand the question well. Do you mean the solder balls of a BGA or Solder balls generated by the soldering process? My comment to you that the oxidation happens always that the solder or metallic surfaces are in contact with the oxygen, and is accelerates by heat.

Open contacts on balls from BGA, can happens by several causes and reasons but is hard that opens by oxidation by second reflow. Normally the second reflow accentuates the problems that happened on the first reflow or other kind of problems.

image
Andres Rojas
Engineering Director / Master IPC Trainer (MIT)
AMMSA Solutions
More than 20 years of technical experience in the electronics industry in roles ranging from Process & Project Engineer to engineering manager and Technical Applications Engineer for Latin Americas. IPC Master Trainer, International speaker and consultant.

Regarding the question following question, "Can solder balls oxidize and create opens during a second reflow if the circuit board assemblies were properly cleaned after the first reflow?" This question raises more question than a simple answer.

Typically solder balls would not cause this, however what type of solder ball and what type of component. Solder balls in a halo around the solder joint would not, but this also may cause insufficient solder in the solder connection. It depends on the severity of the solder balls. In the case of mid chip solder balls (beads) this would be of greater risk to cause some type of lift.

This would be dependent of board type, solder bead size solder bead location and final geometry of solder connection. Then there is the question regarding cleaning. What chemicals are used in cleaning.

Is it possible that an aqueous cleaning has trapped water under components and the steam generated in second side reflow is causing the lifting? What type of board is used in this assembly? Flex, rigid multilayer, hybrid combination all could play into board movement during reflow and the solder connection could have been compromised during cool down or reheating.

image
Karl Seelig

Deck Street Consultants
In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.

It is assumed that the second reflow is meant the second side of the Printed Circuit Board. In soldering the intermetallic compounds (IMC) appear as a solid layer on the interface between the tin and copper, commonly referred as intermetallic layer or diffusion zone.

Next to the copper will be the thin layer of Cu3Sn which is further covered with a thicker layer Cu6Sn5. At normal soldering temperatures, these layers are solid, but above 415oC, they begin to dissolve in the molten solder.

Submicron size voids can get developed either at the interface between the IMC and copper land or within the IMC. These voids will get developed, if the solder joints are exposed to elevated temperature such as in high temperature life tests and thermal cycling.

In the particular case, it is the reflow temperature exposure only and duration is very much limited. Moreover, if the second side reflow is carried out immediately after the first side reflow and proper cleaning or the cleaned PC board assembly is kept in nitrogen purged desiccators (due to anticipated delays) before second side reflow, neither oxidation nor open joints can occur.

image
Dr. Kuttiyil Thomas Oommen Tharakan
Group Director
Vikram Sarabhai Space Centre
Dr. Tharakan’s experience is in the area of avionics package production including reflow, test vector generation, formal verification of VHDL and verilog designs, screening of EEE components, quality control of electronic packages, and indigenisation of EEE components.

I'll assume we're talking about solder spheres on a BGA device. Solder spheres can oxidize to some degree in a 2nd reflow pass but if the solder joint was properly formed in the first reflow, when the solder balls go liquidus in the second reflow the wetting forces should cause the BGA to maintain contact with the PWB. A Head-in-Pillow defect would have occurred in the first reflow pass and wouldn't be exacerbated by the second pass, although you may be seeing it after the 2nd pass. I'd confirm that there is no opens after 1st pass.

image
Kevin Mobley
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group
Kevin has over 30 years of experience in process and manufacturing engineering serving in both EMS and OEM companies. Expertise includes all aspects of SMT as well as wave solder and CCA materials such as PCBs, solder material, and component finishes. Kevin has developed processes for thousands of assemblies from stencil printing to conformal coating and testing.

Your question didn't provide much information about the assembly, so I'm going to assume you mean an assembly with BGA and the solder balls that forms the solder joint for it. If your process for the first reflow are under control and the solder joints created in it are verified and fulfills the requirements, there should not be any issue in the second reflow. There will be some oxidation but it should not be enough to cause any opens.

I have done a lot of tests with daisy chain units on this scenario, where I have actually gotten quite the opposite result. BGA solder joint that was open after first reflow created a functioning solder joint during the second. However, this is definitively not anything I would count on (and I never tested the strength and reliability on them) - the most important thing is still to have an overall process control and control of the first reflow to get the desired result. To verify it I would do some x-ray analysis and maybe a cross section.

image
Anki Forsberg
Senior Quality Engineer - PCB Assembly
Axis Communications
Over 27 years of experience within the PCB assembly area. Hands on experience troubleshooting SMT on a global basis as well as implementing and setting up new processes/technology/equipment. ESD questions is also a key area.

Please find the Below causes of generate of solder ball during reflow soldering.
  • Solder ball generate due to moisture affected on PCB surface
  • You have to backed the PCB before production
  • Check the PBC manufacturing date code, if PCB manufacturing date above three month or six month
  • Please check the MSDS of PCB handling manufacturing

There is possibility of Solder ball get oxidised while 2nd pass reflow if amount of flux accumulated with solder ball,

image
Raju Wagh
Senior Engineer SMT
Advance Power Display system Ltd Mumbai
Mr. Raju Wagh has worked in the electronics industries for the past 20 years focused in TV manufacturing, mother boards, mobile, LED lighting, power supply manufacturing and more. His specialty is in SMT production and processes
Submit A Comment

Comments are reviewed prior to posting. You must include your full name to have your comments posted. We will not post your email address.

Your Name


Your Company
Your E-mail


Your Country
Your Comments



Free Newsletter Subscription
Circuitnet is built for professionals who bear the responsibility of looking ahead, imagining the future, and preparing for it.

Insert Your Email Address