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November 8, 2022 - Updated November 2, 2022 - Originally Posted Baking Old PCBs Prior To ReflowOur standard procedure for new bare PCBs is to bake at 125C if the date code is greater than six months old. These PCBs are sealed in plastic from the supplier. The packages have desiccants and indicator cards with moisture levels below 10%. I understand opened packs, but all of these have been sealed. To avoid delamination during reflow, is it required to bake these PCBs before use? We have reviewed J-STD-033 and found no reference to this issue. J.M. |
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I will go ahead and bake just for good measure. Moisture hides in weird places like inside plated through-holes.
Senior Manufacturing Engineer Northrop Grumman Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.
If sealed, desiccated, and monitored with indicator cards, there’s no need for baking older PCBs as mentioned in the question. Not only is it unnecessary, but if not baked in an inert atmosphere, certain surface finishes will oxidize making soldering more difficult and/or jeopardizing solder joint quality/integrity.
President Colab Engineering A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.
The life span of a PCB is not only about the exposure to humidity, and not everything can be solved by baking. The finish of the best PCB's allow them shelf life assuming they were sealed and stored in a controlled environment. I suggest to follow the IPC to the letter, as the quality of the finish can affect the quality of the product. It is also a good idea to have an additional visual inspection and look for evidence of corrosion or increased roughness of the pads.
CTO Cybord Electronic and Mechanical Engineer with 25 years experience in technology development. CTO of Cybord, developing electronic component authentication technology using deep-tech visual inspection and big-data. Author of a book and 24 papers.
I would NOT make it a hard fast “rule” to always bake boards. Baking can exacerbate oxidation (IPC J-STD-033D 4.2.7.1) on solderable finishes. Sealed dry packaging is good for 1 year (IPC-1601, 5.2). If the 10% dot on the HIC has had significant color change, the seal has been compromised (IPC-1601, 5.3) and then baking may be useful/required. Storage of MSL components (including bare boards/PCBs) in a dry cabinet <=5%RH, is considered equivalent to storage in a dry pack with unlimited shelf life (IPC J-STD-033D, 5.3.3.2).
Senior Project Engineer Electronic Controls Design Inc Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 39 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes and well as moisture sensitive component storage solutions.
You are baking the boards to ensure that there is no moisture, if they are sealed and all the moisture indicators say that there is no moisture then you should be OK. But just to be on the safe side you can always run a bare board through your reflow to see if it is OK. Pre-baking can also induce a risk of warpage on the boards so you might want to consider this as well when factoring in what you want to do.
Global Product Champion Henkel Electronics Richard Boyle is a Global Product Champion at Henkel Electronics. He has over 25 years experience in the electronics assembly industry and is responsible for the global technical service of all of Henkel's solder materials.
You did not specify what the base material is for the PWB- FR4 vs polyimide for example. If your supplier has baked the boards to remove all moisture prior to shipment and you have left them in their original moisture proof packaging then it might be redundant to bake again. However if you cannot guarantee that the supplier has done this then it is a major risk, especially for polyimide type materials and you need to continue baking.
President S T and S Testing and Analysis Gerald O'Brien is Chairman of ANSI J-STD 003, and Co Chairman of IPC 4-14 Surface Finish Plating Committee. He is a key member of ANSI J-STD 002 and 311 G Committees Expert in Surface finish, Solderability issues and Failure analysis in the PWA, PWB and component fields.
The temperature and duration to which a bare PCB has to be subjected to baking depends on the thickness of the PCB and the conditions of storage. Here also one has to observe time-temperature relationship. Generally, for a 2mm PCB it is 110oC for 4 hours or 93oC for 8 hours. The packages need to be stored in vacuum sealed/ nitrogen sealed moisture barrier bags. A level of not greater than 10% RH with the plastic sealing is also bound to absorb moisture, unless it is vacuum/nitrogen sealed. Moreover moisture ingress can happen once it is opened for inspection, phases of movement for reflow soldering, post-reflow inspection, manual soldering of relays, leaded devices etc. There are various reasons for delamination other than humidity build up such as - Construction of PCB w.r.t resin content, types of fiber glass cloth used, adhesion and long term thermo chemical compatibility between the resin system and copper, impact on CTE both in z-axis and y-axis, a very high resin content pre-preg layer adjacent to very-low resin content layer, resin-rich and glass-rich areas within a pre-preg area, excess heat etc. To minimize the entrapped moisture it is always advocated to carry out baking as given above. Moreover, for any soldering operation on the card whether it is manual soldering or rework the cards shall be properly conditioning by thermal baking.
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. Senior Member of the IEEE
You didn’t mention what type of base material or surface finish the PCB’s have. Especially the surface finish of the solder pads are interesting since this may affect the shelf life. From my point of view I would not use a standard procedure to bake everything that has a date code older than 6 months. Most surface finishes are specified to handle at least 12 months of shelf life in unopened packaging (with desiccants and humidity indicator card). Two relatively common surface finishes, OSP and Immersion Ag, are specified with the 6 month limit in shelf life (however some PCB manufacturer states that Immersion Ag can handle 12 months as well). So - depending on the surface finish of your PCB and given that the PCB packaging are unopened, not damaged in any way, and that the humidity indicator doesn’t show any signs of humidity in the packaging. If it is not for example OSP or Immersion Ag as surface finish, I would not bake these PCB’s after 6 months. To be safe I would ask the PCB supplier for the shelf life they specify for the type of surface finish you use on your PCB’s.
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.
If the boards are sealed with desiccant and the humidity indicator card is below 10% you shouldn’t have to bake the boards. Repeated baking can oxidize the surface finish and increase the intermetallic growth before the board is soldered. If the board goes through multiple reflows and a rework or two the intermetallic growth may be a concern.
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.
Refer to IPC-1601 for answers to all questions regarding baking of circuit boards, including methods for determining optimum bake times and temperatures. You do NOT need to bake at 125C, the experts are correct in that baking at that high of a temperature will degrade solderability somewhat. 105 deg. C is a good general baking temperature that will work for 99.9% of all PWBs. J-STD-033 is intended for baking of components only.
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.
If there is no concern regarding moisture presence in PCBs, I consider the process unnecessary and even harmful as you can create issues on the exposed metal finishes.
Engineering and Operations Management Independent Consultant Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com. |
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