|Ask the Experts|
January 15, 2018
We have been experiencing delamination on a small percentage of circuit board assemblies during the reflow process. We believe we have properly addressed the issues regarding proper handling and storage of the bare board prior to assembly. Can you point to any other potential causes of delamination or any other steps we should take to prevent it?
|Expert Panel Responses|
To find proceduresfor the proper stocking and moisture protection of electronic components, youcan refer to IPC standards. (IPC/JEDEC J-Std-033C) If you are looking for guidance tohandle printed circuit boards the more recently published IPC 1601provides somelevel of standards for stocking and moisture protection for printed boards,including caution against baking. Even if youare paying attention to handling and floor time, the packaging quality of PCBsas received from the manufacturers can have a significant effect. Often asimple foil bag or an ESD bag is used instead of a well spec'd Moisture BarrierBag (MBB) with desiccant and HIC (Humidity Indicator Card). With suchpacking, boards are likely to arrive having already absorbed moisture, andstored like this they will be useless after a short time. Desorption processdesiccant drying cabinets, such as those pioneered by Totech (http://www.superdry.info/) can be used fortheir careful drying. Theprocedure is simple and cost-effective. The previously absorbed moisture isremoved from the printed board without temperature stress and with acontrolled, revertive drying process in an atmosphere of less than 0.5 g/m 3vapor content (in effect, a moisture vacuum). In addition,oxidation is stopped by the removal of the electrolytic water molecules.Because these storage systems work at room temperature, (or accelerated withmild heat), boards need not be removed until ready for processing. Dried andstored in this way even oxidation sensitive OSP coated printed boards (which1601 states should never be baked) are protected and can be used over longperiods of time without the risk of delamination and with consistent wetabilitycharacteristics.
I'd be very suspicious of defects in either the adhesive or the surface it's being laminated to. The adhesive defects can be due to insufficient adhesive laydown or the existence of gels in the adhesive. It is also possible that the adhesive composition is non-uniform, which might allow delamination zones to occur. Also, surface contamination on the substrate will result in insufficient wetting between the adhesive and the surface, resulting in delamination. I assume that the lamination pressure was uniform when adhesive was laminated to substrate? The smallest speck of dust can also cause a focal point for delamination to occur. There are some analytical means available to troubleshoot these ideas, but you need access to a good chemical/analytical lab to do some thermal analysis, and some FTIR spectroscopic analysis. In this case, before I can get more definitive, I absolutely will want to see samples to do comparisons between "good" and "bad" in the lab.
Please review your bake out time and temperature for your bare boards. Rule of thumb is 105C for 6 hours and the only caveat is to make sure we do not overtsack the pcb in the bake out chamber. The pcb should be separated into small stacks to insure the boards reach temperature for atleast 4 hours to insure moisture is driven out of the laminate.
VP Engineering Services
STI Electronics Inc.
You should verify the temperature profiles of your pcb assy during the reflow process. If they are confirmed within tolerance, show them to the pcb supplier(s), together with the delaminated boards. It should then be the pcb laminates problem.
Managing Director, Asia Pacific
Delamination is generally a result of moisture content, PCB lamination controll problems with Epoxy system and/or inherent stress that is evident in the PCB. Because the lamination system is suspect, first place to start would be your humidity and moisture control systems. From here, I would also check your preheating system and ensure you are not surpassing your PCB laminate system temperatures.
Capital Equipment Operations Manager
Specialty Coating Systems
Delamination, unfortunately, is a fairly common problem for modern circuit boards, especially high-density, multi-layer boards or those composed of advanced materials such as PTFE (which is inherently difficult to bond to other layers.) Your boards are probably delaminating because, during processing, they are thermally and/or mechanically stressed beyond their limits of adhesion. One way to reduce or eliminate this problem would be to increase the bonding strength between all the layers of the board, so that they don't delaminate. This can be accomplished using a suitable plasma treatment on the layers of the board before they are bonded together. For example, increased adhesion (and decreased delamination) can be achieved by treating the individual layers of the board using an Argon-based plasma in a chamber in a low-vacuum environment, prior to the layers being bonded together to form the final board. The plasma treatment can clean the layers and remove micro-contamination that interferes with bonding strength (this process is sometimes referred to as "plasma cleaning" the layers prior to bonding). The plasma treatment can also activate the surface of the layers before bonding together to make a finished board, so that each layer adheres more strongly to the other layers around it. Plasma treatments are particularly useful and effective when trying to bond advanced materials like PTFE or various flexible materials together such as polyimide, in order to prevent delamination and increase board quality. Contact March Plasma Systems if you'd like to try some plasma cleaning solutions for your particular case.
Global Marketing Manager
What we find is material damage can be grouped into four types that include adhesive delamination, cohesive cracks, crazing and material decomposition. The first thing to do is to determine the type of material damage that you are experiencing. Material decomposition is obvious in that it produces a blackening of the dielectric material and in extreme cases black balls of carbon on the surface of the PWB usually next to holes. This occurs when one uses low end material in a lead free application and, in reality, it is a rare type of material damage. Crazing is a separation between the individual glass fibers and the dielectric material. This type of material damage is not visible upon macroscopic examination but it is seen as a silver separation running down groups of glass fibers in microsections. The silver sheen is from air gaps between the epoxy and the individual glass fibers. This type of material damage is particularly found when the grids size, the hole to hole spacing is .020" or less. This type of material damage is a concern in conductive anodic filament (CAF) formation. The two most common types of material damage include adhesive delamination and cohesive cracks. What we find is that adhesive delamination and cohesive cracks are most often not visible upon macroscopic examination. The material damage occurs in the central zone of the PWB and that there is no visible expression of the material damage. If you can see the delamination after assembly then it is often in the outer layers of the laminate. In adhesive delamination this is a separation between two laminated surfaces. It is usually between the B-stage and C-stage, B-stage and copper or between the glass bundles as a group and epoxy. This type of failure looks like a blister on cross section and usually ends in a single point that rarely branches. This is the type of material damage that may respond to baking to drive down the amount of volatiles, like water, that is trapped within the dielectric material. The problem is I have never seen baking get rid of delamination. If you divide material that is prone to delaminate into two groups, bake one group and don't bake a group, I find that the baked boards will often still delaminate. It is rare that baking makes a significant difference in delamination. What I suggest is that you try baking at 105 C for four hours. More baking can cause a breakdown of the material that could increase the material damage. The forth type of material damage is cohesive cracks. Cohesive cracks tend to produce cracks that go through the C-stage and B-stage layers and through glass bundles. These cracks tend to go off on any direction and frequently branch into multiple points. What appears to happen is the material become over cross linked, shrinks, builds up tress and then cracks. There are many factors that cause cohesive failure, like problems in lamination or material that is beyond it's shelf life. One of the most common causes is lead/free assembly and rework. Assembly temperatures of 260 C are at the limit of what most modern materials can stand. Of the 24 material we just tested for a consortium we had 15 the showed material damage. What you should do is to consider changing the material to one that is more robust in your assembly environment. Also is would be prudent to limit the temperature that you are using for assembly and the number of rework cycles. This means to profile your assembly temperature to give a good solder joint for the largest and smallest components at the lowest temperature possible.
PWB Interconnect Solutions
In order to solve this problem, more data is needed. ThePCBs can have moisture trapped from the supplier or there might be afabrication issue depending on the materials used. Like the assembly materials,the PCB fabrication materials have shelf lives and going beyond the shelf lifeon materials like pre-preg can cause issues, one of them being delamination. Now improper handling in your assembly process can causean issue like this by itself or in a combination with the PCB fabricationprocess. So it would be reallyinappropriate to point out an issue with missing data. A cross section on fewboards would give you more information so I would suggest that you do thatbefore changing a lot of other things.
Engineering and Operations Management
By far, the most common type of delamination isdue to moisture uptake. Since allpolymeric materials absorb moisture, eventually they will reach equilibriumwith the environment. The typical environment for assembly is in a wellcontrolled operations facility that is typically around 50-60% relativehumidity. Unfortunately, this is not a good controlled environment to storeboards. It is common practice forassembly houses to open the package and inspect the product before putting itback in the warehouse. Within weeks, the boards will reach an equilibrium andwill be susceptible to moisture induced delamination. There are some materials that less sensitivethan others, but poor handling and storage can result in delamination. At IPC/APEX 2015 in San Diego, I spoke to a customerwho described this very case. Boardsprocessed fine upon receipt, but after 8 months in the warehouse unsealed, theboards delaminated. The classic patternwas delamination between layer one andtwo next to a large copper plane. Theuser baked the boards and the problem was solved. Baking can be used if performed properly andsolderability is tested as well, but the best solution and least time consumingis maintaining good product packaging to protect boards that are stored forlong periods of time. IPC-1601 is anexcellent guideline and provide the necessary information to significantlyreduce assembly delamination.Michael J. Gay, Isola Laminate Systems, USA
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