|Ask the Experts|
November 2, 2018
Specific Solder To Control Dendritic Growth
|Expert Panel Responses|
It is not normal that a customer would specify a specific paste, although most customers don't have enough knowledge of solder or soldering to specify a paste beyond leaded or lead free. It is perfectly reasonable to specify a paste if you have special requirements. Most EMS manufacturers don't want to change pastes because of change over time, cross contamination concerns, and other processes involved (application, reflow, cleaning, etc.). That being said, make the EMS use what you specify (probably at some extra cost), or let them prove their paste is a suitable substitute.
M.O.L.E. Line Product Manager
Electronic Controls Design, Inc. (ECD)
Most of the time PCB fabricators operate on very tight budgets. Asking the fabricator to use a different type of solder paste might represent a loss of revenue for them. Your sourcing department will need to "re-negotiate" the terms of your manufacturing contract in order to add the use of a specific product brand.
Senior Manufacturing Engineer
It's certainly not unheard-of for a customer to specify which solder paste to use. Contract manufacturers tend to not like this because, as you can imagine, if all customers did it, the contractor would have a nightmarish time managing all of the different materials. If you're a large enough customer to have your own SMT line dedicated to you, then usually it's not an issue. I expect that the paste you are using is a no-clean paste, and the reason that one paste in particular works well for you is that it has a very benign residue. There are two alternatives you can consider:
No its not unusual to find this specified to CEMS but what is unusual is that the fault has been caused by Solder Pastes or rectified by changing pastes. Most Electromigration I have witnessed was caused mostly by poor PCB fabrication processes leaving highly conductive residues on the PCB. Especially if the PCB was HASL finished or roller tinned.
Technical Sales Manager
BLT Circuit Services Ltd
It is normal for an end user or OEM to specify the IPC J-STD-004 classification for the flux used in the solder paste (e.g. ROL0). It is not that common for the end user to specify the manufacturer, brand and part number of the solder paste. Specifying only the IPC J-STD-004 classification allows the PCB fabricator to choose a solder paste that meets the classification and works best in their process. Specifying the manufacturer, brand and part number limits the PCB fabricator to work only with that particular paste.
While a particular solder paste might solve the dendritic growth issue, there might be other process issues associated with that solder paste. This could result in yield issues or additional process costs for the PCB fabricator. If every end user or OEM specified a particular paste for their products, then the PCB fabricator could potentially have dozens of solder pastes in their process. Most PCB fabricators would prefer the freedom to choose the exact brand and part number of solder paste which works well in their process.
Specifying a specific type of solder paste of your choosing to electronics suppliers to be compliant especially in a Class 3 application is not unusual. In addition, do you identify both bare board and assembled boards cleanliness requirements ?
I recommend Ion Chromatography (IC) testing over Resistivity of Solvent Extract (ROSE) testing. Specifically, require Deionize Water (DI) compliance greater than 8 Megaohms for final rinse on both bareboard fabricators and assembled manufacturers be measured at point of dispose in last rinse finger not bulkhead. Have both your bareboard fabricators and assembled boards manufacturers perform IC testing per IPC-TM-650-2.3.28 and stipulate maximum anions, cations, weak organic acids(WOA) levels allowed, see link below:
INCREASING RELIABILITY - Through Predictive Analysis on page 80 http://www.magazines007.com/pdf/SMT-Nov2016.pdf#page80
I can only speculate on why the resistance to your stipulation? I typically test many flux types, bareboard fabricators boards, as well as cleaning chemistry even when no clean required and subsequent cleaning required to educate my customers for agreement. Many times the flux type is not the culprit.
VP of Advanced Technical Operations
The dendritic growth is is a serious problems that is often addressed with defluxing of the PCB. A change in solder paste is not necessarily the answer and the end user of the pcb might not always qualify the ideal solder paste. We see this issue as the conformal coating supplier too often.
Para Tech Coating, Inc.
It is not normal for a customer to specify the solder paste. There are a number of reasons for your EMS providers to want to maintain control of this specification. The process engineers have spent time determining which solder paste will work best in their process.
You say that there was a dendrite growth, and the blame was placed on the solder paste, but this is only partially true. These dendrites were forming due to an interaction between a number of factors including the solder mask, solder paste, and cleaning process used on your assemblies. By changing the paste, you changed the chemistry, and corrected the issue at one supplier, but you could be creating defects with your other suppliers by forcing this change onto them.
Supply Chain Engineer
It is not uncommon for contract assemblers to use multiple solder pastes based on customer requests. However, doing so can cause issues for procurement, storage and processing which might make the contractor reluctant to accommodate your request. Sourcing a specific paste can result in higher material cost availability and shelf life issues. Also, assembly processes are usually optimized around a specific paste, incorporating a different paste can introduce undesirable process variables that you're assembler wants to avoid.
Director of Product Management
Generally what I see is solder type is recommended, not specific brand or part number. However, I do run across customers that are specific to the manufactures part number of the paste. This is best communicated on the BOM. The challenges this can create is,
Director of Corporate Quality Assurance
Delta Group Electronics Inc.
The answer is quite often linked to economics. If the CM buys all or a large amount of paste from a single supplier, they can often get a volume discount. If they buy several different solder pastes from several different suppliers, they typically end up paying a higher price. So, if the paste that you want to use is not one that the CM is purchasing in high volume, it simply costs them more to build your product.
Technical Support Engineer
I hope you found that the root cause was due to paste residue, but the reality is that any of the no clean and water soluble fluxes will cause parasitic leakage and corrosion (dendrite) growth. It is how the material is processed for no cleans to ensure that the residue has reached the thermal conditions for the volume of material to create the benign residue. I do not believe that switch to a different paste is going to solve this problem. I suspect that the reflow process was too cool for the flux to create a complete benign residue. I suggest that you look at the failures with localized C3 extractions and Ion Chromatography in both the dendrite areas and in the non-dendrite areas to assess the amount of extractable flux.
Just because FTIR shows flux residue doesn't mean that it's the root cause. It could be the leads, bare board or an external issue with flux dripping back on your boards from a vent stack. Oh and the reason that CMs are pushing back on using a solderpaste that they are not familiar with may be all the work that goes into validating a new material for all the normal operational, stenciling, tack, slump and reflow profiling work that has to be done with any new material. Many CMs will have a couple materials that all of their customers use and all the work is done and meets the performance needs. New materials are not just a drop in.
President/Senior Technical Consultant
Dendritic growth or electromigration of conductive surfaces is based upon the conductivity of the laminate material and the contaminant on the surfaces of the laminate. In the past dendritic growth was always related to having conductive fluxes left behind after the soldering operation. Although these fluxes were dried the residues were of a conductive nature and would readily absorb moisture from the environment. When power was applied to the product, the residues would created a conductive path to grow the dendritic conductor.
Asking for a subcontractor to use a particular material such as solder or solder paste for a particular function is many times based upon the amount of business they will get from building the product. So, the resistance to change solder paste is based upon the development of the existing processes, including the qualifications of materials, processes, and environment, so to change could be very expensive, hence the push back.
I would recommend finding out what the contaminant is between those land areas and finding a solvent to remove those residues. Many times manufacturers are using a low solid content flux (i.e., No-clean flux) and if the thermal profile is not high enough, the flux activators will not volatize off the board and will remain behind and are still active which will create this condition for dendritic growth.
Vice President, Technical Director
It is easier said than done. You can ask your supplier for specific solder paste to be used and you will pay the price for it. The resistance can mainly come from the trouble that something like this can cause in a manufacturing process. Think about this - 1 of your 100 customers is asking you to do the same thing with their boards going through a wave soldering machine - are you going to have a different solder pot for this customer, change the solder (hundreds of pounds every time) or even buy a new machine only for this?
Engineering and Operations Management
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