|Ask the Experts|
November 18, 2020
Challenging Cleaning Problem
We are designing an ECG monitor that uses an embedded phone-modem LGA module. This phone modem is shielded, however, it is not hermetically sealed. The manufacturer requires that the board not be washed, or that a method of the user's choice be used to prevent ingress of contaminants, wash water, etc.
The same board will also have ECG circuits with high impedance differential amplifiers. The ECG circuits must avoid conductive leakage. Normal industry practice on ECG/Instrumentation boards is to always clean.
The board has several BGA processors and ASICs, uses 3 to 5 mil traces, has 0402's, and could possibly have a need for BGAs on both primary and secondary board sides.
What is the best way to assemble the LGA phone module onto the board in a low volume scenario, 1000 to 3000 units/month?
|Expert Panel Responses|
Given that you really would prefer to clean due to your high-impedance circuitry, you need to find an economical, reliable way to protect the LGA module, or alternatively to mount the LGA module using localized soldering after cleaning the rest of the board. Either process flow would work, however I would suggest that the first might be more economical.
One possibility would be encapsulating the LGA module with a moderately high viscosity, low-modulus (elastomeric) compound after soldering and prior to cleaning. The downsides here are:
My expertise is EHS, not electronic design or assembly. However, there is a cleaning method that may be used for your module described below. That is using carbon dioxide "pellets" (i.e. dry ice particles) to clean the module after assembly.
This approach eliminates the use of water, and because solid CO2 sublimes into a gas, there's no residual left on the assembly. I believe the suppliers of liquid CO2 would be a good starting point for more information on this cleaning method.
The use of no clean flux will create residue opportunities that must be designed out of the assembly process. To make this work it is not flux alone that you must worry about it is the connector cleanliness, component cleanliness and PCB cleanliness then you have to work on flux under the large and small components that is fully heat activated and insulative. I would be happy to discuss these options and design risks with you off line.
President/Senior Technical Consultant
For this critical unit, the obvious cleaning method is vapor cleaning. Vapor cleaning uses a nonflammable hydrocarbon that is boiled to create a pure solvent vapor. The pure solvent vapor fills a chamber in a machine called a vapor degreaser (or vapor defluxer in this case). The machine has a Vapor Trap of cooling coils that operate around -15 F around the top of the unit.
As the parts are lowered into the hot vapor, the condensation of the pure solvent vapor on to the parts washes away the contaminants and they simply drip off the parts down into the machine. A small hand held spray wand can be used to lightly blast solvent to the underside of the BGA or Flip Chip, etc.
The parts are slowly raised up out of the pure hot vapor and they dry as they come up to room temperature passing by the cooling coils and out of the degreaser /defluxer.
In your specific case, there are 2 concerns. First, the solvent selected must be compatible with the parts being cleaned. Normally we would recommend a strong defluxing solvent such as n-propyl bromide (nPB), but in this case it intuitively seems that you are going to have some sensitive plastic parts on board. So, one of the mild solvents such as a fluorinated solvent WITHOUT trans, dichloroethylene in it, would probably suffice.
Manufacturers such as Dupont's Vertrel SMT, 3M's NOVEC HFE7200, Petroferm's LENIUM FRA, or Solvay's Solkane HFC365mfc would be perfect. The cleaning process for each assembly could be as fast as 90 seconds to be totally clean and dry.
Since no water is involved in this process, there should be little if any concern with the LGA Module also being exposed to the mild fluorinated solvent. But you could always pass this by the customer's materials department and they can determine any issues that may exist.
The equipment to be used could be either a small bench top vapor degreaser, such as a Lab Kleen with a small spray wand, or a small common Baron Blakeslee MLR120. Finally, only if necessary, the LGA module could be wrapped up in a small teflon or nylon bag, clasped shut with teflon lacing tape or the equivalent.
In summary, your first step is to see if your customer would accept cleaning in a fluorinated vapor defluxing vapor, such as one of the four that I mentioned above. Then obtain a sample or find a location to try out the cleaning process, and then finally once proven, find the suitable equipment and solvent to continue the process.
I'd be happy to assist if desired. Happy cleaning!
If possible, assemble the board with the ECG circuits with high impedance differential amplifiers, BGA processors and ASICs before attaching the LGA module. Clean the board following the current assembly process.
Following the cleaning process, post bake the boards to remove any trace or entrapped moisture. Attach the LGA module last using a no-clean flux that requires no cleaning.
If cleaning what no-clean residue remains is absolutely required then a precision organic base self-rinsing solvent cleaner(not IPA) should suffice. Cybersolv 141-R has proven to be very effective cleaning this type of flux residue.
Technical Expert Sales Support
Localized soldering of an LGA will be painful. The cleaning options mentioned by others should be explored. Another option to look at is an ultra-low residue/solids no-clean paste flux that leaves less than 1% residue post-reflow, as opposed to the 40-50% residue with standard no-clean fluxes.Karthik Vijay, Indium Corporation
The ultra-low residue flux comes with the caveat of needing nitrogen during reflow. But if the very low flux residue levels do not affect impedance and the LGA is not subject to water cleaning, then it is worth a shot.
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