Ask the Experts
July 11, 2018
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
Giventhat you really would prefer to clean due to your high-impedance circuitry, youneed to find an economical, reliable way to protect the LGA module, oralternatively to mount the LGA module using localized soldering after cleaningthe rest of the board. Either process flow would work, however I would suggestthat the first might be more economical.
Onepossibility would be encapsulating the LGA module with a moderately highviscosity, low-modulus (elastomeric) compound after soldering and prior tocleaning. The downsides here are:
A two-step process whereby the part is underfilled and then encapsulatedcould avoid the trapped-air problem, at the expense of an additional processstep and material.
- You would need to eliminate (or plug) through-vias in the PWBunder the LGA to keep cleaning solution from getting in from the reverse side
- You would then trap air under the part which would expand andcontract, exerting forces on the part and the encapsulant when heated orcooled.
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.
My expertise is EHS, not electronic design or assembly. However, there is acleaning method that may be used for your module described below. That is usingcarbon dioxide "pellets" (i.e. dry ice particles) to clean the module afterassembly.
This approach eliminates the use of water, and because solid CO2sublimes into a gas, there's no residual left on the assembly. I believe thesuppliers of liquid CO2 would be a good starting point for more information onthis cleaningmethod.
Lee Wilmot has 20+ years doing EHS work in the PCB/PCBA industries, including environmental compliance, OSHA compliance, workers compensation, material content declarations, RoHS & REACH compliance. Active on IPC EHS committee and c-chaired committees on IPC-1331, J-STD-609A on labeling & marking, IPC-1758 on packaging and others.
The use of no clean flux will create residueopportunities that must be designed out of the assembly process. To makethis work it is not flux alone that you must worry about it is the connectorcleanliness, component cleanliness and PCB cleanliness then you have to work onflux under the large and small components that is fully heat activated andinsulative. I would be happy to discuss these options and design riskswith you off line.
President/Senior Technical Consultant
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.
For this critical unit, theobvious cleaning method is vapor cleaning. Vapor cleaning uses anonflammable 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 coolingcoils that operate around -15 F around the top of the unit.
As the parts arelowered into the hot vapor, the condensation of the pure solvent vapor on tothe parts washes away the contaminants and they simply drip off the parts downinto the machine. A small hand held spray wand can be used to lightlyblast solvent to the underside of the BGA or FlipChip, etc.
Theparts are slowly raised up out of the pure hot vapor and they dry as they comeup 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 selectedmust be compatible with the parts being cleaned. Normally we wouldrecommend a strong defluxing solvent such as n-propyl bromide (nPB), but inthis case it intuitively seems that you are going to have some sensitive plasticparts on board. So, one of the mild solvents such as a fluorinatedsolvent WITHOUT trans, dichloroethylene in it, would probablysuffice.
Manufacturers such as Dupont's Vertrel SMT, 3M's NOVEC HFE7200, Petroferm's LENIUM FRA, or Solvay's Solkane HFC365mfc would beperfect. The cleaning process for each assembly could be as fast as90 seconds to be totally clean and dry.
Since no water is involved in this process, there should be little if anyconcern with the LGA Module also being exposed to the mild fluorinatedsolvent. But you could always pass this by the customer's materialsdepartment and they can determine any issues that may exist.
Theequipment to be used could be either a small bench top vapor degreaser, such asa Lab Kleen with a small spray wand, or a small common Baron BlakesleeMLR120. Finally, only if necessary, the LGA module could be wrappedup in a small teflon or nylon bag, clasped shut with teflon lacing tape or theequivalent.
In summary, your first step is to see if your customer would accept cleaning ina fluorinated vapor defluxing vapor, such as one of the four that I mentionedabove. Then obtain a sample or find a location to try out thecleaning process, and then finally once proven, find the suitable equipment andsolvent to continue the process. I'd be happy to assist if desired.Happy cleaning!
Rick Perkins is a chemical engineer with more than 33 years of Materials & Processes experience. He has worked with Honeywell Aerospace in high-reliability manufacturing, as well as with several oil-field manufacturing companies. He also has a good understanding of environmental, health, and safety regulations.
If possible, assemble the board with the ECG circuitswith high impedance differential amplifiers, BGA processors and ASICs beforeattaching the LGA module. Clean the board following the current assemblyprocess. Following the cleaning process, post bake the boards to remove anytrace or entrapped moisture. Attach the LGA module last using a no-clean fluxthat requires no cleaning.
If cleaning what no-clean residue remains isabsolutely required then a precision organic base self-rinsing solvent cleaner(not IPA) should suffice. Cybersolv 141-R has proven to be very effectivecleaning this type of flux residue.
Technical Expert Sales Support
Charlie Pitarys has over thirty years of industry experience and has been with KYZEN for twenty-one years. Charlie is a former Marine and a retired Sargent First Class in the Army Reserves. His previous employers include Hollis and Electrovert. Charlie continues to use his expertise on cleaning processes and machine mechanics to help KYZEN customers and partners improve their cleaning operations.
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. 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.
Karthik Vijay, Indium Corporation