|Ask the Experts|
November 6, 2017
Flux Residue Causing Test Issues
We are having issues with flux residues that are impacting in-circuit and functional test? We use a no-clean flux that is applied by spray application. After wave, we experienced many incomplete readings due to flux residues on connector pins. We have reduced the usage of flux to as low as possible and still have the problem. Our test guys suggest brushing the connectors, but that will impact the labor cost. Any comments?
|Expert Panel Responses|
"No Clean" is a complete misnomer. If you want solid test results you will need to clean off the residue. The tradeoff is higher labor to ensure higher quality. We have even talked with customers that are (gasp) cleaning "no-clean" flux, in-line with washing systems, to get rid of residues.
Not sure what incomplete readings mean. Are you referring to an ICT issue due to flux residue on the connector pins? Flux Residue & ICT No-Clean flux residues are mostly designed to be test-probable. The residue could be soft & gummy for easy probe penetration or hard & brittle so that it shatters upon probe contact. In addition, the residue needs to be designed to minimize probe maintenance and prevent clogging of the probes that in turn causes false failures. The solids content in the flux could be a factor in this case. The higher the solids, more residue is left behind; but a higher solids content also makes soldering easier. There have been a lot of advances in probe technology as well. Even though a clamp-shell fixture limits the amount of force a probe can exert, there are spring loaded probes which when untorqued can better penetrate the flux residue.
Technical Manager - Europe
No clean flux has been a bane of the in-circuit test world ever since its introduction. There are several approaches to help, but no cure the problem: 1) Reduce usage of flux to the bare minimum. This usually causes a conflict between the solder process engineer who finds that the more flux he/she uses the better the quality of the solder joints. The test engineer on the other hand has to live with the residual flux and its effect on test probes. There are a number of solder/flux vendors who have formulated "no residue" fluxes that can help. They may cost more than conventional "low residue" flux but can save the customer in the long term from the hidden cost of re-test and rework. Contact vendors such as Kester, Indium, Nihon, etc. 2) Change probe styles, materials and spring forces. a. There are a number of probe head styles that have been created to deal with high flux environments. Many of the head styles have more aggressive/"self-cleaning" designs that can help. b. The traditional plating on the probe tip is gold. But many customers find that switching to a harder material such as steel can provide a more reliable contact over extended use. c. The probes are available with higher spring forces that can help punch through flux residue and oxidation such as found on OSP coated boards. d. Contact vendors such as Ingun, IDI, QATech and Everett Charles Technologies for more information. 3) Use a test fixture system that supports re-cycle on fail. Most commercial automated test system vendors can provide the ability to automatically cycle the PCBA in the fixture upon a failure. Customer-developed systems can do the same if properly programmed. Some systems can cycle the fixture prior to test as an effort to punch thru flux residue. 4) Use "dry well" fixtures. Pneumatic-style fixtures or vacuum box fixtures that don't expose the probes to vacuum-related air flow can be more reliable since the air movement past the probes can exacerbate the effect residual flux by attracting dust onto the probes. Some customers with vacuum fixture have also found that having the tester too close to the soldering system can result in flux-laden vapors being sucked into the fixture! Hope these ideas are useful.
No-clean type of flux residue does not necessarily mean it would not require a wash process. No clean paste or flux refers to a "low solid content" type material actually. It is still highly recommended defluxing the no-clean post solder flux residues as you do with OA (water soluble), RMA type flux residues with an aqueous based or solvent based cleaning agents. It can be expected that after soldering, the presence of activators, resin/rosins, or even thixotropic residue could impact your IC and functional test. Try removing the flux residues and re-conduct the IC /functional test to see if you would pass. Based on our past field experience, variation of flux amount applied on the board surface did not make any difference in terms of flux removal rate, in the end, you would still form a resin layer that entraps the activator, rosin, flux agents that could cause reliability failures. Brushing the connectors would not provide you consistent results as from one operator to another, the cleaning results or degree of brushing would differ. In order to increase the reproducibility of the cleaning results, an automated system such as industrial dishwasher / cabinet type batch cleaner or if your throughout is higher maybe a conveyroized in-line cleaning equipment.
Application Technology Manager
Flux residue on connector pins at times can prove challenging to resolve. It appears you are running a true No Clean line as you do not mention any aqueous or solvent cleaning post wave. In a spray flux application without a cover slip over the connector it is possible to get overspray to settle within the connector body and on the pins but usually a greater amount will wick up around the outside of the pin through the connector. What remains is flux residue that has not volatized completely as it never touches the solder or sees a cooler pre-heat exposure in part by the mass of the connector itself. Just dry brushing the connector pins will not suffice. The brush will load up with flux and be transferred from one connector to another. If you can physically see the flux under low magnification, it will probably look like white crystals clinging to the pins surface you will have to set up a manual bench top cleaning process with an effective cleaning product. Something quick, efficient and simple as an aerosol flux cleaner solvent available through many sources could work well for your particular requirement. A review of the total cost for cleaning versus fixing in-circuit test or field failures will shed light on the true benefit of cleaning. If you need further assistance in this matter do not hesitate to contact me.
Technical Expert Sales Support
First question is are you running lead or lead free and is the residue left behind tacky? If the answer is leaded and tacky then you might want to check into your flux to make sure you are using a rosin free No Clean Flux. If it is lead free then you might want to play with some fluxes that might have a lower amount of rosin content. T ypically there is around 2% solids in a No Clean Flux associated with the activation package so anything above that might be due to a rosin type material.
A good No Residue flux will not interfere with Test, we do see allot of residues from Solder Pallets and Solder resist that cause allot of test issues as these are not pin probe testable residues such that they are tacky and don't break down easily. Check to see under magnification if you have a greasy/oily residue which can sometimes be in a droplet form over the PCB and Connector. Also you could test the Solder Resist to see if that is cured sufficiently
Technical Sales Manager
BLT Circuit Services Ltd
In my opinion, regardless of the path you are going tofollow, you will add cost to your process
Engineering and Operations Management
One issue often ignored when recommending all these new spring probes with their higher spring force and more aggressive head styles is that cycling the board in the test fixture. If the board is cycled more than a couple times these new aggressive probe styles will literally punch holes through the solder and into the fab itself.Sliced annular rings are not uncommon. Boards rejected by OEM as damaged have been seen.Matt B.Turpin, Datest, USA
If you are having test issues, (ex:false alarmdefect issues), this is probably involved by an accumulation of glued flux onyour testing pins. Because of that , I guess you are running your wavesoldering process under air atmosphere... Because, from our experience, usually, companies thatswitched their process to nitrogen wave soldering atmosphere, for costreduction reasons (dross, flux , rework, etc...) , discovered also some otherbenefits including: "clear up test issues"... Flux residues doesn't stick anymore to their testing pins.That could be explained by an evolution of the textureflux residues, involved by the non-oxidation of these residues. It is also thesame for Reflow under nitrogen. So simply try to switch your wave process to nitrogen,you could use less flux. Sure you willadopt it!DUTOURNIER Bertrand, AIR LIQUIDE FRANCE INDUSTRIE, France
|Submit A Comment|
Free Newsletter Subscription
Circuitnet is built for professionals who bear the responsibility of looking ahead, imagining the future, and preparing for it.
Insert Your Email Address