| 'If You Clean, You Must Rinse' |
 |
 |
| Written by Terry Munson | |||
| Wednesday, 30 April 2008 19:00 | |||
Our expert clarifies his stance on brush cleaning.
It has been brought to my attention (“The Big Brush Off Revisited,” April 2008) that I was unclear in a December 2007 case study on localized brush cleaning.1 To recap, I am not suggesting localized cleaning is good or bad, but if you clean, you must rinse. If you solder or repair with a no-clean flux and don’t clean, you must ensure the flux (flux pen, bottle or core wire) is completely heat-activated, guaranteeing it becomes a benign residue, but not until then. During repair, rework and hand solder, flux is used to prepare the soldering surface. Now, let us address what happens after surface prep and with flux after soldering. Issues that need to be addressed during repair, rework, selective and hand soldering beyond soldering: 1. A soldering operator must understand more flux does not improve soldering (although sufficient flux does). 2. Two flux choices are available: cored wire, and cored wire with liquid flux in a pen or bottle to assist in wetting. 3. When using flux to prepare the surface, remember flux has a low surface tension and will travel everywhere – to nearby components, or the opposite side of the board through small vias. 4. A general rule is no-clean fluxes need to be heat-activated to become benign residues. The flux supplier identifies the activation temperature. Also, just because the flux evaporates at room temperature doesn’t mean it is completely benign. 5. When the only area that gets heat is the repaired or hand solder joint, the remaining no-clean flux around and under nearby components is moisture-absorbing and conductive. Dendrites can grow with no-clean flux residues and 1.5V difference (Figure 1).  6. Some of you are saying, We brush clean only because test pads at ICT have a problem with the no-clean flux. Remember to rinse effectively in these and surrounding areas. This requires localized testing and is not a visual inspection issue. 7. This residue can be dealt with in one of three ways: a. Do nothing and expect residues to become benign because of magic. We see this a lot and are surprised when clients bring us these failures. b. Localized brush cleaning/can of cleaning solvent. When cleaning with a solvent in a localized area, if you do not rinse the contamination, you are simply relocating it to nearby areas. Brush clean, then rinse/flush in a controlled fashion, using a solvent rinse or a steam approach. If you cleaned an area, then rinse the area as well. Remember, low-standoff parts hold a lot of fluid by strong capillary forces. c. Secondary heating of the nearby areas to transform the no-clean flux from a conductive moisture-absorbing flux to a benign residue. This can be done in a controlled cure oven, or even with a heat gun in a stationary fixture and thermocouple, to avoid thermal damage. How to close this little discussion? When cleaning with any localized method, rinse effectively and use a localized analysis method to assess effectiveness. References
Terry Munson is with Foresite Inc. (residues.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . This column appears monthly.
|
|||
| Last Updated on Thursday, 29 October 2009 18:32 |
Columns
| An Early Spring? |
The second half of 2011 is behind us, literally and figuratively. |
| Read more... |
| ‘An End-to-End Solution for Assembled HDI Rigid-Flex’ |
Multi-Fineline Electronix (MFLEX) and AT&S last week announced an agreement to share their respective printed circuit board technologies. Under the agreement, AT&S's high-density interconnect (HDI) rigid PCBs and MFLEX's flex board capabilities will be offered to respective customers, and the two will partner on rigid-flex technology. |
| Read more... |
Features
| SMT Stencils from a Production Perspective |
Is the slew of new materials, coatings and processes truly unique, or just the same old hype? |
| Read more... |
| Implementing Good Test Coverage and Eliminating Escapes |
Experimental data reveal a strong correlation between SPI and AXI. |
| Read more... |
Search
Search
Login
CB Login
Language
Language
Products
Custom Products & Services Offers Work Benches
These modular work benches offer quality and versatility for work areas that include network labs, assembly areas, shipping stations, security and command consoles, or as computer repair benches. Can...
These modular work benches offer quality and versatility for work areas that include network labs, assembly areas, shipping stations, security and command consoles, or as computer repair benches. Can...
