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www.us-tech.com Keep Your Cleaning Processes Separated Continued from previous page
ly consists of the removal of organic acids. As such chemical constituents are only effectively removed with alkaline cleaning agents, combining both of these processes will ultimate- ly lead to cleaning problems. Prede - fining the cleaning process window can be very helpful. Generally speaking, a wider
process window leads to more flexibil- ity within the cleaning process and ultimately, better results. The amount of raw solder paste removed during a stencil cleaning operation is signifi- cantly higher than the amount of flux removed from PCBs. The raw solder paste excess, which is suspended in the cleaning agent during normal operations, could adversely affect PCBs once introduced into a stencil cleaning process for defluxing. To make matters worse, minute spheres of solder paste (solder balls) can be easily trapped under components and lead to product failures in the field.
Elevated Temperatures Furthermore, defluxing opera-
tions are typically conducted at ele- vated temperatures. Applying these process settings to a stencil cleaning process can lead to stencil material compatibility problems related to high temperatures. During a recent global lead-free
evaluation, the objective was to spec- ify a defluxing and stencil cleaning process. The process engineer in charge had been mistakenly informed by other industry sources that a single product could meet both requirements. After initial stencil cleaning trials and sufficient expo-
sure of the traditional PCB cleaning agent to the stencil mesh, adhesive and foil, the stencil frame darkened and the foil delaminated. This was traced back to the fact that an alka- line material, typically used for defluxing of printed circuit boards, was suggested to the customer for both, stencil cleaning and defluxing purposes. The customer acknowl- edged that the only solution would be to install two separate processes (i.e. stencil and reflowed misprinted PCB cleaning) and use cleaning agents specifically designed for each task. The designated stencil cleaning agent was tested according to the customer’s specifications. Furthermore, independent SIR
(surface insulation resistance) test- ing with designated SIR coupons val- idated the product’s cleaning per- formance in accordance with Belcore Standard GR-78 CORE, Issue 1, Section 13.1.3 and was subsequently approved for global use.
Stencil Cleaning Care Stencil construction materials
are typically prone to failure under certain conditions and can lead to problems with adhesion from foil to mesh and from mesh to frame. Increasing the temperature, which is typically required for surfactant-based defluxing products, accelerates this delamination process. On the other hand, using stencil cleaning products to remove flux residues will provide unsatisfactory results, i.e. the process window is too narrow. Consequently, white residues may be left on the sub- strate’s surface, which is a clear indi- cation of insufficient cleaning. There is also a generally over-
looked cleaning process. For double- sided misprinted SMT assemblies, which are misprinted on one side and reflowed on the other, oftentimes a customized process is needed. Here, post-reflow defluxing and solder paste removal can actually be accomplished,
Test #
1 2
Solder Paste Type
NC Lead-free RMA
3 NC Lead-free 4
No-Clean
5 Water soluble 6 NC Lead-free 7
RMA
8 Water soluble 9
No-Clean 10 Water soluble
of setting up the most economical production line can become a monu- mental task for some manufacturers. Industry cleaning experts have set up state-of-the-art technical centers where proper process support includes hands-on cleaning demon-
Stencil
Elapsed Time after Printing
5 min. 5 min.
2 hours 2 hours 2 hours 4 hours 4 hours 4 hours 4 hours 4 hours
given adequate equipment and process window. However, the user has to consider the surface finish at all times. Organic solder protection (OSP) layers, for example, can become affected under certain conditions. Temperatures, cleaning agent formu- lations, exposure times and other variables can easily diminish OSPs. There are specific analytical
tests, such as OSP thickness meas- urement equipment, to measure the organic solder protection layer after the cleaning cycle in order to ensure that the assemblies are still viable. Dealing with the complexities of cleaning processes as well as the task
Delamination Yes -
No
- - - - - - - - -
Stencil compatibility and cleaning results with VIGON®
SC 202 at 25 percent concentration.
strations and trials. This help should include the assessment of any process limitations as well as ample assistance in topics related to other process specifics as well as economic requirements. Such a methodical approach can help engineers and companies differentiate between short-term as well as long-term cost savings and ultimately lead to a suc- cessful cleaning process. Contact: Zestron America,
11285 Assett Loop, Manassas, VA 20109 % 703-393-9880 fax: 703-393-8618 E-mail:
infoUSA@zestron.com Web:
www.zestron.com r
Soldered Stencil & Misprinted PCB
Cleaning
July, 2011
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