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Cleaning Underneath 4 Mil Standoffs
December 31, 1969 |Estimated reading time: 6 minutes
The miniaturization process has not reached its physical limits in the electronics industry. The increase in component complexity results in a continuous decrease in standoff spacing between the components and the substrate's surface (currently 0.002 to 0.004", or 2 to 4 mil). This article looks at mechanical agitation cleaning systems for best cleaning results.
The question of which cleaning process can provide the required cleanliness levels under low standoff components must be addressed. The contacts found under components such as BGAs, microBGAs or CSPs pose mechanical barriers, meaning capillary penetration of the cleaning and rinsing agent is hampered.
As a result, the number of additional requirements for cleaning increases almost exponentially. A cleaning process should not only allow the cleaning media ample access to capillary spaces, but it also, among other things must lift off the residues and remove the latter from under the respective components.
A common notion implies that traditional spray- and immersion-based processes, using water- and/or solvent-based cleaning agents, are conditionally effective. Apart from the traditional mechanical assistance through ultrasound, spray-under-immersion and spray-in-air, this view prompted equipment processes that rely on centrifugal, high-pressure, vacuum and vapor-phase support.
(Left) Simulation of a CSP (0.5 mm = 0.02", 0.75 mm = 0.03"). (Right) SIR test comb.
To effectively simulate the influence of mechanical barriers on BGA-type connections, glass coupons were glued and soldered on top of a bumped CSP contact pattern, to which flux had previously been applied (Figure 1a).
Additional test substrates also were prepared to take electrical cleanliness levels into account. Solder paste and flux were applied on the standard comb structures before glass coupons were placed over the patterns and soldered.
The cleaning agent and processes were combined based on process feasibility (Table). This guaranteed that the product-characteristic and process-specific parameters were taken into account.
The criteria used to evaluate the results obtained were based on optical, ionic and electrical (SIR) cleanliness.
- Optical inspection was performed via 40X magnification, in accordance with IPC standard 610A.
- Ionic contamination measurements were performed with a 75 percent IPA/25 percent water mixture, in conformance with IPC test method 650. The glass coupon geometries and surfaces were not part of the surface area determination. The measured values for the ionic contamination can be seen as benchmark values. Three to five measurements for identical components were necessary to determine statistically meaningful values. For a measured surface area of 12 × 10", accuracy and reproducibility had a standard error rate of less than ±2 percent.
- Electrical cleanliness was performed based on SIR tests according to J-STD 001. Climatic storage was performed in a climatic exposure test cabinet. Temperature was 104°F/40°C; humidity was 92 percent rh; and the duration was four days.
- Surface insulation resistance was measured continuously using a measuring unit, with strain voltage of 10 V and measured voltage of 100 V.
An RMA flux-containing paste was chosen, which would be removable under identical mechanical assistance with all cleaning agents used during this study. The reference standoff height in this selection process was 0.004" (4 mil).
Results were evaluated based on the following questions:
What was the influence of the process type with regard to the cleaning result under components with a consistent standoff (0.004", 4 mil), non-varying cleaning cycle times and constant bath temperature? The individual processes showed good cleaning results underneath the glass coupons, given that the cleaning agent was able to dissolve the flux residues. The trend observed indicated best results for the ultrasonic HFE co-solvent, spray-in-air and spray-in-air in-line processes. For pressurized spray-under-immersion cleaning, however, residues remained in the border area of the coupons. In spray-assisted stencil cleaning equipment the effectiveness of the cleaning agent is reduced to its capillary and creeping ability. Consequently, clear visual residues remained in and around the adhesives in the corner of the glass coupon.
The different processes illustrate that besides the recirculative and spray-assisted stencil cleaning equipment, no significant differences regarding cleaning ability under capillaries and components were observed. To optimize the results for the latter two, one could alter the process parameters.
Figure 2. Test substrates with different standoffs prior to and post cleaning. (left column) BEFORE CLEANING at 50 µm, 100 µm and 200 µm (right column) AFTER CLEANING at 50 µm, 100 µm and 200 µm
What was the standoff effect on the cleaning result underneath components? The cleaning result between the smaller (0.002", 2 mil) and bigger standoff spacing (0.008", 8 mil) only slightly differed from the results obtained using the 0.004" (4 mil) standoff. Therefore, there was neither an improved result with higher standoffs nor a worse outcome for lower ones (Figure 2).
What was the influence of time and temperature in reference to the cleaning result underneath components for equal amounts of contamination and standoff? A temperature increase led to a drastic acceleration in cleaning, whereas an increase in the cleaning cycle time only marginally affected the results. Therefore, the conclusion is that a process optimization is easier achieved through increased temperature than a change in time. However, not the result but only the cleaning rate affects the overall outcome.
What is the impact of the component geometry in regard to the cleaning result under components? Using flux jelly, glass coupons were glued and soldered on top of pre-bumped CSP areas (30 and 20 mil, respectively) to simulate the influence of mechanical barriers of BGA-type connections. The standoff is predetermined by solder ball height, and was found to be 0.006" (6 mil, for the 0.03" pitch) and 0.002" (2 mil, for 0.02" pitch size), respectively. After comparing the cleaning results to all previous experiments, there was no significant difference. The conclusion is that the influence of mechanical barriers for a given grid pattern are negligible, and only the overall geometry of the capillary spaces is relevant.
Figure 3. SIR curves after ultrasonic dip tank cleaning and climatic storage.
An optical inspection for evaluation of the cleaning result proved to be the most simple and conclusive. For all ionic contamination measurements performed, values below 0.3 µg NaCl Eq./cm² were found. This confirms the results obtained during optical inspection. The electrical cleanliness levels underneath the glass coupons were ascertained via SIR test (Figure 3).
The relationship between component size and cleaning result also was studied. SOICs, as well as 1206, 0805, 0603 and 0402 ceramic capacitors were part of this analysis. The distance of the residues under the SOICs as well as the 0603 and 0402 capacitors did not indicate a problem for any medium or process. However, explicit flux and paste residues were noticed for 1206 and 0805 ceramic capacitors. This can be attributed partly to the smaller standoff (<0.001", 1 mil) when compared to flip chip and microBGAs. The capillary forces of the cleaning agent are no longer sufficient to ensure adequate rinsing under the component. Consequently, the cleaning process for standardized conditions reaches the application limitations, observed for 1206 and 0805 chip capacitors.
Conclusion
The study shows no classical "winner" for mechanically supported cleaning methods, since most technologies differed only in nuances. Penetration of capillary spaces between base substrate and component by the noncontaminated cleaning agent was found to be less problematic. This study also determined that the actual rinsing ability of the cleaning medium underneath the components was altered, mostly because of its now compromised surface penetrating properties. This led to the conclusion that an efficient rinsing after cleaning is important, and even critical, to overall process performance.
Apart from the particular standoffs, component geometry, as well as its material of construction, heavily influence the limitations of cleaning under components.
For those reasons, BGAs, microBGAs and CSPs, often considered difficult to clean, are relatively unproblematic. Due to their more specific capillary behavior, however, 1206 and 0805 ceramic capacitors are critically sized components.
Andreas Muehlbauer, Ph.D.; Helmut Schweigart, Ph.D. and Stefan Strixner may be contacted at ZESTRON America Corp., 21641 Beaumeade Circle, Suite 315, Ashburn, VA 20147; Web site http://www.zestron.com.