The Miniaturization Paradox Print E-mail
User Rating: / 0
PoorBest 
Written by Dr. Mike Bixenman   
Monday, 09 May 2011 12:28

A tradeoff of higher functioning electronics devices is that cleaning becomes necessary.

Many governments and non-government organizations support increasingly stringent regulations over air quality, water discharge, recycling practices, and toxic materials within products. But the subsequent change in materials creates a series of tradeoffs.

As Jensen and Lasky1 report, the removal of halogens from solder pastes and fluxes will have the greatest potential impact to the board assembly process. Halogens in soldering materials aid the soldering process by providing a powerful oxide removal, which enhances wetting. The transition to Pb-free alloys that do not wet as well as eutectic SnPb means the enabling technology will be the flux composition.

Cleaning agent suppliers strive to improve cleaning performance with less active chemistry. Advances in aqueous cleaning agents have targeted no-clean flux
residues, the choice of many high-reliability assemblers that clean post-assembly. But flux compositions are moving targets as solder suppliers formulate new flux compositions aimed at smaller components.

Smaller circuit board features and components render hardware more prone to environmental effects.2 Over the past 10 years, the size of electronics has been reduced over 70%. For flip chip ICs, miniaturization amounts to greater than 90%.3 Tighter spacing increases the electric field. At constant voltage, the electric field between conductors rises inversely with the conductor spacing. As the distance between conductors reduces, the risk of electrochemical migration increases.4 

Miniaturization makes proper cleanliness more important, yet more difficult.3 As spacing between conductors reduces, increased frequency and finer pitch spacing are contributing time-to-failure factors.4 Smaller spacing tends to be more difficult to clean, potentially resulting in a buildup of contaminants.4

Bumiller, Pecht, & Hillman reported5 on the potential for electrochemical migration, as spacing on boards and component pitch decreases. At low contamination levels (0 to 2 µg/in2 of chloride ions), dendritic growth was mainly found on the 0.00625" spacing comb structure, with infrequent appearances on the 0.0125" comb structure, and no occurrence on the 0.025" comb structure. At 5 to 20 µg/in2 chloride ions, dendritic growth was found on both the 0.00625" and 0.0125" spacing comb structures, with infrequent appearance on the 0.025" spacing comb structure. At 50 µg/in2 chloride ions, dendritic growth was no longer distinguishable.

With cleaning an enabler for reliability, a number of design-for-cleaning factors are worth consideration. Influential cleaning factors include the soldering process, solder flux composition, soldering temperatures, baking boards post-solder but pre-cleaning, under-clearance depth of components, static cleaning rate and cleaning equipment. Variations in any of these factors can and do influence the cleaning rate.

Each alloy composition has unique properties such as melting point, hardness, and solid to liquid transition phases. Poor base metal wetting results in a poor solder joint. To control the rate of oxidation, critical process variables such as the soldering flux composition, temperature control, soldering atmosphere, and flux compositions must be optimized. These factors influence the cleanability of the flux residue.

Pb-free, no-clean flux compositions are formulated with higher molecular weight flux vehicles that improve thermal stability and oxygen barrier during soldering. Flux residues from these higher molecular weight compositions have a greater degree of product-to-product variation from suppliers, form hard resinous barriers, and are more difficult to clean.

The reflow solder profile progressively starts by evaporating flux volatiles, initiates flux activation, raises components to be joined to a temperature sufficiently consistent for the solder to flow evenly onto all surfaces, and reflows the paste over board finishes to facilitate solder connections. Temperature excursions and the time exposed to liquidus solder temperatures influence cleaning properties.

Exposing flux residues to excessive heat over long periods of time can polymerize, oxidize and harden flux residues. Changes to normal process procedures can alter the nature of the flux residue. A residue that is cleanable when processed under normal conditions might not be when exposed to elevated temperatures over an extended time.

As component pitch reduces, use of leadless packages increases. The distance from the board surface to the bottom side of leadless components is consistently less than 0.002". Component miniaturization decreases conductor spacing. The problem then is, during solder reflow, flux underfills the bottom side of the component. To clean the residue, the cleaning process must break through the flux dam to create a flow channel. For many no-clean flux residues, penetrating low-clearance gaps requires a cleaning agent that matches with the flux residue, impinging forces that deliver the cleaning agent to the residue, wash temperature and wash time.

A tradeoff of green materials and miniaturization is the risk of ion migration from flux residues that bridge the conductors. Soldering processes must be designed to provide increased clearance under components and control the temperature exposure.

Dr. Mike Bixenman is cofounder and CTO of Kyzen Corp. (kyzen.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

References
1. T. Jensen, and R. Lasky, “Challenges Toward Implementing a Halogen-Free PCB Assembly Process,” IPC Apex, April 2010.
2. ASHRAE Technical, Gaseous and Particulate Contamination Guidelines for Data Centers, www.docstoc.com/.../Gaseous-and-Particulate-Contamination-Guidelines-for-Data-Centers.
3. D. Minzari. et al., “Electrochemical Migration on Electronic Chip Resistors in Chloride Environments,” IEEE Transactions on Device and Materials Reliability, vol. 9, no. 3, September 2009.
4. E. Bumiller and C. Hillman, “A Review of Models for Time-to-Failure Due to Metallic Migration Mechanisms,” white paper, www.dfrsolutions.com/uploads/white-papers/Time-to-Failure_Metallic_Migration.pdf.
5. E. Bumiller, M. Pecht and C. Hillman, “Electrochemical Migration on HASL Plated FR-4 Printed Circuit Boards,” SMTA Pan Pacific Symposium, February 2004.

Last Updated on Monday, 09 May 2011 18:19
 

Columns

Eastern-US: China’s New Competitor?

Parity emerges among EMS Factories from Asia, Mexico and the US.

For the first time in years we see parity in the Eastern US among EMS factories from Asia, Mexico and the US. This EMS market condition will permit American OEMs (the EMS industry refers to OEMs as customers) to have more EMS pathways to choose from. Now more than ever, such EMS assignments will require deeper investigation relating to the OEMs’ evaluation of manufacturing strategies.

Read more...
 
The Human Touch

For those who count on the electronics industry for big feats, it’s been a remarkable couple of years.

Read more...
 

Features

Advances in Concentration Monitoring and Closed-Loop Control

Contaminated bath water skews refractive index results. New technology can accurately measure aqueous cleaning agent concentration.

Read more...
 
Circuits Disassembly: Materials Characterization and Failure Analysis

A systematic approach to nonconventional methods of encapsulant removal.

Read more...
 

Search

Search

Login

CB Login

Language

Language

English French German Italian Portuguese Russian Spanish
 

Products

Panasonic Debuts PanaCIM Maintenance with Augmented Reality
PanaCIM Maintenance with Augmented Reality software provides instant communication and information to factory technicians -- when and where it is needed -- so they can respond to factory needs more...