| PWB Cleanliness, Part 2 |
 |
 |
| Written by Terry Munson | |||
| Thursday, 31 January 2008 19:00 | |||
Localized extraction clearly shows residue volumes at isolated spots.
Ed.: This continues a discussion begun last month.We can define cleanliness from an observational or theoretical stance, but what do the data say? This is not to say a general understanding of the overall board is wrong, but rather that we need to understand the cleanliness of the critical areas. Using results from a large investigation of process variables, let’s look at two process variables on Umpire Test coupons.  Group 1 used dirty (high chloride and bromide residues from HASL) incoming boards processed with water-soluble solder paste and wave flux, then cleaned with DI water. Group 2 used dirty (high chloride and bromide residues from HASL) incoming boards processed with water-soluble paste and wave flux, then cleaned with DI water with 3% saponifier added to the wash solution. The testing plan included localized extraction of the 80-pin TQFP using the C3 extraction system; then the 68-pin LCC breakaway coupon was ROSE-tested and the total board was bag extracted for ion chromatography and modified ROSE testing. Group 1 (No. 26) Cl Br WOA OM600 SMD Bag extraction (total) 5.1 7.24 5.04 - ROSE testing (LCC) - - - 3.7 µg/in2 NaCl equiv Modified ROSE - - - 9.3 µg/in2 NaCl equiv 80-pin TQFP leads 9.51 14.57 11.43 - (C3 results 41 sec. = dirty) Group 2 (No. 72) Ch Br WOA OM600 SMD Bag Extraction (total) 1.05 3.22 1.53 - ROSE testing (LCC) - - - 3.0 µg/in2 NaCl equiv Modified ROSE - - - 8.1 µg/in2 NaCl equiv 80-pin TQFP leads 2.65 0.57 4.40 - (C3 results: 151 sec. = clean) Tables 1 and 2 show the Umpire Assembly SIR results for both groups.   The results reflect only a single board from each group of a much larger experiment. These individual boards are a good representation of the results of each group. Saponified cleaning (using low pressure) of the water-soluble flux on a dirty incoming bare board does a good job of removing ionic residues from the assembly. Conclusions The difference between bag and localized extraction of the boards showed a clear difference between the samples. The sample from Group 1 bag extraction shows acceptable chloride levels as defined by Foresite’s recommended limits (less than 6.0 µg/in2) after cleaning, but the localized level of contamination is high for chloride and bromide, with poor C3 electrical response (which correlates to assembly SIR performance). The comparison between Group 1 and Group 2 reveals the same issues: Bag extraction of the total board shows a lower ionic level, while localized extraction shows a pocket of ionic residue that, in this case, is acceptable and shows good electrical performance under C3 and SIR testing. But comparing the differences between Groups 1 and 2 for the localized residue pockets gives a better residue understanding than do the total board in a bag or ROSE and modified ROSE when compared to SIR performance. A localized extraction gives a clear view of the residue volume between pads on a critical circuit, while the general assembly cleanliness is a different data point, revealing gross surface residues over the entire board, a reflection of process conditions. Terry Munson is with Foresite Inc. (residues.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . His column appears monthly.
|
|||
| Last Updated on Wednesday, 06 February 2008 08:49 |
Columns
| European RoHS Enforcement Explained |
A series of workshops next month on compliance with RoHS and other directives will help US companies looking to break into the European market. |
| Read more... |
| Believing Foxconn Means Suspending Belief |
The Foxconn makeover is in full swing, with the latest this piece from the New York Times that supposes that the world's largest ODM is worried that Apple -- yes, Apple -- might be bringing it down: |
| Read more... |
Features
| Managing Your ESD Program |
The processes are as important as the tools. |
| Read more... |
| SMT Reflow Oven-to-Oven Repeatability |
How to adjust an oven so a single recipe will work across multiple ovens for an individual product. |
| Read more... |
Search
Search
Login
CB Login
Language
Language
Products
Alpha Introduces Lumet LED Materials
Lumet LED materials are for die attach and package, package on board, Luminaire module, power driver/supply and control systems. Lumet P33 Pb-free, no-clean solder paste for LED package-on-board...
Lumet LED materials are for die attach and package, package on board, Luminaire module, power driver/supply and control systems. Lumet P33 Pb-free, no-clean solder paste for LED package-on-board...
