PWB Cleanliness, Part 2 Print E-mail
User Rating: / 0
PoorBest 
Written by Terry Munson   
Thursday, 31 January 2008 19:00

Localized extraction clearly shows residue volumes at isolated spots.

Process Doctor 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.

Image

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.

Image

Image

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

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

KIC Debuts K2 Thermal Profiler
K2 thermal profiler has plug-and-play hardware and a graphical user interface said to make profiling both quick and easy. Enables each thermocouple to use its own unique process window, while...