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Services: Process Assessment
Design of Experiment (DOE) for Process Residue Baseline Cleanliness Assessment
This dendrite grew, shorted, and failed a medical device in the field. >View larger image

Root Cause: corrosive and moisture-absorbing board fabrication and low solids flux residues in an office environment operating with only 2.5 volts. How robust is your assembly process at removing or encapsulating both assembly and fabrication residues?


Foresite Baseline Process Assessment Package
Umpire Mixed Technology Test Board

Purpose
Electronic assemblers are experiencing field performance problems due to processing and fabrication residues that are corrosive and conductive. These residues cause electromigration (metal migration) and electrical leakage failures (No Trouble Found — NTF returns) under typical operating conditions (humidity levels at 50% and higher). Historical industry-standard process cleanliness tools and test methods (IPC TM 650 2.3.25 Resistivity of Solvent Extract) are not adequate gauges of product cleanliness when testing today’s low solids and water-soluble fluxes. Due to poor extraction conditions these tools give false low levels and do not identify if a residue is corrosive or insulative. The need for a process assessment approach that identifies the contaminants and determines their electrical effects has lead to our development of this assessment package.

Goals
The goals are to determine if the residues from an electronic assembly process are electrically detrimental and to define (type and level of contaminant) the thoroughness of cleanliness required to build long lasting reliable hardware.

Overview
This assessment package will take an innovative approach toward understanding the residue effects of product assembly and rework processes. Using a component driven designed electrical assessment board (Umpire) with specific fabrication cleanliness levels, we will be able to evaluate the electrical effects associated with the manufacturing processes and materials.

The strategy applied in this package uses the Foresite Umpire board as the vehicle to evaluate the residual cleanliness using ion chromatography (IC) and electrical performance assessment with a modified surface insulation resistance (SIR) testing. This modified SIR technique will measure electrical resistance lead to lead and hole to hole. Three temperature/humidity conditions (35C/85%RH, 65C/85%RH, 85C/85%RH) will be applied for a one week period. This package has 16 SIR measurement sites. These sites are designed to measure the component pad to pad, lead to lead and entrapment effects on a variety of component styles (BGA, LCC, QFP, DIP, PGA, 1206 chip, 0805 chip, and comb patterns (B24, Bellcore)).

In addition, the Foresite-supplied Umpire board is provided with three levels of fabricator cleanliness typical of HASL processing (chloride levels ranging Level 1=2-3 ug/in2, Level 2=6-9 ug/in2, and Level 3=16-20 ug/in2). The three levels of contamination are used to determine the robustness of the assembly process in dealing with a typical range of fabrication contaminants. These test boards will be built using the customers established procedures for mixed assembly techniques, topside SMT, bottom side SMT (chipbonder adhesive), wave solder, and rework hand soldering.

Assessment Package Deliverables
  • Determine if the process residues are electrically detrimental (areas below the component, lead to lead, and on the open comb patterns).
  • Determine the type and quantity of residues associated with each process step.
  • Determine if the process is robust against fabricator contamination.
  • Evaluate entrapment effects of component packaging.
  • Evaluate process interaction effects.
  • Establish cleanliness requirements based on electrical performance.
  • Establish a residual and electrical performance baseline from which to make improvements.
  • Evaluate the electrical and ionic effects of the temporary soldermask material.
  • Detailed comprehensive report including photo documentation.
Assessment Package Matrix
The following matrix shows the required samples and associated testing.
Fifteen Umpire boards will be used to perform the following tests


  SIR tests IC tests
1. Bare unprocessed Umpire boards (L1-3) 3 3
2. Processed Umpire boards (SMT/B-SMT/Wave/Hand) 12 24
3. Bare unprocessed boards of Customer 0 3
4. Samples of solderpaste, flux and cored soldered 0 3
5. Water samples If cleaner is used (wash, rinse, incoming) 0 3
Total tests 15 36

Assessment Package build kit will include the following
  1. Process documentation traveler and assembly procedure document
  2. Stencil (on loan from Foresite)
  3. Placement coordinates Gerber file disk
  4. Two assembly packages, labeled Setup and Product
  5. Setup package includes 2 Umpire boards, 2 parts kits and instruction set
  6. Product package includes 12 Umpire Assemblies + 3 unprocessed control boards and 12 parts kits (no parts placed on control boards).
  7. Parts Kit includes
    • BGA component (272 IO (25 mil spacing)) (2 in Setup and 12 in Product)
    • LCC component (68 IO (25 mil spacing)) (4 in Setup and 24 in Product)
    • QFP component (80 pin (10 mil spacing)) (2 in Setup and 12 in Product)
    • PGA socket (172 pin (50 mil spacing)) (2 in Setup and 12 in Product)
    • DIP component (16 pin (25 mil spacing)) (8 in Setup and 48 in Product)
  8. Pre-labeled Kapak Bags for sample protection after processing
  9. Sample collection containers for flux, solderpaste, and cored solder
  10. Water collection containers for cleaner samples (wash, rinse and incoming)
  11. Project shipping kit and check-off sheet
Additional samples defined in matrix to be provided by the customer
  1. Bare unprocessed boards from customer’s inventory (3 separate boards), record part numbers on traveler, and place boards in supplied labeled bag.
  2. Samples of solderpaste (1-2 grams), flux (0.5-1.0 oz), and cored solder (3 inches)
  3. Water samples (if cleaner is used) from wash, rinse, and incoming feed water
Assessment Package Setup
The following will detail the building and processing details.

Setup Build Procedure — 2 Umpire boards
Umpire Board (Top) Umpire Board (Bottom)
  1. Remove the setup packet containing boards S-1 and S-2, parts kit and Gerber part placement data
  2. Take board S-1 as the primary set up board, Setup printer, Setup topside component coordinates
  3. Print topside S-1, place topside parts provided in kit (BGA, 2 LCC’s, QFP), see build sheet #1, and inspect board
  4. Reflow (Clean if required by established procedures) and inspect S-1
  5. Setup bottom side adhesive coordinates, setup part placements coordinates
  6. Apply adhesive on bottom side and place resistor parts on S-1 board using the 1206’s and 0805’s from customers stock (not included in parts kit)
  7. Glue Cure and inspect
  8. Hand place through hole components in S-1 provided in parts kit (PGA, 4 DIP’s and connectors)
  9. Wavesolder S-1 (Clean if required by established procedures) and inspect
  10. Rework operation on three areas on S-1 (Bellcore, B-24-3, and Head–2 patterns)
  11. Each pattern will use cored solder and flux applied with a soldering iron on all exposed solder surfaces of the patterns, care should be taken not to bridge the combs. Standard debridging techniques are acceptable at removing bridges.
  12. Inspect S-1 for standard assembly criteria
  13. Attach thermal couples to S-1 per instructions and profile reflow oven, glue cure oven, wave solder and cleaning system (if possible). If any modifications are needed to optimize board processing make the changes before building S-2.
  14. Build S-2 steps 1-13 using the parameters documented in step 14, inspect and then document all parameter settings (on traveler) prior to production build
Assessment Package Build Procedure
Build Procedure — 12 Umpire boards
  1. 1. Remove the setup packet containing boards B-1 to B-12, parts kit
  2. 2. Print topside B-1, place topside parts provided in kit (BGA, 2 LCC’s, QFP per board), see build sheet #1, and inspect board
  3. 3. Print and place topside parts on boards B-2 to B-12
  4. 4. Reflow all boards (Clean if required by established procedures), inspect B-1 to B-12, and document any defects on defect sheet
  5. 5. Apply adhesive on bottom side and place resistor parts on boards B-1 to B-12 using the 1206’s and 0805’s from customers stock (not included in parts kit)
  6. 6. Glue cure all boards, inspect and document any defects on defect sheet
  7. 7. Hand place through hole components in B-1 to B-12 provided in parts kit (PGA, 4 DIP’s and connectors per board)
  8. 8. Wavesolder all boards, (Clean if required by established procedures), inspect and document any defects on defect sheet
  9. 9. Rework (all boards) operation on three areas on B-1 to B-12 (Bellcore, B-24-3, and Head –2 patterns) and document any defects on defect sheet
  10. 10. Each pattern will use cored solder and flux applied with a soldering iron on all exposed solder surfaces of the patterns, care should be taken not to bridge the combs. Standard debridging techniques are acceptable at removing bridges.
  11. 11. Inspect B-1 to B-12 for standard assembly criteria and document any defects on defect sheet
  12. 12. Place all samples into labeled Kapak bags for shipment to Foresite Inc.
Analysis of the Assemblies
Elements of Ion Chromatography analysis
(per IPC TM 650 2.3.28):

  1. The Extraction
    is a solvent/water system (IPA 75% / DI H20 25%) extraction that is done at 80oC (+0.5oC) for 1 hour (+ 1.0 min) in KAPAK™ bag suspended inside a water bath.
  2. The analytical system
    is a Dionex DX120 or equivalent, where 3 to 5 mls of extract solution is injected into the Ion Chromatograph that will separate the ionic and organic elements based on molecular weight and valence charge strength. This output (conductance changes) is measured and plotted over time. Typical sample runs range anywhere from 5 to 30 minutes.
  3. The Species Identification
    is done using known standards (NIST traceable) to identify the specific elution time for each species. Typical species identified on an anion column are fluoride, chloride, methane sulfonic acid (MSA), bromide, nitrate, nitrite, phosphate, formate, acetate, abeitic acid (rosin), sulfate, and weak organic acids (WOA). Typical WOA low residue flux activators are succinic acid, gultaric, malic, and other similar weak acids that we combine into one peak labeled as WOA. With typical cation residues identified as sodium, potassium, ammonia, calcium and magnesium.
  4. The Data Analysis
    is the most important element in the proper identification of type and level of extractable residues, because it identifies what their impact on reliability will be. This data analysis forms the foundation for understanding the type and level of each residue and it
The Extraction is a solvent/water system (IPA 75% / DI H20 25%) extraction that is done at 80oC (+0.5oC) for 1 hour (+ 1.0 min) in KAPAK™ bag suspended inside a water bath.

The analytical system is a Dionex DX120 or equivalent, where 3 to 5 mls of extract solution is injected into the Ion Chromatograph that will separate the ionic and organic elements based on molecular weight and valence charge strength. This output (conductance changes) is measured and plotted over time. Typical sample runs range anywhere from 5 to 30 minutes.

The Species Identification is done using known standards (NIST traceable) to identify the specific elution time for each species. Typical species identified on an anion column are fluoride, chloride, methane sulfonic acid (MSA), bromide, nitrate, nitrite, phosphate, formate, acetate, abeitic acid (rosin), sulfate, and weak organic acids (WOA). Typical WOA low residue flux activators are succinic acid, gultaric, malic, and other similar weak acids that we combine into one peak labeled as WOA. With typical cation residues identified as sodium, potassium, ammonia, calcium and magnesium.

The Data Analysis is the most important element in the proper identification of type and level of extractable residues, because it identifies what their impact on reliability will be. This data analysis forms the foundation for understanding the type and level of each residue and it’s corrosive behavior (or insulative) and it’s reliability effect on field performing hardware.

Areas of ionic analysis by IC on the Umpire board are as follows:
  • Standard mixed processing assessment area
    the LCC area break off coupon (68pin (25 mil spacing)) with a 6 mil comb pattern
  • Rework assessment area
    the break off coupon of the Bellcore comb pattern with 50-mil spacing
  • Temporary soldermask assessment area
    the through holes on the LCC break off coupon labeled E2, E1, Gnd, M1, and M2
Surface Insulation Resistance (SIR) testing will be done per Foresite work Instruction

  1. All 15 umpire boards will be placed into a single chamber (3 unprocessed control boards + 12 processed assemblies) after visual and photo documentation has been taken.
  2. Initial resistance measurements will be taken at ambient conditions and recorded.
  3. The boards will be exposed to three temperature levels, while at a constant humidity during the 168 hour conditioning exposure.
  4. Bias of 50 volts will be applied throughout the test, (the bias will be applied after the boards have reached equilibrium at each new temperature condition)
  5. Six measurements (minimal) will be taken each day with a 100-volt measurement voltage and a one-minute soak prior to making the measurement.
  6. Each board has 16 measurement sites for SIR and 42 measurements of these sites will be taken over the 168-hour period for all 15 boards. (15 boards x 16 SIR sites=240 sites x 6 measurement a day x 7 days=10,080 SIR data points collected during the 168 hour test)
  7. Areas of the board that will be evaluated by SIR
    1. BGA area (272 IO (30mil spacing))
    2. LCC1 area (68pin ( 25 mil spacing))
    3. LCC2 area with a 6 mil comb pattern
    4. QFP1 area (80 pin (10 mil spacing))
    5. QFP 2 area with a 6 mil comb pattern
    6. PGA socket (174 pin (50 mil spacing))
    7. DIP 2 area 1 (16 pin (25 mil spacing) with Y comb patterns
    8. DIP 2 area 2 (16 pin (25 mil spacing) with Y comb patterns
    9. DIP 1 area 1 (16 pin (25 mil spacing) hole to hole
    10. DIP 1 area 2 (16 pin (25 mil spacing) hole to hole
    11. B-24-3 pattern for Rework by
    12. B-24-1 pattern for standard wave solder effects
    13. B-24-2 pattern for standard soldermask effects
    14. Head 2 pattern (topside) will be Reworked
    15. Head 1 pattern will measure bare board contamination effect
    16. Head 3 pattern will be used to assess the temporary soldermask
    17. Bottom Side SMT will be biased and visually inspected for dendrites with parts removed
  8. Data Comparisons will be made between the L1, L2, and L3 effects on all the component /comb areas and between SMT-top, SMT-bottom, Wave Solder, and Hand Soldering. Other comparisons between the component areas described above and the varying environments will show the differences between class 1, 2, and 3 conditions. Visual and microscopic inspection of all assemblies will be documented after testing. Comparisons will be made between standard processing and areas covered with temporary soldermask materials.
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