The key to success: examine key parameters for a successful no-clean assembly.While investigating a medical product sensitive to stray voltage stray voltage accumulation of low voltages in the metalwork of a milking parlor due either to leakage from poor wiring or to poor earthing (grounding) with no outlet for static electricity. Very small voltages cause restlessness and a fall in milk yield. , my company discovered that the original bare hot-air solder solder (sŏd`ər), metal alloy used in the molten state as a metallic binder. The type of solder to be used is determined by the metals to be united. Soft solders are commonly composed of lead and tin and have low melting points. Hard solders (i. level (HASL (language) HASL - SASL plus conditional unification. ["A Prological Definition of HASL, A Purely Functional Language with Unification Based Conditional Binding Expressions", H. Abramson in Logic Programming: Functions, Relations and Equations, D. DeGroot et al eds, P-H 1986]. ) boards passed all process validation and accelerated environmental stress screening Environmental stress screening (ESS) refers to the process of exposing a newly manufactured product or component (typically electronic) to stresses such as thermal cycling and vibration in order to force latent defects to manifest themselves by failure during the screening process. (ESS (1) (Electronic Switching System) A large-scale computer from Lucent used to route telephone calls in a telephone company office. The 5ESS is a Class 5 central office switch, and the 4ESS is a Class 4 tandem office switch. ) when built up. However, when the full production units were run, they showed early failures in the field and were classified as no trouble found (NTF NTF No Transaction Fee NTF National Turkey Federation NTF No Trouble Found NTF National Transfer Format (UK Geographic Data Standard) aka BS7567 NTF Nigeria Trust Fund NTF National Transonic Facility NTF Noise Transfer Function ). Investigating NTF Failures The NTF numbers continued to increase and a detailed investigation took place. The units showed failures in boxed assemblies prior to shipment. Three months into product launch, the assemblies were tested for ionic i·on·ic adj. Of, containing, or involving an ion or ions. ionic pertaining to an ion or ions. ionic medication iontophoresis. cleanliness Cleanliness See also Orderliness. Cleverness (See CUNNING.) Berchta unkempt herself, demands cleanliness from others, especially children. [Ger. Folklore: Leach, 137] cat continually “washes” itself. . The assemblies showed levels of bromide bromide, any of a group of compounds that contain bromine and a more electropositive element or radical. Bromides are formed by the reaction of bromine or a bromide with another substance; they are widely distributed in nature. that were 10-15 times higher than the original qualification data. All failures showed no visible corrosion or leakage pathways and passed testing when baked for three hours at 85[degrees]C. Electrical measures confirmed the failures. We began a detailed investigation of the bare board and assembly process. With a 9% failure rate--growing every month--understanding what variables affected the circuit performance was important. A data review shows key differences with the weak organic acid (WOA WOA Wacken Open Air (music festival) WOA Work of Art WOA Western Orthopaedic Association WOA Web Offset Association (Nashville, TN) WOA World Airways, Inc (ICAO code) ), bromide, chloride and sulfate sulfate, chemical compound containing the sulfate (SO4) radical. Sulfates are salts or esters of sulfuric acid, H2SO4, formed by replacing one or both of the hydrogens with a metal (e.g., sodium) or a radical (e.g., ammonium or ethyl). levels. The current production data showed these levels to be much higher. What was creating the failures and why is there a difference between the two groups? Bare Boards The data from the original group was low in all the anions reported. The current production boards show higher chloride, bromide and sulfate levels, indicating a poor cleaning process and possibly a different line. The boards for production came from a new, high-volume automated process line that was not yet qualified. The level of bromide from the HASL flux can cause the stray voltage leakage problem. The levels of chloride and sulfate are only slightly elevated; when the bromide level is reduced, the other residues will also be reduced. Surface-Mount Technology/Reflow The surface-mount technology Surface mount technology (SMT) is a method for constructing electronic circuits in which the components (SMC, or Surface Mounted Components) are mounted directly onto the surface of printed circuit boards (PCBs). reflow (1) The process of heating and melting the solder that has been screen printed onto a printed circuit board in order to bond chips and other components to the board. Surface mount chips (SMT) use the reflow method. Contrast with wave soldering. See also reflowable text. data show similar results for the reported ionic levels. The levels are typical for the no-clean paste and thickness used. The levels of ionics from the reflow step pose minimal risk for electrical leakage or corrosion problems. Wave and Hand Soldering soldering Process that uses metal alloys with low melting points to join metallic surfaces without melting them. Tin-lead solders, once widely used in the electrical and plumbing industries, are now replaced by lead-free alloys. The product passed and performed well when exposed to high humidity. But the current production units show much higher WOA levels that can cause the stray voltage due to excess flux that has not been completely heat-activated. Since the bromide from the HASL flux can also cause these failures, we must evaluate clean, bare boards with the current parameters and the reduced optimal parameters from applying the flux during wave and hand soldering. Conclusions The cleaned bare boards show lower ionic residues when built using the current production specs (SPECificationS) The details of the components built into a device. See specification. , but fluxing parameters have little effect on the final performance because they still fail. When we use the current bare boards and reduce the fluxing parameters, the boards still fail testing. Only when the cleaned bare boards are built with optimized flux levels do we see good performance and results similar to the original qualification samples. [ILLUSTRATION OMITTED]
TABLE 1: ESS testing results highlight the crucial parameters.
all values are Ion Chromatography
[micro]g/i[n.sup.2]
Sample Description Cl- Br S[O.sup.2-.sub.4] WOA
Original Qualification Boards
and Data
Bare HASL Boards Original 0.27 1.36 0.18 0.00
HASL / SMT No Clean Paste 0.21 4.36 0.00 9.35
Reflow
HASL / SMT Reflow / Selective 0.24 5.12 0.00 77.37
Soldering No Clean wave
HASL / SMT Reflow / Selective 0.19 5.39 0.00 99.36
Soldering / Hand No Clean
Current Production Samples
Bare HASL Boads 3 months later 3.54 14.77 1.54 0.00
HASL / SMT No Clean Paste 2.45 18.91 0.00 11.59
Reflow
HASL / SMT Reflow / Selective 2.27 20.12 0.00 182.36
Soldering No Clean Wave
HASL / SMT Reflow / Selective 2.05 21.39 0.00 223.74
Soldering / Hand No Clean
Assessment of Changes from
Current Production
Cleaned bare HASL boards 0.29 1.08 0.11 0.00
HASL / SMT Reflow / Soldering / 0.21 4.57 0.00 208.86
Hand / Current Parameters
Cleaned bare HASL boards 0.22 1.11 0.09 0.00
HASL / SMT Reflow / Soldering / 0.20 5.02 0.00 105.36
Hand / Reduced Parameters
Bare HASL Boads 3 months later 3.21 16.54 1.33 0.00
HASL / SMT Reflow / Soldering / 2.15 20.98 0.00 97.48
Hand / Reduced Parameters
all values are ESS
[micro]g/i[n.sup.2] Testing
Sample Description
Original Qualification Boards and 65C/85%
Data
Bare HASL Boards Original not tested
HASL / SMT No Clean Paste Reflow not tested
HASL / SMT Reflow / Selective not tested
Soldering No Clean wave
HASL / SMT Reflow / Selective Pass
Soldering / Hand No Clean
Current Production Samples
Bare HASL Boads 3 months later not tested
HASL / SMT No Clean Paste Reflow not tested
HASL / SMT Reflow / Selective not tested
Soldering No Clean Wave
HASL / SMT Reflow / Selective Fail
Soldering / Hand No Clean
Assessment of Changes from
Current Production
Cleaned bare HASL boards not tested
HASL / SMT Reflow / Soldering / Fail
Hand / Current Parameters
Cleaned bare HASL boards not tested
HASL / SMT Reflow / Soldering / Pass
Hand / Reduced Parameters
Bare HASL Boads 3 months later not tested
HASL / SMT Reflow / Soldering / Fail
Hand / Reduced Parameters
Terry Munson is with Foresite Inc., Kokomo, IN; (765) 457-8095; email: ResiduGuru@aol.com; Web page: www.residues.com. |
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