Is white residue a reliability risk? Not all white residues pose a threat to performance.I am frequently asked if white residue is a problem. The answer depends on what the source of the white residue is and what caused it to turn white. [TEXT NOT REPRODUCIBLE IN ASCII ASCII or American Standard Code for Information Interchange, a set of codes used to represent letters, numbers, a few symbols, and control characters. Originally designed for teletype operations, it has found wide application in computers. ] The white flaky flaky - (Or "flakey") Subject to frequent lossage. This use is of course related to the common slang use of the word to describe a person as eccentric, crazy, or just unreliable. residues seen on assemblies are often due to flux and moisture. The variables that affect product performance include: the cleanliness of the bare board; whether the flux is no clean or water soluble; heat activation of no-clean fluxes; and cleaning. White Residue on No-Clean Assemblies Brush cleaning with water or isopropyl alcohol isopropyl alcohol: see isopropanol. (IPA IPA - International Phonetic Alphabet ) can leave a white residue that is possibly conductive and moisture absorbing. Rework occurs in sensitive areas of the assembly, creating added concern about the residue's harmfulness. Extra flux reacting with an area of surface-mount paste can also leave a white residue. But if fully heat activated, the flux residue is benign and not moisture absorbing or conductive. This is true only for assemblies with clean bare boards; dirty hot air solder 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 still pose a risk of electrical leakage. Possible Residue Causes on Cleaned Assemblies If white residue is on the topside surface between pads, it is most likely due to partial cleaning of the paste flux residues. If the residue is on the bottom side leading or trailing edges, the thicker flux buildup on the bottom side probably has been only partially cleaned. Case Study In one case study, we evaluated heavy white residue on the topside of an assembly around the reworked surface-mount static random access memories Static random access memory (SRAM) is a type of semiconductor memory. The word "static" indicates that the memory retains its contents as long as power remains applied, unlike dynamic RAM (DRAM) that needs to be periodically refreshed (nevertheless, SRAM should not be confused with (SRAMs). Each assembly showed visible residue after the hot gas surface-mount rework. The customer did not know if the residues would be harmful. Each assembly was processed with no-clean solder paste Solder paste (or solder cream) is a mix of small solder particles and flux. It is used extensively in the automated soldering processes wave soldering and reflow soldering. , then the SRAM See static RAM. SRAM - static random-access memory quad flat pack (QFP (Quad FlatPack) A square, surface mount chip package that has leads on all four sides and comes in several varieties. PQFP (Plastic QFP) may refer to all of the following QFP types. All quad flatpacks use gull-wing leads, except for the CQFP, which stick straight out. ) was removed before new SRAMs were placed and soldered using a hot gas 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. system. [ILLUSTRATION OMITTED] To determine the effects and residue type, our lab used surface insulation resistance Surface insulation resistance is a property of the material and electrode system. It represents the electrical resistance between two electrical conductors separated by some dielectric material. (SIR) testing and ion chromatography Ion-exchange chromatography (or ion chromatography) is a process that allows the separation of ions and polar molecules based on the charge properties of the molecules. per IPC (1) (InterProcess Communication) The exchange of data between one program and another either within the same computer or over a network. It implies a protocol that guarantees a response to a request. protocols. Ten SRAM areas were isolated to examine with SIR testing, and 10 separate SRAM areas were isolated to examine with ion chromatography. Our analyses showed that the residue was benign and that leaving it on the board would be better than trying to remove it. Conclusions If white residues are caused by flux residue from the rework process, they can function as a protective and insulative in·su·la·tive adj. Serving to insulate or keep safe: the insulative value of an animal's fur; insulative packing materials. barrier on the board. Good testing procedures and understanding of process residues will help you to understand if a white residue poses a reliability risk. [FIGURE 1 OMITTED]
TABLE 1: SIR and IC testing proves this customer's white residue is
benign.
Ion Chromatography (IC)
Sample Description (* separate areas Cl Br S[O.sub.4.sup.2-]
were used for IC & SIR testing sites)
Recommended Bare Board 2 6 3
Cleanliness Limits
No Clean Assembly 3 12 3
Reworked assemblies with white
residue
Area #1 1.29 9.27 2.45
Area #2 1.15 9.07 1.58
Area #3 1.08 8.46 2.39
Area #4 reference area 1.36 9.34 1.01
Saponified and steam clean boards
Area #1 0.28 3.79 0.74
Area #2 0.39 3.65 0.56
Area #3 0.24 3.71 0.64
Area #4 reference area 0.33 3.65 0.49
Bare boards prior to assembly
Area #1 1.95 2.94 2.04
Area #2 2.05 3.03 1.77
Area #3 1.87 3.21 1.36
Area #4 reference area 1.54 2.58 2.08
Ion SIR
Chromatography
(IC)
Sample Description (* separate areas WOA 85[degrees]C
were used for IC & SIR testing sites) /85%RH
Recommended Bare Board 0
Cleanliness Limits
No Clean Assembly 150
Reworked assemblies with white 168 hrs
residue
Area #1 147.30 Pass
Area #2 135.24 Pass
Area #3 140.39 Pass
Area #4 reference area 59.89 Pass
Saponified and steam clean boards
Area #1 23.26 Pass
Area #2 24.15 Pass
Area #3 23.38 Pass
Area #4 reference area 20.15 Pass
Bare boards prior to assembly
Area #1 0 Pass
Area #2 0 Pass
Area #3 0 Pass
Area #4 reference area 0 Pass
* All ion chromatography data is in [micro]g/i[n.sup.2]
Terry Munson is with Foresite Inc., Kokomo, IN; (765) 457-8095; email: ResiduGuru@aol.com; Web page: www.residues.com. |
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