【SMT资料】FluxSolder ability（英文）

1、fluxes & solder ability http:/ fluxes & solderability what is a flux? what does it do? different types of fluxes? flux testing & classification what is solderability? what can effect solderability? how do we measure solderability? http:/ what is soldering? in order to get a solder joint, the surface

2、s to be soldered must be free from dirt and oxides. oxides are formed during the hot soldering process. therefore a flux must be used. the flux cleans and protects the joint during the soldering process. http:/ flux a fluxs job is to remove the oxide and other surface contaminants from the materials

3、 to be soldered and the solder itself. if this is not done the solder cannot wet the surfaces to be soldered. the flux will also form a protective layer over the joint, preventing further oxidation during the soldering process. the flux will also aid heat transfer. http:/ flux substratesubstratesubs

4、trate http:/ types of fluxes fluxes may be found in various forms: solid flux within a solder wire liquid flux for flow solder machines thick flux medium within a solder paste gels or pens for rework the same principles apply for all the above http:/ solder wires http:/ liquid fluxes http:/ liquid f

5、luxes usually based on alcohol (typically 90-98%) may be rosin free or rosin containing usually sprayed onto the board the board is then heated (to evaporate the solvent) then passed over a “wave” of molten solder newer fluxes are water based. http:/ http:/ http:/ http:/ http:/ foam fluxer http:/ fl

6、ux spray nozzle http:/ http:/ http:/ ideal operating conditions http:/ liquid fluxes http:/ http:/ http:/ pioneer retro-flow nozzle circa 1973 stationary area solder bath uni directional laminar wave http:/ dual wave technology http:/ solder paste 0.5mm powder particles flux medium http:/ http:/ flu

7、x medium a flux medium imparts certain characteristics to a solder paste activity rheology screen & tack life residue characteristics http:/ flux medium usually rosin based (50-70%) activators (usually halide) solvents viscosity modifiers http:/ rosin usually extracted from pine trees may be solid o

8、r a thick liquid at room temperature chemical composition may vary from year to year natural product that has been used for soldering since roman times it has a natural fluxing action may be chemically treated to form modified rosins http:/ physical properties of rosin l melts (softens) at 80c ester

9、s & polymers slightly different l water solubility low l resistant to moisture l electrical resistance high l absorbs oxygen turns yellow, then brown reaction products catalyse process http:/ advantages of modified rosin higher softening point as high as120o c (248 o f) increased thermal stability c

10、lear residues better chemical stability consistency (lot to lot performance) residues tack free and dry http:/ acid reactions of rosin ll ll in solvent and by fusion rc-oh + mx - rc-om + hx m = sn, pb, cu x = oxide, hydroxide, carbonate metal rosinates * are soluble in rosin (lead 40 w/w %, copper 8

11、 w/w %) * are invisible in rosin (lead salt - clear copper salt - red/brown) * neutralize acidity of rosin * increase softening temperature of rosin oo http:/ activators for rosin carboxylic acids - stronger complexants - more reactive to metal oxides - include: long chain fatty acids (palmitic, ste

12、aric) polycarboxylic acids (adipic, succinic) monocarboxylic acids (acetic acid) amine hydrohalides - rupture oxide films activators soluble in rosin metal salts soluble in rosin http:/ flux activators caboxylic acids rosin monocarboxylic acid ( r-c02h) dicarboxylic acid ( h02c-r-co2h halides fluori

13、de f- chloride cl- bromide br- iodide i- astatide at- http:/ carboxylic acid activation (r-co2h)2 + mo = (r-co2)2m + h2o metal salts thus formed disolve safely in rosin http:/ halide activation r3-nhcl equilibrium between r3-n + hcl 2hcl + mo = mcl2 + h2o metal salts thus formed disolve safely in re

14、sin http:/ halide activators very effective - high yields high halide pastes must be cleaned no-clean materials should pass appropriate tests (eg. ipc, bellcore, j-std) http:/ http:/ various industry protocols for example . j-std-004 (january 1995) replaced ipc-sf-818 (january 1988) bellcore gr-78-c

15、ore issue 1 (september 1997) replaced tr-nwt-000078 issue 3 (december 1991) http:/ various industry protocols these test . aspects of composition interaction with copper (corrosion?) surface insulation resistance (how conducting are flux residues?) electromigration (do flux residues give rise to met

16、allic dendrite formation?) http:/ various industry protocols other tests . corrosion tests outside of these protocols resistivity of water extract (an indication of composition) ionic contamination on soldered pcb http:/ j-std-004 flux induced corrosion (copper mirror) halide (cl- and br-) test with

17、 agcro4 paper fluoride test quantitative halides flux solids (non-volatiles) flux corrosion sir http:/ copper mirror thin copper coating (50 nm) on glass 23c, 50% rh, 24 hrs removal of copper film? http:/ copper corrosion flux solids used to reflow alloy on copper coupon 40c, 93% rh, 10 days corrosi

18、on? http:/ j-std-004 sir comb pattern printed reflowed using standard profile 85c, 85% rh, 7 days, 50v dc bias surface resistance measured with 100 v dc reverse polarity 24, 96, 168 hours 1 x 108 passmark http:/ j-std-004 sir comb http:/ sir ? assesses propensity of flux residue to absorb water and

19、create a conducting pathway between conductor tracks governed by residue, pattern, temperature, rh and bias voltage http:/ bellcore electromigration comb pattern printed reflowed using standard profile 65c, 85% rh, 21 days, 10v dc bias sir measured with 100 v dc same polarity 96 (“pre-soak), 500 hou

20、rs if the surface resistance falls by more than a decade, the flux fails also, there must be no dendritic growth http:/ electromigration ? conditions are designed to promote dendrite formation eg copper ions diffuse towards cathode where they are reduced to elemental copper. http:/ dendritic growth

21、http:/ all no-clean fluxes should be tested to a suitable protocol which will indicate the long term reliability of the residues under adverse conditions. all multicore fluxes are test under elevated temperature & humidity to the ipc and bellcore sir and electromigration tests. other protocols may b

22、e used on request. http:/ solvents a blend of solvents is often used to maximise the solubility of the flux components and give the best balance between printing and reflow characteristics a high boiling solvent will prolong tack & stencil life a low boiling solvent will give good reflow results wit

23、h little slump much research goes into the optimum blend of solvents http:/ gelling agents impart suitable rheology contribute to slump resistance the metal content of the paste will also effect these characteristics. http:/ flux: summary cleans substrate, component and powder to allow soldering to

24、take place may use acid, halide or a combination of the two as the “active ingredient” must be tested to the appropriate protocol if used as a no-clean flux http:/ solderability intermetallic component substrate solder http:/ solderability solderability relates to how well a component will solder un

25、der production conditions. there are three aspects to be considered wettability of metallisation resistance of metallisation to dissolution resistance to soldering heat http:/ wettability of metallisation mainly determined by the condition of the surface of the metallisation effected by oxidation, s

26、urface contaminants and films heat sink effects will contribute will effect the spread or wicking of solder http:/ resistance of metallisation to dissolution during soldering part of the substrate surface will be dissolved the rate of dissolution will depend on temperature, time, quantity of molten

27、solder, composition of solder and composition of metallisation low dissolution rates will give slow soldering high dissolution rates may result in de-wetting or solder contamination http:/ substrate dissolution solder partly dissolves the surface being soldered too much dissolution can contaminate t

28、he solder or remove the solderable coating rate can be reduced by loading the solder with constituent of substrate. eg. cu when soldering to cu (savbit) ag when soldering to ag/pd (sn62) http:/ dissolution rates auvery fast agfast cu, electroless nimoderate pdslow ni, ptvery slow http:/ resistance t

29、o soldering heat will the component withstand the soldering process? (e.g thermal shock) if a component is incapable of seeing the temperatures require to obtain a good solder joint it is unsolderable. http:/ solderability testing the simplest solderability test is the “dip & look” test this is non-

30、quantitative test flux component with a test flux, dip in molten solder & inspect only able to recognise very poor solderability components http:/ wetting balance in order to obtain an accurate idea of how solderable a component (or board) is, you must have control of all parameters alloy type alloy

31、 temperature pre-heat speed of immersion dwell time speed of withdrawal http:/ wetting balance the wetting balance should be capable of controlling all these parameters whilst making accurate measurements throughout the duration of the test in order to determine how well the solder wets (wetting for

32、ce) how fast the solder wets (wetting speed) if there is any evidence of de-wetting http:/ wetting balance very sensitive micro-balance copper coupon (fluxed) solder bath http:/ wetting balance f t http:/ wetting balance f t http:/ wetting balance f t http:/ wetting balance f t http:/ wetting balanc

33、e f t http:/ wetting balance f t http:/ wetting balance f t max wetting force time to 2/3 max force evidence of de-wettinghttp:/ de-wetting de-wetting is the withdrawal of solder from the surface after initial wetting has occurred a thin layer of solder will be evident over most of the surface with

34、irregular shaped “droplets” of solder here and there de-wetting will be caused by contamination of either the solder, substrate or component de-wetting will be influenced by soldering temperatures and times http:/ wetting balance f t the graphs are often shown this way up you can clearly see the speed and force of wetting evidence