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SMT electronics assembly manufacturing forum.


Solderability of BGA and PCB

#2421

Solderability of BGA and PCB | 20 November, 2000

Sirs, Please do me a favor to answer the following questions, thanks a million! 1. Will the baking of BGA or PCB cause more oxidation before the SMT process? 2. What is the acceptable Life Time of solderability at customer site? The time that the solderability becomes oxidized and causes poor contact. 3. Is any recycled solder paste acceptable? What is the standard? Do we need to consider the oxide in the paste? If yes, what is the percentage?

Thanks again!

Gyver

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Michael Psrker

#2422

Re: Solderability of BGA and PCB | 20 November, 2000

First question I have for you is: Are you assuming that standard precautions are in place at your facility to minimize moisture content i.e. air- deionizers, vacuum sealed packaging, humidity control in storage and manufacturing?

You do not mention geographic region you are in. Some parts of the world have high humidity, causing bigger concerns. We are also coming into the winter months which will also raise the possibility of humidity in the air. These are seemingly plain, obvious questions - but are commonly overlooked by non-technical types. In other words, did you apply common sense before you asked these questions?

To be specific regarding your question # 1: Baking will create some oxidation, it is a natural occurance. Normally, the amount of oxidation is negligible, barely measurable and is easily corrected by the reaction with flux. Flux in solder is there to clean, etch or prepare the mating surfaces, it dissolves the oxidation. Depending on your environment (high humidity), you may need to consider a more aggressive flux to overcome oxidation. Check with your paste supplier - they should be able to tell you what is normal for your region.

Question #2 - expected Life of Product should give you some indication of solderability expectations. Generally, you should warranty for craftsmanship defects as you cannot control the abuse of your work in the outside world. Can you guarantee that the product will always be safe from humidity or other sources that will accelerate oxidation?

Question #3 - recycled solder paste? Who is doing the recycling? Are you receiving this from a certified supplier? If so, the amount of oxidation present should be near zero, as long as the flux is still active. If you are recycling your own, from old dried out paste, you will probably be doing yourself more harm than good by trying to save a few bucks here. Can you guarantee the right amount of flux to solder spheres? Can you accurately measure your paste viscosity? If you say no to either of these questions, pass on using recycled solder pastes.

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#2423

Re: Solderability of BGA and PCB | 27 November, 2000

Gyver: Lemme take a wack [if you�ll excuse the expression, given direction of the later discussion] at your questions.

Q1. Will the baking of BGA or PCB cause more oxidation before the SMT process? A1. It certainly will not cause less oxidation, will it? Here�s how baking affects your components to make them less solderable: * Heat makes most materials more active. This heat allows more oxygen to bond with the metal of your solderable surface. These metal oxides form an unsolderable crust that must be removed by flux. The more crust, the less solderability, the more � * Unsolderable intermetallic compounds that formed when the solderability protection was added to the components grow faster when heated and eventually will consume the surface, making it totally unsolderable.

Q2. What is the acceptable Life Time of solderability at customer site? The time that the solderability becomes oxidized and causes poor contact. A2. Dunno. A variety of factors affect the life time of solderability. Among them are: * Different types of solderable surfaces retain the ability to be soldered differently than others. * Processing methods affect the ability of solderable surfaces remain solderable. * Approach to protecting components from heat, humidity, and bad environmental factors affects solderability. Q3. Is any recycled solder paste acceptable? What is the standard? Do we need to consider the oxide in the paste? If yes, what is the percentage? A3a. What do you mean by "recycled solder paste"? Do you mean: * Unused paste that reclaimed by a paste recycler? This is very good stuff!!! It costs about 1/3 of regular paste and passes our in-bound tests. [Haaa, got ya goin� on that one, right? ;-)] * Unused paste that�s been sitting on a stencil, lost its luster, and is now sluggish? The longer solder paste is exposed to the environment the less it becomes. Solder balls oxidize, volatiles evaporate, flux absorbs water, etc etc. We have talked about solder paste handling guidelines previously on SMTnet [check the fine Archive]. Further, most paste suppliers provide guidelines to their customers. A3b. There is no standard. A3c. You need to be aware that paste oxidizes as you work with it. A3d. Who knows? For every handful of pennies you save by using lousy paste, you will pay $50 for a board that requires hours of rework. Get a grip!!!

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#2424

Re: Solderability of BGA and PCB | 27 November, 2000

Mike: It gets more humid in the winter? [I can see it�s true (not that I�d doubt you) cause I�m looking a a humidy map of the USA, right now] How does that work outin Californy? [Do the ocean breezes move the humid air in-land in the summer? Er what � a charmed life?]

Around here [in the great midwest] it gets less humid in the winter, because warm air can hold more water vapor than colder air. [The atmosphere always contains some moisture in the form of water vapor; the maximum amount depends on the temperature. The amount of vapor that will saturate the air increases with a rise in temperature. At 4.4� C (40� F), 454 kg (1000 lb) of moist air contains a maximum of 2 kg (5 lb) of water vapor; at 37.8� C (100� F), the same amount of moist air contains a maximum of 18 kg (40 lb) of water vapor.]

Thank you Mr. Science!!! Tada

We are going through IC lotion like people are slathering it on their bodies. [Hmmm, that sounds interesting. Howcumm I�m not invited? OK OK, I know why I�m not invited. I�m here talking to you guys, rather than swinging from the chandeliers down at Jimbo�s.]

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Michael Parker

#2425

Re: Solderability of BGA and PCB | 27 November, 2000

Prof. Whoopee - amazing facts that you find on your 3-D BB, it has completely confounded my comrade, Chumley.

Plainly spoken that around the SF Bay Area a.k.a. Silicon Valley(ever hear of the famous SF Fog?)during the winter months, near the coast and in the flats, our moisture content in the air (the stuff that promotes oxidation) increases in the morning and disappears in the afternoons (the fog burns off). That holds mostly true because our year round ambient temperature is approx. 65 degrees F. Once you get into the higher elevations (the Sierra Nevada Mountains), the ambient air temps lower significantly, humidity goes near zero and ionic charging is high. The only thing high around here is the old hippies who love their Humboldt County home grown, you can spot those types right away from their Grateful Dead t-shirts, gold chains around their necks with the ever present zircon encrusted tweezers prominately displayed. Yes, life on the Gold Coast is charmed. Demographically, you will find more electronics manufacturing in the potentially higher humidity zones (the flats)cause that is easier to control than the unpredictable qualities of static electricity in the air (in the mountains). We would rather rust our parts slowly than kill 'em dead with a good zap of electricity. Am I ramblin' on enough or should I continue?

faithfully yours, Tennessee Tuxedo

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