SMT007 Magazine

SMT-July2014

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10 SMT Magazine • July 2014 uncontrolled recycling was being carried out by uneducated individuals. While all is not yet well in that regard, that "hole" in the system is being addressed by businesses, NGOs and gov- ernments around the world. Not to be forgotten is the fact that the im- pact of the EU's decision has been significant and far-reaching and it has caused the industry to spend needlessly many tens of billions of dol- lars diverting the considerable talents of count- less talented engineers and scientists around the world to make products that are unfortunately proving less reliable and arguably less environ- mentally friendly electronics than those built with tin lead solders. The Devil We Know, Disrobed While soldering (especially tin-lead solder- ing) holds many benefits in terms of offering a means of mass assembly of electronic com- ponents to printed circuit boards and is funda- mentally simple, its application in the assembly and manufacture of electronic products of the present age is much more complex and fraught with opportunity for defects to be generated, but it is also the a demon we have elected to live with, for as the old saying suggests that "Bet- ter is the devil we know." The devil we know is at least familiar and the simple truth is that humans are creatures of habit and most of us abhor change. There is a lingering question in the current situation? Is dealing with the devil we know on a daily basis really worth the price we are paying? Following is a recitation of some of the many types of solder and solder-related defects that test and inspection is tasked with finding before a product reaches market. Bear in mind as these defects are recounted and reviewed that the cost of defects rises as a product moves fur- ther from the manufacturing line. a) Opens: Opens are discontinuities gener- ated in the soldering process that can be mani- fest in assembly in a number of ways. For exam- ple, a bent or lifted lead on a QFP component, missing solder ball on a BGA, insufficient solder on an LGA or the warpage of the component during the high-temperature, lead-free reflow process can all result in an open circuit. b) Shorts: Solder shorts are bridges of sol- der between one or more component leads on an assembly. As component lead pitch con- tinues to drop, the incidence of short circuits increases. Presently, the threshold of pain for most assembly is experienced when the lead pitch drops below 0.5 mm. c) Insufficient Solder: Insufficient solder is a condition where the amount of solder in a solder joint is less than desired or specified con- tractually through industry specifications or customer requirements. d) Excessive Solder: Excessive solder is obviously the opposite of the condition of in- sufficiency and is again measured against agree- ments. It also introduces a wild card because it is not what reliability testing is based on. e) Solder Cracking: Solder cracking is an obvious concern as it could result in a la- tent open circuit condition. Good during prod- uct test before shipping but then failing in the field. f) Tin Whiskers: Tin whiskers are small metal projections emanating from a solder joint. They can grow up to 15 mm long and given the fine pitch of today's components, they are a significant concern. There are also challenging because they are typically a latent defect that shows up unpredictably. Past research indicat- ed that the addition of lead to tin solder alloys would mitigate the formation of whiskers; how- ever, with the ban on lead in electronic solders the incidence of whiskers is on the rise. g) Poor Wetting/Dewetting: Good wet- ting is manifest by the presence of uniform coat of solder on both the leads of the compo- nent and terminations of the printed circuit to which they are joined. In areas of poor wetting or dewetting the solder thins appreciably in ar- eas leaving only a thin silvery sheen. h) Voids: Voids are defect which are often difficult to detect without use of special equip- ment such as an X-ray apparatus. The chal- lenge with voids is that they represent potential weakness in the solder joint owing to their in- consistent nature. Voids can be found both in through-hole and surface mount components. In the case of surface mount components the voids are often extremely small and are some- times referred to as champagne voids. ELECTRoniC inTERConnECTionS ElECTRoNIC ASSEMBly WITH SolDER continues

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