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Ask and Answer -- October 23, 2002
December 31, 1969 |Estimated reading time: 3 minutes
Ask and Answer is a forum where SMT professionals with questions can meet up with those with the answers. If you know the answers to any of the questions below, or have questions of your own, please e-mail them to christinef@pennwell.com, referencing the reference number below each question, and we will publish them in a future column. Please include your name and the location you are writing from. SMT Online reserves the right to edit responses. Look for Ask and Answer every Wednesday.
QUESTIONSMike Bywater writes: Is it really worth going lead free? The impact on the environment for mining, refining, processing waste disposal and cleaning appears from current reports to be greater than the impact of small percentage of lead output going in to landfill. Has everyone missed the point of reducing the burden on the environment as a whole? Reference Number: 102301
George Lucas writes: Are surface mount metal film resistors and diodes susceptible to cracking like surface mount multilayer ceramic capacitors and what would be the expected failure modes for a cracked surface mount metal film resistor when exposed to a high humidity environment? Reference Number: 102302
Cheryl Johnson writes: I am getting ready to do my first flip chip on flex design and need to know if there is any good reference material I can use to find out the following (or if someone can just tell me the rules of thumb!): 1) "Balls" are ~3 mils as close as 7 mil center to center. 2) What size pad (1:1?) do I put on my copper layer? 3) Soldermask vs coverlay -- and what kind of openings for each? 4) Can traces be 3-4 mils as well? 5) What kind of notes would be different from standard flex? 6) Would anyone have a sample fab/gerber file set (even in pdf) that they could show me? Thank you. Reference Number: 102303
John Noel Villordon of China writes: I am working in an EMS company that manufactures electronic products and assemblies using surface mount technology. I would like to know how long a time can a standard SMA PCB/PCBA (thickness equal to 1 mm) be exposed in ambient/normal manufacturing temperature (21 to 24 degrees C) & humidity (40 to 60 RH) such that the accumulated moisture is still negligible? What is the moisture sensitivity JEDEC level of a standard double-sided SMA PCB? Thank you. Reference Number: 102304
Ruud Lobel of the Netherlands writes: Is there any rule of thumb or another way to know what is the ideal plated through-hole diameter vs. PCB thickness when trying to fill the holes with solder? I need this for improving the thermal conduction below a power device. What's best: applying solder (paste) to the holes, reflow and mount the SMD afterwards or do(ing) this in one single pass? Or are there other low-cost solutions that can be used instead of solder filled vias? Thank you. Reference Number: 102305
Les Pearson of California, Md., writes: I work for a company that repairs a RF test set for the Department of Defense. Essentially the unit is made up of 24 RF submodules. Each submodule has several hybrid ceramic substrates with gold print and SMT components on top. The bottom surface is glued in a shallow aluminum block cavity using conductive epoxy. Removal of the substrate for rework is difficult, expensive and time consuming. In trying to rework the substrate while attached to the aluminum block, I am unable to apply enough localized heat to effectively cause solder to flow before the heat rapidly dissipates.
Is there any new technology, technique, ideas that might help resolve my problem? Any help would be greatly appreciated. Thank you. Reference Number: 102306