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Ask and Answer -- November 6, 2002
December 31, 1969 |Estimated reading time: 4 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.
QUESTIONSOrlando Pontes from Brazil writes: We intend to reballing BGAs using the reballing fixtures method so we have some questions regarding the stencil design used for paste printing and balls position:
- What (is) the appropriate thickness for the stencil to be used for paste printing to solder the new balls on (the) BGA.
- What (is) the appropriate design for the stencil (aperture size and thickness) to be used for position the balls (Ball Size: 20mils, Ball composition,63SN37PB) on the BGA.
Thank you. Reference Number: 110601
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: 110602
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: 110603
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: 110604
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: 11065
ANSWERSMike Bywater wrote on October 23: 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?
And the answer:
David Bergman writes: I'm not sure if that is the question at this point. The question should be: "Do you want to sell your products in Europe?" or "Do you want to compete green with Japan?" No one argues that the volume of lead used in electronics is a small portion of the whole. An IPC/EIA/EPA study is looking at some of the life cycle costs involved with changing over from lead bearing to lead free. This should be completed in 2003.
The argument used to be "It can't be done." That is no longer valid. The current argument is "It doesn't make sense." Maybe not, but July 1, 2006 is becoming the most likely date when it will have to be done. For those in Asia that want an update on the status of lead-free, IPC and JEDEC are holding a conference in Taipei, Taiwan, December 9-11.
Les Pearson of California, Md., wrote on October 23: 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.
And the answer:
John Parker writes: I don't have any experience with this, but it looks like some of the Zephyrtronics components might work. The aluminum cavity could be heated from the bottom with the pre-heater, and the hot air pen used for the de-soldering. Info is at www.zeph.com. The RF system sure wasn't designed for maintenance!