Circuits Assembly - August 2008 - (Page 37)

How Clean Must Bare Boards Be? The fabrication process is a prime source of contaminants. drill bits break fiber bundles (Figure 3). High levels of any ask why we need clean bare boards for sulfuric acid from etchants can be found on the innera “cleaned assembly” process. The answer is layers and, should delamination or poor processing most cleaning systems are designed to remove occur, can cause leakage and only the easy-to-get-to flux shorting. residues. But take, for instance, 4. Most bare boards have underneath low-standoff liquid photoimageable (LPI) areas, where it is difficult to soldermask. LPI epoxy mask reach some flux residues. Why has a range of porosity directly compound the issue of high related to the mask cure. The chloride levels from a HASL more the mask is cured, the flux or high sulfate residues tighter it becomes. Likewise, from trapped etchant below the less the mask is cured, the these components as well? more porous it will be. Mask To better understand what cure ranges from just above contamination levels must be tacky to hard cured. The probmet, we should understand lem is the mask must be at the where contamination resides Figure 1. Plugged vias can fill with etch residue. lower to middle end of the cure and from where it comes. Conwindow so as not to experience tamination on a bare board problems from over-curing or resides on and in the following flaking after three to five solareas: dering conditions. Soldermask 1. Metal surfaces (pads with a HASL flux and finish and plated holes) form thin will absorb a large amount of films of dry residue (including flux during the solder exposure. in hole and via barrels). The This is why the post-HASL smaller diameter the via, the cleaning processes must use more difficult the rinsing of good DI water and an effective these etch residues. If the via is soaping agent. SMOBC (solplugged on one end, this adds dermask over bare copper) is a cup effect, making rinsing Figure 2. So-called micropores hold residues a good process for minimizing more difficult (yet still pos- from etch, scrub and rinse processes. the metal exposure of the platsible) (Figure 1). ing process, but must be con2. Laminate surfaces withtrolled for effective cleaning. out soldermask. These areas So how clean should a bare have micropores in the top board be? Some will apprecilaminate, the result of copper ate the response, “It depends,” foil being pressed into the butbut we found that with multer coat of the laminate, then tiple localized extractions etching. This process leaves using an ionic cleanliness tester small (0.001"-0.003") micropover plugged vias, fine-pitch ores deep in the top surface pad areas and in soldermask(Figure 2). These indentations covered areas, we are able to hold residues from the etch, scrub and rinsing fabrica- Figure 3. Plating residues can get trapped behind understand the effects of much of the fabrication residue. Using tion steps prior to soldermask the barrel on loose weaves or when dull drill bits break fiber bundles. ion chromatography for the application. acetate, formate, chloride, bro3. Laminate surface on the mide, nitrate, sulfate, sodium, potassium and ammonia innerlayers also goes through etch and chemical rinses levels, we can see the process effects and have estabafter stack-ups are drilled and holes plated. Plating resilished limits for good assembly performance based on dues can wick up inside the fiberglass bundles, getting clean bare boards. trapped behind the barrel on loose weaves or when dull n circuitsassembly.com Circuits Assembly AUGUST 2008 Process Doctor M Terry Munson is with Foresite Inc. (residues.com); tm_foresite@ residues.com. This column appears monthly. 37 http://residues.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - August 2008

Circuits Assembly - August 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Screen Printing Better Manufacturing Auditing a Fabricatior Cutting Machine Programming Time Simultaneous Acoustic Imaging and Surface Mapping Tech Tips Soldering Test and Inspection Process Doctor Pb-Free Lessons Learned Getting Lean Component Advances Product Spotlight Ad Index Assembly Insider Techincal Abstracts

Circuits Assembly - August 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.