Fluxless oxide-reduction soldering and die bonding replace chemical flux with a reducing atmosphere, so the joint forms clean and there is no post-bond flux residue to remove. In advanced packaging, the fastest-growing back-end segment, the reducing agent is most often formic acid vapor carried in nitrogen, with forming gas (hydrogen in nitrogen) as the alternative route. At bonding temperature, formic acid reduces copper and tin oxides to bare metal plus carbon dioxide and water, and the mechanism runs in two steps: formate forms on the oxide roughly between 150 \u00b0C and 200 \u00b0C, then decomposes above that to leave a wettable surface. The result is oxide-free, void-minimized, residue-free joints.<\/p>
Two process families sit in scope, and both put the same demand on the reducing-gas stream: formic acid reflow soldering (solder bumps, die attach, power devices) and fluxless thermocompression and hybrid bonding (fine-pitch, copper-to-copper, chiplets, and high-bandwidth memory). What makes flow control here different from metering an inert gas is that the reagent cannot be measured accurately where it is used. The reducing dose has to be established without measuring the vapor, then carried to the chamber without loss.<\/p>\n[\/et_pb_text][et_pb_text admin_label=”App Note Header – Challenge” module_class=”article-section-label” _builder_version=”4.27.7″ _module_preset=”default” header_2_font=”||||||||” header_2_text_color=”#808080″ header_2_font_size=”28px” custom_margin=”||1rem||false|false” custom_css_free_form=”selector span {|| border-bottom: 1px solid #000;|| padding-bottom: 5px;||}” global_module=”46104″ saved_tabs=”all” global_colors_info=”{}” theme_builder_area=”post_content”]
Challenge<\/p>\n[\/et_pb_text][et_pb_heading title=”Formic acid cannot be metered like an inert gas” _builder_version=”4.27.7″ _module_preset=”default” title_level=”h2″ global_colors_info=”{}” theme_builder_area=”post_content”][\/et_pb_heading][et_pb_text _builder_version=”4.27.7″ _module_preset=”default” custom_margin=”||2rem||false|false” global_colors_info=”{}” theme_builder_area=”post_content”]
The reducing power is set by concentration, and concentration is a process variable in exactly the way soak temperature is: fluxless processes run the nitrogen carrier at a few percent formic acid vapor, with the patent literature describing a band of about 3 to 15 percent<\/a>. Formic acid dominates advanced packaging for a thermal reason: hydrogen in nitrogen reduces copper and nickel oxides effectively only at 350 \u00b0C and above, while tin-based alloys reflow well below that, so forming gas is right for gold-tin and high-lead die attach and wrong for most of the back end.<\/p> Getting that concentration wrong is costly in either direction. Under-dose, and residual oxide leaves incomplete wetting and a higher void fraction under X-ray or acoustic microscopy; a good atmosphere holds voiding under 1 percent<\/a> on direct bonded copper. Over-dose, and the excess produces tin steaming<\/a>, a fog of redeposited tin and tin-oxide particles that settles on the surfaces around the joint. Neither failure shows up at the tool; it surfaces later as a shifted void population across a whole lot, after the die is attached, when the only options left are scrap or rework.<\/p> What makes it hard is that the concentration cannot be read where it is used. Formic acid vapor associates into hydrogen-bonded dimers, and the monomer-to-dimer split shifts with temperature and partial pressure, so the mixture\u2019s effective molar mass, viscosity and thermal conductivity all move with the operating point, and it is not represented in the standard gas-property data that flow instruments rely on. Any instrument that infers flow from those properties, a laminar differential pressure meter or a thermal mass flow controller, reads the combined stream against a surrogate gas and carries a systematic, concentration-dependent offset. A Coriolis meter is the exception, because it measures mass directly and needs no gas model, but at the dilute, near-atmospheric density of a reducing-gas stream a gas-phase Coriolis measurement is impractical. So no instrument in the vapor returns an absolute concentration, and the dose has to be established another way.<\/p> Delivery is a materials problem as much as a thermal one. Formic acid attacks brass, aluminum and fluoroelastomers such as FKM, so the wetted path is stainless steel with an elastomer chosen for the service. The vapor condenses anywhere the path falls below the mixture\u2019s dew point, which puts the whole atmospheric-side run inside a heated zone, instruments included. A controller sitting at room ambient in a 75 \u00b0C line is a dense metal body with a large surface area, and it cools below the rest of the line.<\/p>\n[\/et_pb_text][et_pb_code _builder_version=”4.27.7″ _module_preset=”default” custom_margin=”||3rem||false|false” global_colors_info=”{}” theme_builder_area=”post_content”] Options<\/p>\n[\/et_pb_text][et_pb_heading title=”Where the concentration gets fixed” _builder_version=”4.27.7″ _module_preset=”default” title_level=”h2″ global_colors_info=”{}” theme_builder_area=”post_content”][\/et_pb_heading][et_pb_text _builder_version=”4.27.7″ _module_preset=”default” custom_margin=”||2rem||false|false” global_colors_info=”{}” theme_builder_area=”post_content”] Once formic acid is the reagent, the architectural decision is where the concentration gets fixed: thermodynamically, by saturating a carrier over the liquid, or gravimetrically, by metering the liquid and the carrier independently and letting their ratio define it.<\/p>\n[\/et_pb_text][et_pb_code _builder_version=”4.27.7″ _module_preset=”default” global_colors_info=”{}” theme_builder_area=”post_content”] The sparged route fits a concentration that is fixed and validated by joint quality; the metered route fits one that has to be set or defended as a number.<\/p>\n[\/et_pb_text][et_pb_text admin_label=”App Note Header – Selection” module_class=”article-section-label” _builder_version=”4.27.7″ _module_preset=”default” header_2_font=”||||||||” header_2_text_color=”#808080″ header_2_font_size=”28px” custom_margin=”||1rem||false|false” custom_css_free_form=”selector span {|| border-bottom: 1px solid #000;|| padding-bottom: 5px;||}” global_module=”46106″ saved_tabs=”all” global_colors_info=”{}” theme_builder_area=”post_content”] Selection<\/p>\n[\/et_pb_text][et_pb_heading title=”Instrumenting the two delivery routes” _builder_version=”4.27.7″ _module_preset=”default” title_level=”h2″ global_colors_info=”{}” theme_builder_area=”post_content”][\/et_pb_heading][et_pb_text _builder_version=”4.27.7″ _module_preset=”default” custom_margin=”||2rem||false|false” global_colors_info=”{}” theme_builder_area=”post_content”] Both routes are built from Alicat controllers, and differ mainly in how many instruments there are and where they sit.<\/p>\n[\/et_pb_text][et_pb_heading title=”The sparged route: one controller on the mixed stream” _builder_version=”4.27.7″ _module_preset=”default” title_level=”h3″ global_colors_info=”{}” theme_builder_area=”post_content”][\/et_pb_heading][et_pb_text _builder_version=”4.27.7″ _module_preset=”default” custom_margin=”||2rem||false|false” global_colors_info=”{}” theme_builder_area=”post_content”] The sparged route is the simplest way to deliver the reducing gas, and on a fixed recipe it is frequently the right one. Nitrogen is bubbled through a temperature-controlled pot of formic acid, and a single corrosion-resistant MCS on the combined stream holds the total reducing-gas flow to the tool. There is no liquid line, no pump, and no vaporizer to maintain: one controller does the whole job.<\/p>\t\t
\t\t\t\t \t\t\tArchitecture<\/th>\t\t\t\t Where the dose is fixed<\/th>\t\t\t\t What limits run-to-run repeatability<\/th>\t\t\t\t When to pick it<\/th>\t\t\t<\/tr>\t\t<\/thead>\t\t \t\t\t\t Sparged carrier: nitrogen saturated over liquid formic acid, mass flow control on the combined stream<\/td>\t\t\t\t In the acid pot. Concentration follows the liquid\u2019s saturation pressure at pot temperature; the controller sets total stream flow only.<\/td>\t\t\t\t Pot temperature control and sparge efficiency, both upstream of the measurement and invisible to it.<\/td>\t\t\t\t Concentration is fixed and already validated by joint quality, and the pot is well controlled. Simplest reagent handling: no liquid line, no pump, no vaporizer, and one instrument instead of two.<\/td>\t\t\t<\/tr>\t\t\t \t\t\t\t Metered: independent liquid and carrier mass metering into a vaporizer<\/td>\t\t\t\t In the ratio of two independently measured mass flows. No saturation step sits in the chain, so no pot temperature for the dose to follow.<\/td>\t\t\t\t The two mass measurements, and whether the vaporizer takes the liquid fully into the gas phase.<\/td>\t\t\t\t Concentration is itself a recipe variable, or the concentration window has tightened and the atmosphere has to be defensible in a process record. Costs a vaporizer, a liquid line and a reservoir to maintain.<\/td>\t\t\t<\/tr>\t\t<\/tbody>\t<\/table><\/div>[\/et_pb_code][et_pb_text _builder_version=”4.27.7″ _module_preset=”default” custom_margin=”||2rem||false|false” global_colors_info=”{}” theme_builder_area=”post_content”]