{"id":39022,"date":"2026-11-14T15:50:51","date_gmt":"2026-11-14T07:50:51","guid":{"rendered":"https:\/\/www.bestpcbs.com\/blog\/2026\/11\/thermal-grease-pump-out\/"},"modified":"2026-11-14T15:50:51","modified_gmt":"2026-11-14T07:50:51","slug":"thermal-grease-pump-out","status":"publish","type":"post","link":"https:\/\/www.bestpcbs.com\/blog\/2026\/11\/thermal-grease-pump-out\/","title":{"rendered":"Thermal Grease Pump-Out: Why an MCPCB Runs Hotter"},"content":{"rendered":"<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_89 ez-toc-wrap-left counter-hierarchy ez-toc-counter ez-toc-grey ez-toc-container-direction\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\" style=\"cursor:inherit\">Table of Contents<\/p>\n<span class=\"ez-toc-title-toggle\"><a href=\"#\" class=\"ez-toc-pull-right ez-toc-btn ez-toc-btn-xs ez-toc-btn-default ez-toc-toggle\" aria-label=\"Toggle Table of Content\"><span class=\"ez-toc-js-icon-con\"><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">Toggle<\/span><span class=\"ez-toc-icon-toggle-span\"><svg style=\"fill: #999;color:#999\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewBox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #999;color:#999\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewBox=\"0 0 24 24\" version=\"1.2\" baseProfile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/span><\/span><\/a><\/span><\/div>\n<nav><ul class='ez-toc-list ez-toc-list-level-1 ' ><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/www.bestpcbs.com\/blog\/2026\/11\/thermal-grease-pump-out\/#Thermal_grease_pump_out\" >Thermal grease pump out<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/www.bestpcbs.com\/blog\/2026\/11\/thermal-grease-pump-out\/#Thermal_paste_dry_out\" >Thermal paste dry out<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/www.bestpcbs.com\/blog\/2026\/11\/thermal-grease-pump-out\/#Thermal_interface_material_thickness\" >Thermal interface material thickness<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/www.bestpcbs.com\/blog\/2026\/11\/thermal-grease-pump-out\/#Thermal_grease_vs_thermal_pad\" >Thermal grease vs thermal pad<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/www.bestpcbs.com\/blog\/2026\/11\/thermal-grease-pump-out\/#Phase_change_thermal_interface_material\" >Phase change thermal interface material<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"https:\/\/www.bestpcbs.com\/blog\/2026\/11\/thermal-grease-pump-out\/#Thermal_cycling_test_procedure\" >Thermal cycling test procedure<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-7\" href=\"https:\/\/www.bestpcbs.com\/blog\/2026\/11\/thermal-grease-pump-out\/#Thermal_resistance_measurement\" >Thermal resistance measurement<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-8\" href=\"https:\/\/www.bestpcbs.com\/blog\/2026\/11\/thermal-grease-pump-out\/#Heatsink_mounting_pressure\" >Heatsink mounting pressure<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-9\" href=\"https:\/\/www.bestpcbs.com\/blog\/2026\/11\/thermal-grease-pump-out\/#FAQ\" >FAQ<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-10\" href=\"https:\/\/www.bestpcbs.com\/blog\/2026\/11\/thermal-grease-pump-out\/#Prepare_an_aluminum_PCB_thermal_review\" >Prepare an aluminum PCB thermal review<\/a><\/li><\/ul><\/nav><\/div>\n<div class=\"yzp-no-index\"><\/div><p>If an aluminum PCB runs hotter after repeated power cycles, first check whether the temperature rise is across the board-to-heatsink interface. Thermal grease pump-out is one possible cause: grease moves away from part of the loaded contact area. Grease at the edge alone is not proof. Compare the initial and aged thermal response under the same conditions, then inspect the contact footprint on a representative sample.<\/p>\n<figure><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/www.bestpcbs.com\/blog\/wp-content\/uploads\/2026\/11\/thermal-grease-pump-out-cover.jpg\" width=\"1200\" height=\"640\" alt=\"Aluminum circuit board beside a heat spreader with a thin thermal compound imprint\" style=\"display:block;max-width:100%;height:auto\"><figcaption>Illustrative interface, not evidence of a measured failure.<\/figcaption><\/figure>\n<h2><span class=\"ez-toc-section\" id=\"Thermal_grease_pump_out\"><\/span>Thermal grease pump out<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Repeated heating and cooling can change the gap between mating surfaces as they expand or warp. A mobile grease can move laterally as that gap changes and leave part of the hot area less well covered. <a href=\"https:\/\/research-hub.nlr.gov\/en\/publications\/degradation-characterization-of-thermal-interface-greases-3\/\" rel=\"nofollow\">Experimental work on thermal interface greases<\/a> describes this degradation under cycling. The risk depends on the material and assembly, not just where paste is visible after disassembly.<\/p>\n<p>Consider an LED driver on an aluminum-core board bolted to an enclosure. A uniform line of grease around the perimeter at first assembly can be ordinary squeeze-out. A growing temperature difference through the joint, paired with changed coverage in cycled samples, is stronger evidence. Opening a joint can disturb the imprint, so compare to an initial unit opened the same way.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Thermal_paste_dry_out\"><\/span>Thermal paste dry out<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Dry-out is a change in the paste\u2019s condition, whereas pump-out is movement out of the active interface. They can occur together. A dry-looking area does not show how or when it developed. Review storage, operating temperature and time as well as cycling, and ask the supplier what evidence is appropriate for the material.<\/p>\n<p>When the diagnosis matters, compare an unaged reference and a cycled unit. A separate time-at-temperature exposure, if agreed by the qualification team, can help distinguish changes driven by aging from those driven by repeated movement. Neither trial by itself proves the field failure mechanism.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Thermal_interface_material_thickness\"><\/span>Thermal interface material thickness<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>There is no universal bond-line thickness for PCB-to-heatsink joints. Measure or bound the assembled gap at its relevant locations, not just the dispensed layer or uncompressed sheet. For equal area and conductivity, a layer twice as thick has twice the ideal bulk thermal resistance, excluding contacts, spreading and voids.<\/p>\n<p>For an illustrative 20 \u00d7 20 mm interface, assume 2 W\/(m\u00b7K) and a uniform gap of 0.10 mm. The material-only estimate t\/(kA) is 0.125 K\/W. At 0.20 mm it is 0.250 K\/W. These are model inputs, not product specifications. If the real joint bows, averaging the gap may hide a thick region under the device.<\/p>\n<figure><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/www.bestpcbs.com\/blog\/wp-content\/uploads\/2026\/11\/thermal-grease-pump-out-scope.jpg\" width=\"1200\" height=\"640\" alt=\"Side view of a circuit board and its interface on an aluminum heatsink\" style=\"display:block;max-width:100%;height:auto\"><figcaption>Illustrative assembly showing the location of the interface, not a measured bond-line thickness.<\/figcaption><\/figure>\n<h2><span class=\"ez-toc-section\" id=\"Thermal_grease_vs_thermal_pad\"><\/span>Thermal grease vs thermal pad<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Use grease as a candidate for a thin, repeatable interface. A compliant pad can accommodate a larger tolerance stack, but its compressed thickness, contact resistance and force matter. Do not choose a pad only from its thermal conductivity rating or replace a required insulator with grease.<\/p>\n<table>\n<caption>Grease vs. pad vs. phase-change interface<\/caption>\n<thead>\n<tr>\n<th scope=\"col\">Candidate<\/th>\n<th scope=\"col\">When to evaluate it<\/th>\n<th scope=\"col\">What to verify<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Grease<\/td>\n<td>Flat mating parts and a controlled thin joint.<\/td>\n<td>Initially wetted area and performance after cycling.<\/td>\n<\/tr>\n<tr>\n<td>Compliant pad<\/td>\n<td>A tolerance gap that needs a resilient filler.<\/td>\n<td>Compression, loading, thermal resistance and any required isolation rating.<\/td>\n<\/tr>\n<tr>\n<td>Phase-change material<\/td>\n<td>A controlled interface that reaches the material\u2019s activation conditions.<\/td>\n<td>Assembly instructions, initial conditioning and aged performance.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h2><span class=\"ez-toc-section\" id=\"Phase_change_thermal_interface_material\"><\/span>Phase change thermal interface material<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>A qualified phase-change interface can soften through its designed temperature range and improve contact. It is not a universal solution for pump-out. Review the chosen product\u2019s activation conditions, loading and thickness. If the interface never reaches the required temperature or the gap is too large, the candidate may not establish the intended contact.<\/p>\n<p>Compare materials on independently assembled samples of the same board and enclosure. Do not interpret a later test on a reworked unit as a material-only comparison: cleaning, gap, fastening and assembly sequence may also have changed.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Thermal_cycling_test_procedure\"><\/span>Thermal cycling test procedure<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Choose a test that reproduces the movement and heat path of interest. External temperature cycling heats the assembly from its surroundings, whereas operating power cycles heat it from the active components. Document temperatures at relevant locations, fixture, clamp method, cycle count and inspection intervals. Do not substitute a generic cycle count for the intended service condition.<\/p>\n<table>\n<caption>Evidence for an aged aluminum PCB interface<\/caption>\n<thead>\n<tr>\n<th scope=\"col\">Observation<\/th>\n<th scope=\"col\">Check next<\/th>\n<th scope=\"col\">What helps distinguish it<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Hot from first startup<\/td>\n<td>Gap, pressure and initial coverage.<\/td>\n<td>Repeatable fresh assembly and flatness check.<\/td>\n<\/tr>\n<tr>\n<td>Interface temperature difference grows with cycles<\/td>\n<td>Coverage loss, gap changes and material aging.<\/td>\n<td>Matched thermal measurements plus untouched reference units.<\/td>\n<\/tr>\n<tr>\n<td>The enclosure is also hotter than before<\/td>\n<td>Airflow, cooling load or mounting boundary.<\/td>\n<td>Matched power, ambient and enclosure measurements.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h2><span class=\"ez-toc-section\" id=\"Thermal_resistance_measurement\"><\/span>Thermal resistance measurement<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Measure the assembly at stable, comparable power and cooling. Monitor points that bracket the interface when practical. State the sensor locations and whether the power used in the calculation represents heat through that path. If there are meaningful parallel paths, temperature difference divided by total power is an assembly comparison, not an isolated interface property.<\/p>\n<p><a href=\"https:\/\/store.astm.org\/d5470-17r24.html\" rel=\"nofollow\">ASTM D5470<\/a> provides a defined material test method for thermal transmission properties. It does not recreate the board flatness, fixture or duty cycle. Record both initial and aged behavior and apply the agreed thermal margin to the representative assembly.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Heatsink_mounting_pressure\"><\/span>Heatsink mounting pressure<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Clamping pressure affects how the material wets or compresses and how wide the operating gap remains. Screw torque is not a pressure map: fastener friction, clamp spacing and board stiffness can change the load under the heat source. If a fresh assembly already runs hot, check enclosure flatness, fastener sequence and board support against the approved drawing before blaming material aging. Do not tighten beyond the approved limits.<\/p>\n<p>Keep the same mounting conditions for candidate comparisons, and define the aged thermal result the assembly must meet. The <a href=\"https:\/\/www.bestpcbs.com\/blog\/2026\/11\/aluminum-pcb-enclosure-thermal-pad-gap\/\">aluminum PCB pad gap guide<\/a> explains how to check the static fit; a fit check alone does not qualify the aged joint.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"FAQ\"><\/span>FAQ<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<h3>Does grease at the edge prove pump-out?<\/h3>\n<p>No. It may have been squeezed out when assembled. Compare reference and aged samples.<\/p>\n<h3>Can higher conductivity fix a thick gap?<\/h3>\n<p>Not by itself. Thickness, contact resistance and coverage still matter.<\/p>\n<h3>Does tightening the screws always help?<\/h3>\n<p>No. It can change board bow or load. Follow the approved mounting requirements.<\/p>\n<h3>Is a dry-looking imprint conclusive?<\/h3>\n<p>No. Opening the joint may change it, and appearance cannot establish a cause.<\/p>\n<h3>Can a pad substitute for an electrical insulator?<\/h3>\n<p>Only when qualified for the required insulation and assembly conditions.<\/p>\n<h3>Should I compare temperatures at equal air temperature only?<\/h3>\n<p>No. Also match power, cooling, sensor positions and assembly conditions.<\/p>\n<h3>Can I approve a candidate using only its initial result?<\/h3>\n<p>Not if the risk is degradation during cycling. Measure after representative aging.<\/p>\n<h3>Can grease compensate for a warped board?<\/h3>\n<p>Do not assume so. Check the actual gap and the material\u2019s qualified operating range.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Prepare_an_aluminum_PCB_thermal_review\"><\/span>Prepare an aluminum PCB thermal review<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>Send Best Technology the board stack, enclosure drawing, interface material and temperature records at <a href=\"mailto:sales@bestpcbs.com\">sales@bestpcbs.com<\/a>. Identify the initial and aged conditions so the board and assembly evidence can be reviewed separately.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Distinguish thermal grease pump-out from dry-out and poor assembly. Compare thermal interfaces and verify aluminum PCB cooling after cycling.<\/p>\n","protected":false},"author":32827,"featured_media":39021,"comment_status":"open","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_uf_show_specific_survey":0,"_uf_disable_surveys":false,"footnotes":""},"categories":[174],"tags":[114,9121,9378,9379],"class_list":["post-39022","post","type-post","status-publish","format-standard","hentry","category-bestpcb","tag-aluminum-pcb","tag-thermal-cycling","tag-thermal-grease","tag-thermal-interface-materials"],"acf":[],"aioseo_notices":[],"aioseo_head":"\n\t\t<!-- All in One SEO 5.0.4 - aioseo.com -->\n\t<meta name=\"description\" content=\"Distinguish thermal grease pump-out from dry-out and poor assembly. 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