{"id":349,"date":"2026-09-06T09:00:00","date_gmt":"2026-09-06T09:00:00","guid":{"rendered":"https:\/\/rlxsolutions.com\/blog\/bga-underfill\/"},"modified":"2026-09-06T09:00:00","modified_gmt":"2026-09-06T09:00:00","slug":"bga-underfill","status":"publish","type":"post","link":"https:\/\/rlxsolutions.com\/blog\/bga-underfill\/","title":{"rendered":"BGA Underfill: Materials, Process & Applications"},"content":{"rendered":"

BGA underfill is a polymer material placed beneath a ball grid array package to mechanically support the solder-joint array and redistribute stress between the component and printed circuit board. RLX Solutions<\/a>, a contract electronics manufacturer in Concord, Ontario, supports both capillary and no-flow underfill as part of PCB assembly<\/a>.<\/p>\n

Why BGA Underfill Changes the Mechanical Load on Solder Joints<\/h2>\n

A ball grid array (BGA) connects to a circuit board through an array of solder balls hidden beneath the package. Those joints provide both electrical connection and mechanical attachment. The difficulty is that the package, solder and PCB do not expand and contract at identical rates as temperature changes.<\/p>\n

That mismatch creates shear strain across the solder array. Distance from the package’s neutral point matters because joints farther from the centre generally experience greater displacement during thermal expansion and contraction.<\/p>\n

Underfill changes the load path. Instead of leaving the solder balls to carry most of the mechanical strain between package and board, cured underfill bonds the package body to the PCB and distributes part of that load across a much larger area.<\/p>\n

This distinction explains why underfill is not simply an adhesive added to keep a component in place. Its material properties, coverage beneath the package and cure quality affect how loads move through the assembly.<\/p>\n

Consider a hypothetical 20 mm square BGA mounted on a 1.6 mm PCB. Without underfill, repeated thermal expansion produces displacement between the package and board that must be accommodated largely through the solder joints. Filling the gap couples the package and PCB mechanically. The solder joints remain electrically and structurally significant, but they are no longer acting alone.<\/p>\n

That benefit introduces a trade-off. Once a BGA has been underfilled, access to its hidden joints becomes harder. Rework planning therefore belongs in the underfill decision, rather than being considered only after a failed device needs replacement.<\/p>\n

Capillary and No-Flow Underfill Solve the Same Problem Differently<\/h2>\n

The two processes differ mainly in when the material is introduced.<\/p>\n\n\n\n\n\n\n\n\n\n\n
Decision Factor<\/th>\nCapillary Underfill<\/th>\nNo-Flow Underfill<\/th>\n<\/tr>\n<\/thead>\n
Material placement<\/td>\nAfter BGA soldering<\/td>\nBefore BGA placement and soldering<\/td>\n<\/tr>\n
How material reaches the joint field<\/td>\nCapillary action draws it beneath the package<\/td>\nPackage placement and soldering occur with material already present<\/td>\n<\/tr>\n
Process separation<\/td>\nSoldering and underfill are separate stages<\/td>\nUnderfill is integrated more closely with placement and soldering<\/td>\n<\/tr>\n
Key process concern<\/td>\nComplete flow beneath the package<\/td>\nCompatibility between material, placement and soldering process<\/td>\n<\/tr>\n
Inspection implication<\/td>\nHidden solder joints should be evaluated before underfill restricts access<\/td>\nProcess validation must account for the combined material and soldering sequence<\/td>\n<\/tr>\n
Rework consideration<\/td>\nUnderfill is added after initial solder-joint formation<\/td>\nMaterial is present during initial assembly<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Capillary underfill is dispensed along one or more package edges after the BGA has been soldered. Capillary forces pull the liquid into the narrow gap between the component and PCB, around the solder balls and across the underside of the package.<\/p>\n

No-flow underfill reverses the sequence. Material is applied before component placement. The BGA is then placed and processed with the material present. That removes a separate post-solder capillary-flow operation, but it makes the interaction among underfill chemistry, soldering and process temperature more significant.<\/p>\n

Neither process should be chosen solely because one removes a manufacturing step. The deciding question is whether the material and process sequence produce reliable solder joints and controlled underfill coverage on the actual package and board geometry.<\/p>\n

Board A vs Board B: Identical Layer Count, Different Assembly Risk<\/h3>\n

Take two hypothetical 8-layer boards using the same BGA package.<\/p>\n\n\n\n\n\n\n\n\n\n\n
Factor<\/th>\nBoard A<\/th>\nBoard B<\/th>\n<\/tr>\n<\/thead>\n
PCB layer count<\/td>\n8<\/td>\n8<\/td>\n<\/tr>\n
BGA pitch<\/td>\n0.8 mm<\/td>\n0.5 mm<\/td>\n<\/tr>\n
Via architecture beneath BGA<\/td>\nThrough vias outside pads<\/td>\nVia-in-pad beneath selected BGA pads<\/td>\n<\/tr>\n
Underfill approach<\/td>\nCapillary<\/td>\nCapillary<\/td>\n<\/tr>\n
Hidden-joint inspection need<\/td>\nBGA joints remain inaccessible optically<\/td>\nBGA joints plus via structures increase hidden-feature density<\/td>\n<\/tr>\n
Manufacturing implication<\/td>\nSimpler BGA escape geometry<\/td>\nMore fabrication and assembly processes interact beneath the package<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Layer count does not explain the manufacturing difference. Board B introduces via-in-pad beneath the package. A plated over filled via (POFV) requires resin fill, a copper cap and planarization so the BGA pad presents a suitable mounting surface.<\/p>\n

Underfill does not compensate for a poorly prepared BGA land. If the pad geometry or filled-via surface creates a placement or soldering problem, encapsulating the area afterwards can make the defect harder to access rather than fixing it.<\/p>\n

Material Selection Starts With the Package and Operating Environment<\/h2>\n

Underfill is commonly based on a polymer system formulated to flow or dispense into the package geometry and then cure into a solid mechanical support. Selection is an engineering problem because flow behaviour before cure and mechanical behaviour after cure affect different stages of production.<\/p>\n

Viscosity influences dispensing and flow. In a capillary process, material must travel through a small standoff gap and around a dense field of solder joints. A formulation that does not wet and flow appropriately can leave incomplete coverage or trapped regions beneath the package.<\/p>\n

After cure, mechanical properties become more important. The underfill must transfer load without creating an unsuitable stress condition of its own. Thermal behaviour also matters because the material is now mechanically coupling two structures that respond to temperature.<\/p>\n

Package size, solder-ball pitch, standoff, board construction, expected temperature exposure and rework strategy therefore belong in the material decision.<\/p>\n

A 12 mm BGA at 0.8 mm pitch and a 30 mm package at 0.5 mm pitch should not be treated as equivalent dispensing problems merely because both are BGAs. The larger package gives capillary material a longer distance to travel, while the denser joint field changes the flow paths available underneath it.<\/p>\n

This is why an underfill drawing should communicate more than “apply underfill.” The assembly documentation needs enough information to define which component receives material, the intended coverage and any keep-out constraints that affect nearby components or test points.<\/p>\n

The Capillary Underfill Process Starts Before Dispensing<\/h2>\n

Capillary underfill is a post-solder operation, so the quality of the BGA attachment should be established before material is allowed beneath the package.<\/p>\n

    \n
  1. Print and inspect solder paste.<\/strong> Solder paste inspection (SPI) measures paste volume, area and offset before reflow. A paste-printing defect beneath a BGA is much cheaper to address before the package has been soldered and underfilled.<\/li>\n
  2. Place and reflow the BGA.<\/strong> The solder array forms the electrical and mechanical connections between package and PCB.<\/li>\n
  3. Inspect the hidden joint field.<\/strong> Automatic optical inspection (AOI) cannot directly see the central solder balls beneath an opaque BGA. X-ray inspection can evaluate the hidden array and support void analysis.<\/li>\n
  4. Prepare the assembly for dispensing.<\/strong> The board and component condition must support controlled material flow. The dispense path also needs to avoid unwanted contamination of surrounding features.<\/li>\n
  5. Dispense underfill at the package edge.<\/strong> Material moves beneath the BGA through capillary action rather than being mechanically forced into every gap.<\/li>\n
  6. Complete the specified cure.<\/strong> The cure converts the dispensed material into the mechanical structure that couples package and board.<\/li>\n
  7. Inspect the completed operation.<\/strong> The inspection plan should address the material application and the assembly requirements established for the build.<\/li>\n<\/ol>\n

    The order matters. Underfilling first and trying to establish solder-joint quality later creates an avoidable problem because the encapsulant restricts physical access to the very area that may require rework.<\/p>\n

    Flow Distance Can Turn a Simple Dispense Into a Process Problem<\/h3>\n

    Imagine a hypothetical BGA with a 0.5 mm ball pitch and a 25 mm square body. Material introduced along one edge has substantially farther to travel to reach the opposite side than it would beneath a 10 mm package.<\/p>\n

    Adding more material at the dispense edge does not automatically guarantee uniform filling. Flow is governed by the geometry beneath the component and the material’s behaviour in that geometry. Voids or incomplete fill can remain hidden beneath an apparently satisfactory perimeter fillet.<\/p>\n

    That is the mechanism behind a common manufacturing mistake: judging underfill only by what is visible outside the package.<\/p>\n

    BGA Inspection Should Happen Before Rework Becomes Expensive<\/h2>\n

    BGA solder joints<\/a> cannot be fully assessed through conventional top-down optical inspection because the package itself blocks the view. X-ray inspection is therefore directly adjacent to the underfill process.<\/p>\n

    For an assembly using underfill, the inspection sequence has an additional consequence. Once cured material surrounds the solder joints, removing and replacing the BGA becomes more involved. Detecting a joint problem before underfill avoids adding an encapsulation process on top of an assembly that already requires correction.<\/p>\n

    X-ray can examine BGA joint structures and support void analysis. It does not turn every underfilled assembly question into a single pass\/fail image. The inspection criteria still have to match the workmanship class, component and assembly requirements.<\/p>\n

    For a new build, first article inspection also provides a useful control point. A first article establishes whether the production sequence, placement, soldering, inspection and specified secondary processes produce an acceptable assembly before the production lot is released.<\/p>\n

    That becomes particularly important when underfill is part of the build because the process changes later rework access.<\/p>\n

    Via-in-Pad and Fine-Pitch BGAs Connect Fabrication to Underfill<\/h2>\n

    Underfill is an assembly operation, but dense BGA layouts can make bare-board fabrication part of the same engineering decision.<\/p>\n

    Via-in-pad is one example. Placing a via directly in a BGA pad can free routing space, but the via structure has to provide an appropriate mounting surface. Resin fill, copper capping and planarization prevent the pad from behaving like an open hole during soldering.<\/p>\n

    Microvias create another interaction. A laser-drilled microvia can connect selected layer pairs without using the full-board depth of a conventional through via. Stacked and staggered microvia architectures can provide routing options under dense packages, but they also alter the fabrication sequence.<\/p>\n

    Consider a hypothetical 10-layer board with a 0.4 mm-pitch BGA. One routing approach might use Layer 1 to Layer 2 laser microvias, followed by Layer 2 to Layer 3 connections and buried structures deeper in the stack. The assembly engineer may see one BGA footprint, but the manufacturer sees a sequence of lamination, drilling, plating, filling, planarization, soldering, X-ray inspection and potentially underfill.<\/p>\n

    This is why underfill should not be isolated from design for manufacturability (DFM). The package attachment, pad structure, via architecture and encapsulation process all occupy the same small physical region of the circuit board.<\/p>\n

    Where BGA Underfill Earns Its Place<\/h2>\n

    Underfill is most useful where the mechanical loading expected at the package-to-board interface justifies adding an encapsulation process and accepting the corresponding rework implications.<\/p>\n

    The engineering case becomes stronger when an assembly faces repeated thermal movement, mechanical shock, vibration or another loading condition that places significant strain on the BGA solder array. The exact material and qualification plan should follow the product’s actual environment rather than a generic rule based only on package type.<\/p>\n

    It can also be relevant where package and PCB construction create a large thermal-mechanical mismatch. The mechanism remains the same: cured material redistributes stresses that would otherwise be concentrated more heavily in individual solder joints.<\/p>\n

    Underfill should not, however, be treated as a universal reliability upgrade. Adding polymer beneath a BGA changes manufacturing, inspection and repair. If the assembly does not require that change, the additional process can add cost and restrict rework without solving a meaningful failure mechanism.<\/p>\n

    Design the Assembly for Underfill Before Releasing Production Files<\/h2>\n

    The manufacturer needs an unambiguous definition of the intended process. Leaving the decision until the first assembled boards<\/a> arrive creates uncertainty around material selection, inspection sequence, keep-outs and rework.<\/p>\n

    For a board that requires BGA underfill, the release package should make the assembly intent clear. The bill of materials (BOM), pick-and-place or centroid data and assembly drawing establish the component and placement information, while fabrication files define the PCB underneath it. Stackup and fabrication notes become especially relevant when via-in-pad, blind vias, buried vias or microvias occupy the BGA escape region.<\/p>\n

    The engineer should also decide where underfill sits relative to inspection and functional verification. If a device needs replacement, discovering that before cure avoids unnecessary removal of encapsulant.<\/p>\n

    For example, compare two otherwise identical 6-layer assemblies with a 0.65 mm-pitch BGA. In the first sequence, the board is reflowed, X-rayed and then underfilled. In the second, underfill is committed before hidden-joint inspection is complete. A solder defect on the second assembly now carries an additional removal problem before BGA rework can begin.<\/p>\n

    The difference is procedural, not cosmetic. Process order changes the cost of failure.<\/p>\n

    Underfill Is Part of the Assembly Strategy, Not a Standalone Add-On<\/h2>\n

    The strongest underfill specification connects package geometry, solder-joint inspection, PCB construction, material selection, cure and rework planning before production begins. That prevents the encapsulation step from masking an upstream problem or creating an unexpected downstream constraint.<\/p>\n

    If your printed circuit board uses BGA underfill or another process around hidden solder joints, RLX can review the assembly and fabrication data as part of its engineer-reviewed DFM<\/a> process. Send the Gerbers, BOM, pick-and-place data and assembly drawing with the quote package so the board and assembly requirements can be evaluated together.<\/p>\n

    Frequently Asked Questions<\/h2>\n

    What is BGA underfill?<\/h3>\n

    BGA underfill is a polymer material placed between a ball grid array package and the PCB beneath it. After curing, it mechanically couples the package to the circuit board and redistributes loads that would otherwise be carried more heavily by the solder-ball array. It also changes how the component can later be inspected, removed and reworked.<\/p>\n

    What is the difference between capillary and no-flow underfill?<\/h3>\n

    Capillary underfill is applied after the BGA has been soldered, with liquid material drawn beneath the package by capillary action. No-flow underfill is introduced before component placement and is present during the soldering process. The choice affects the assembly sequence, material requirements, inspection timing and how underfill interacts with solder-joint formation.<\/p>\n

    Can a BGA be X-rayed before underfill?<\/h3>\n

    Yes. X-ray inspection is particularly useful for BGA packages because the solder joints are hidden beneath the component and cannot be fully examined by conventional optical inspection. Performing hidden-joint inspection before a post-solder capillary underfill operation can identify assembly defects before cured encapsulant makes component removal more involved.<\/p>\n

    Does via-in-pad affect BGA assembly?<\/h3>\n

    Yes. A via located directly in a BGA pad changes the mounting surface and therefore has to be considered as part of both PCB fabrication and assembly. A plated over filled via uses resin filling, copper capping and planarization to create a suitable pad surface. Underfill should not be expected to correct an unsuitable pad or via structure.<\/p>\n

    Is underfill required for every BGA?<\/h3>\n

    No. The engineering reason for underfill comes from the mechanical and environmental demands on the package-to-board connection, not simply from the presence of a BGA. Package construction, PCB construction, thermal cycling, vibration, shock, inspection requirements and future rework all affect whether adding underfill makes sense for a particular assembly.<\/p>\n

    What files are useful when quoting a PCB assembly with underfill?<\/h3>\n

    A complete PCB assembly package should include the manufacturing files needed for the board and component placement, including Gerber, ODB++ or IPC-2581 data as applicable, the BOM, pick-and-place or centroid file and assembly drawing. Stackup, impedance targets and fabrication notes should also be supplied where they apply to the circuit board.<\/p>\n","protected":false},"excerpt":{"rendered":"

    BGA underfill is a polymer material placed beneath a ball grid array package to mechanically support the solder-joint array and […]<\/p>\n","protected":false},"author":2,"featured_media":348,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"nf_dc_page":"","site-sidebar-layout":"default","site-content-layout":"","ast-site-content-layout":"default","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"default","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"footnotes":""},"categories":[17],"tags":[],"class_list":["post-349","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-pcb-assembly"],"_links":{"self":[{"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/posts\/349","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/comments?post=349"}],"version-history":[{"count":0,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/posts\/349\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/media\/348"}],"wp:attachment":[{"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/media?parent=349"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/categories?post=349"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/tags?post=349"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}