Underfills

Capillary flow and Pre-applied Underfills for Flip chips and BGA arrays

AVAILABLE DIRECTLY AT CAPLINQ.COM

Liquid Encapsulants Underfills Optically Clear Materials Wafer Level Packaging

Semiconductor and Board level Underfills
Product	Description	Viscosity (MPa-s)	Glass transition temperature (°C)	CTE a1	CTE a2	Cure time (min)	Cure Temp °C	Available	Key attributes
Board level Underfills
Loctite 3508NH	Reworkable cornerbond	70,000	118	65	175	-	245	Yes	Pb free Solder reflow cure Preapplied to the pad corners
Loctite 3517M	Reworkable underfill for solder joint protection	2,600	101	65	191	5	120	Yes	Solder joint protection CSP, Flip chip, BGA packages
Loctite ABLESTIK ACP 3122	Anisotropic epoxy adhesive (Z axis)	22,000	100	-	-	5 seconds	150	Yes	Conductive only in z axis Anisotropic electrically conductive Low temperature cure
Loctite ECCOBOND E1172A	Non reworkable capilary underfill	17,000	135	27	85	6	135	Yes	Uniform and void-free Minimizes induced stress at the solder joint Use with very fine area array devices with 25 μm geometries
Loctite ECCOBOND E1216M	Capillary flow epoxy underfill	4,000	125	35	131	4	150	Yes	Snap, fast and low temp cure Non anhydride curing agents
Loctite ECCOBOND FP4502	Single component liquid epoxy underfill	35,000	115	22	84	30	165	Yes	Forms a rigid, low stress seal Extends thermal cycling performance Low CTE
Loctite ECCOBOND FP4526	Liquid epoxy capillary underfill	4,700	133	33	101	15	165	Yes	Suitable for applications that require high thermal cycling performance For capillary flow on flip chip applications with excellent reliability
Loctite ECCOBOND FP4527	Epoxy capillary underfill	12,000	158	26	87	15	165	Yes	Excellent wettability Low viscosity
Loctite ECCOBOND FP4530	Snap curable fast flowing Underfill	3,500	145	46	150	7	160	Yes	Changes from blue to green upon cure For flip chip on flex applications with a 25 μm gap
Loctite ECCOBOND FP4531	Snap curable non reworkable underfill	10,000	161	28	104	7	160	Yes	Passes NASA outgassing For flip chip on flex applications with a 25 μm gap
Loctite ECCOBOND UF1173	One component void free underfill	7,500	160	26	103	5	150	Yes	Can be jet or needle dispensed REACH/SVHCs compliant High Tg and low CTE
Loctite ECCOBOND UF3810	Reworkable epoxy underfill	394	102	55	171	8	130	Superseded by UF3811	Halogen free Higher Tg version of LOCTITE ECCOBOND UF 3800 Cures quickly at moderate temperatures
Loctite ECCOBOND UF3811	Reworkable epoxy underfill	354	124	61	190	30	110	Yes	Room temperature flow capability Higher Tg version of LOCTITE ECCOBOND UF 3810 Cures quickly at moderate temperatures
Loctite ECCOBOND UF3812	Reworkable epoxy designed for CSP,WLSCP and BGA	350	131	48	175	10	130	Yes	Flows at room temperature with no preheat High tg and high fracture toughness
Loctite ECCOBOND UF3820	Reworkable epoxy designed for CSP,WLSCP and BGA	340	133	51	172	10	130	Yes	Flows at room temperature with no preheat Stable electrical performance under thermal BIAS
Loctite ECCOBOND UF3915	Reworkable underfill designed for WLSCP and flip chip	5,500	125	25	100	7	160	Yes	Compatible with most Pb free solders Halogen free, Snap curable
Loctite ECCOBOND UF8806G	Cyanate ester with superior adhesion	4,500	136	27	97	45+60	195	Yes	1-3mil gap heights Silica filled
Semiconductor Underfills
Loctite ECCOBOND FP5201	BMI/Acrylate for flip chip	21,000	171	31	65	30	165	Yes	Thermal compression bonding Works well with Solder,Au, Cu pads
Loctite ECCOBOND NCP5209	Non conductive paste Underfill	12,500	145	28	80	30 + 120	Ramp to 160	Yes	#1 Non Conductive Paste Underfill Paste version of NCF218 Bump protection of flip-chip devices, with bump pitches less than 100 µm and gaps less than 40 µm
Loctite ABLESTIK NCF218	Non conductive underfill film	-	119	24	190	30 + 60	Ramp to 175	Yes	Film version of NCP 5209 Enables fine pitch, narrow gap Cu pillar TSV, Die to die & Die to substrate
Loctite ECCOBOND UF8830S	Liquid epoxy capillary underfill for BGA	22,120	118	25	100	30 + 120	Ramp to 150	Yes	Achieves MSL L3/L2A and passes all reliability requirements Excellent wettability to different substrates Formulated for tight bump pitch and narrow gap in flip chip BGA applications

Semiconductor and board level Underfills

Underfills are (mostly) epoxy based thermo set materials that contain (mainly) silica fillers and are designed to flow in the gap between the board and the component. They are typically applied after solder reflow and require heat cure.

Underfill materials need to have a low viscosity to allow them to flow under the components by capillary force. Substrate heating is also sometimes required to facilitate a good flow process and allow the capillary force to pull the underfill under the package.

Underfill properties

Thermally, underfills increase the thermal package reliability by distributing the stress across the surface of the die or substrate instead of being concentrated in the solder balls especially during thermal cycling (TC). It is important to point out that the peripheral (outside) bumps are stressed the most.

Mechanically, it is used to increased mechanical shock resistance in various configurations such as button push, drop resistance and bending. Specifically for drop performance, the UF properties provide exquisite improvement.

Finally, underfills reduce moisture absorption and chemical contamination. Moisture can create dendrite formations that would result in electrical shorts and create bridges in places where traditional conformal coatings wouldn't be able to penetrate.

Board level Underfills

Underfills come in two types. Semiconductor (die) level and board level. The board level types are the ones applied between the finished IC package (ie BGA) and the PCB.

There are three main types of board level underfills:

Full Capillary Underfill

Dispense material along edge(s), completely flows under the component
and fills the complete space
Improved thermal and mechanical reliability

Partial Underfill - Corner Bond

Dispensed before reflow
Dispense dots or “L” shape only at the corners
Small improvement in mechanical reliability, not in thermal reliability

Partial Underfill - Edge Bond

Dispense after reflow
Only covers corner fillets
Small improvement in mechanical reliability, not in thermal reliability
Can be UV or thermal Cure.

Standard Capillary underfill process

Capillary underfills have a pretty standard operational process. They are applied and cured post reflow so they don't have to withstand extremely high temperatures. The common steps one would have to follow are:

Stencil print with solder paste
Pick and place the die on the board
Take the package through the Reflow oven
Dispense the underfill by either jetting it or with a classic syringe
Cure the underfill for the recommended time and temperature
Done. You have a thermally, mechanically and environmentally protected semiconductor package

Preparing the substrate

Before applying the underfill we need to prepare the substrate for the application. As a first step we need to clean the substrate with either solvent wash or plasma to improve the surface tension and reduce flux residues.

Afterwards we prebake the board to remover the moisture and any remaining solvents.. Normally this is not needed after reflow but it can be a necessary step when we had the boards stored for long, in order to remove the moisture trapped in the board/components that can create outgassing voids when curing.

Heat that is typically in a range between 60°C and 100°C can reduce the viscosity and the surface tension while improving the wetting properties of the substrate.

Developing a capillary underfill process

Of course it is not as simple as it sounds (is it ever?). In order to find the ideal underfill process there is a series or steps you need to take to optimize it.

To begin with we need to calculate the required volume: L x W x H (gap) in mm. Using this value, times the specific gravity (can be found in the TDS) we get the volume measured in mg. Granted this is a rough calculation, not taking into account the solder bumps, but it is good enough.

Let's say that the calculated volume is 10mg. In this case we start with passes of 1 mg. Dispense 1 pass and wait until the material is under the component, dispense second pass... proverbially rinse and repeat. When the underfill is creating a fillet on the opposite site than the component is finished. Now it is the time to check the underfill quality by flat and X sections.

Next step is to reduce the amount of passes to check the maximum possible speed. Flow out and top die contamination should be the main points of focus and concern . If needed, changing hardware can also be a solution. A small nozzle can result in an unstable dispense process. With jetting the flowrate is determined by the dot weight, so the machine will calculate the amount of dots needed for the programmed dispense pass. For needle dispensing the flowrate is calculated by the amount dispensed per second. The robot speed will be adjusted to ensure the right volume per pass is dispensed.

Dispensing

The dispensing pattern needs to be optimized to avoid air encapsulation. There are I, L and U shaped dispense patterns with one or multiple passes. U is the most dangerous for void entrapment and should be carefully considered. These parameters depend on the ball grid layout.

The main challenge is to make the process as quick but also as good as possible, with quality always coming before speed. Avoiding the overflow and contaminating adjacent components is the main target for this process. A bigger amount per pass will give a faster process but a bigger flow out, so a compromise has to be made between how good an underfill needs to be and how fast you can apply it. Any flow out can impact volume repeatability and create solder extrusion so it is always advised to optimize it and have a dispense system with a built in weight scale for volume control.

Curing

Curing of the Underfill should occur as soon as possible after dispensing since moisture absorption becomes a bigger risk after a couple of hours. Also because of the filler settling we might need a gelling step to increase viscosity.

Thermal mass of the boards and components should be taken into account when settling the curing process. Keep in mind that the curing time and temperature in the TDS are guidelines that don't take the thermal mass into account. Testing needs to be done with thermo couples under the device to track time and temperature and be done with a full oven since the thermal mass will be massively different with one board and with a full oven. As a general rule, over-cure is not possible with time.

Application Notes & Troubleshooting

Flux residue tends to narrow the gaps between bump joints under the die and obstruct the flow of the viscous underfill material, thus, reducing the flux residue should minimize the voids.

Let me share a sample study (picture below) wherein shows that Flux A,B, and C have >30% flux residue exhibits "flow striations" phenomena and had the longest underfill flow time.

While flux D with only 4% flux residue exhibited the shortest underfill flow time with no flow striation, which meant the underfill flowed more evenly beneath the die as compared to the other 3 fluxes when the same underfill material was used under the same process conditions.